Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Returning to Venus

Space Probes have helped us build a good picture of Mars, yet Earth's inner neighbor Venus is still very much a mystery. But, writes Paul Sutherland, a host of new missions are being planned to help us learn more about it.

Image by Anna M., Venus Transit

Nothing was known about the surface of Venus before the Space Age because it is completely obscured by clouds. Scientists once speculated that it might be a raging ocean, or a Sahara-like desert. The first probe to provide answers was NASA's Mariner 2, which flew past Venus in December 1962, discovering that surface temperatures must be extremely high and that, like Uranus, it rotates in the opposite direction to the rest of the planets in the Solar System. Since then, orbiting probes with cloud-piercing radar have produced maps of Venus's surface and Soviet landers have confirmed that conditions are completely inhospitable on the ground. There is neither sea nor desert, but rather a landscape resembling a vision of hell.

Fire and Brimstone

The surface temperature is twice the maximum found in a kitchen oven and the pressure of the poisonous atmosphere is 90 times that at sea level on Earth, which crushed probes making early landing attempts. The first Soviet probe to reach the surface and send back signals was Venera 7 in 1970, which survived for 23 minutes. It was followed by the more successful Venera 8 in 1972, which returned data on the surface temperature and pressure, wind speed and illumination, before being destroyed after 63 minutes. The probes had to be built like submersibles to withstand the air pressure, but their electronics quickly failed in the extreme heat. Subsequent Soviet landers in the 1970s and 1980s sent back crude photos of a rocky landscape.

Helium balloons were released into Venus's higher, cooler atmosphere in June 1985 by two Soviet Vega probes that were on their way to Halley's Comet. They gathered data for 47 hours each as they floated 50km high in the cooler clouds.

NASA's Mariner 10 flew past Venus in 1974 en route to Mercury and managed to image wind patterns in the clouds. This was followed by a dedicated US mission, Pioneer Venus, made up of two spacecraft that arrived in December 1978. An orbiter studied the atmosphere and made radar maps of the surface. The other component was a multiprobe made up of a transporter and four separate probes that were fired into the atmosphere, returning data for an hour.

NASA's next mission, Magellan, carried out extensive radar imaging of Venus from a polar orbit in the early 1990s. Its imaging of almost the entire surface revealed it was covered with volcanoes. Scientists suspect many are still active, but still no one can say for certain.

ESA's first envoy, Venus Express, was launched in November 2005. During the eight-year mission, the spacecraft's swooping orbit brought it low over the cloud tops and revealed big variations in the sulfur dioxide content, suggesting that the volcanoes were still active. Its fuel exhausted, Venus Express was purposefully destroyed in the atmosphere in early 2015.

A Japanese space probe called Akatsuki, launched towards Venus in 2010, looked lost after a fault caused it to fly past the planet. But five years later, mission controllers managed to rescue it and put it into a new, more elongated orbit where it began to survey the atmosphere.

Two NASA missions to the outer planets also gathered data on Venus as they flew past to get a gravitational boost on their long journeys. Galileo shot past on its way to Jupiter in February 1990, taking pictures, measuring dust, charged particles and magnetism, and making infrared studies of the lower atmosphere. Saturn probe Cassini made two flybys in April 1998 and April 1999 when it looked for, but failed to spot, lightning in the clouds.

The Trouble with Landers

Current proposals for future Venus missions are focusing on orbiters and a new generation of balloons and aerial vehicles. Experts see too many difficulties in sending a lander to explore the surface like the Martian rovers. As planetary scientist and Venus expert Dr. Colin Wilson of the University of Oxford explains: "The issue is the heat, because silicon electronics simply don't work at these temperatures. There are studies into building a new kind of electronics using silicon carbide — this is of interest also for use inside car engines and jet engines — but even if you sort that out, you still have the problem of how you are going to power a probe on the surface. Although Venus is closer to the Sun, only one or two per cent of the sunlight at the cloud tops reaches the surface and so solar panels are completely impractical. Radioactive power sources have been suggested, but this would make the mission both expensive and difficult."

Two proposals to explore Venus are on a shortlist of five Solar System projects currently being considered for the next round of NASA's Discovery Program, missions that could launch in the early 2020s. One, called DAVINCI (Deep Atmosphere Venus Investigation of Noble Gases, Chemistry and Imaging) is being studied by NASA's Goddard Space Flight Center. It is an entry probe designed to study conditions between the dense cloud tops and the surface. "It will have much more modern and accurate instrumentation than previous probes," says Wilson. "All their temperature sensors failed in the lower atmosphere so we have hardly any data about atmospheric processes near the surface. DAVINCI is really going to give us a much better understanding of the deep atmosphere of Venus, in particular of its chemistry." The other proposal, from NASA's Jet Propulsion Laboratory, is for a new orbiter called VERITAS (Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy) that aims to produce radar maps of the planet in much higher resolution than before. It has strong European support, including for a French-German infrared camera that will look for hot volcanic material on the surface.

Looking Further Ahead

But a further attempt to get a new spacecraft to Venus will come with a UK-led proposal to ESA for a mission called EnVision. The probe will be another orbiter with advanced radar to detect tiny changes in surface features, at centimeter scale, that could confirm lava flows or similar surface deformation. Looking farther ahead, Venus scientists are keen to see a new generation of balloons or airships to taste the planet's atmosphere. It has been suggested that simple microbial life might exist in the cloud tops, though this is pure speculation. One advanced concept being prepared in the US is for a delta winged aircraft called VAMP (the Venus Atmospheric Maneuverable Platform) to be dropped by an orbiter into the clouds. Once in the atmosphere it would switch to flight phase, spending up to a year maneuvering between the upper and mid cloud layers, gathering data to send back to Earth. During the Venusian day, it would fly in the higher atmosphere, charging its batteries from the sunlight, before dipping to lower regions again at night.

ABOUT THE WRITER
Paul Sutherland is a space journalist, and the author of Where Did Pluto Go? Each month he reports on the latest space research in BBC Sky at Night magazine.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Realm of Ghosts

Will Gater dives into the region of Virgo, one of the most extraordinary swaths of the night sky, to reveal the science behind the faint galaxies that reside there.

Image by Douglas S, M104 — Sombrero Galaxy

Cast your eye across the band of sky between the star Spica and the handle of The Big Dipper on a clear spring night and what do you see? Save for few stars there doesn't appear to be much to write home about — at least not to the naked eye.

Yet this great swathe of the celestial sphere is arguably one of the most extraordinary patches of our night sky as it's peppered with hundreds of galaxies. Virgo is swarming with faintly glowing celestial forms; there are delicate spiral wisps, fuzzy ellipticals and even great clusters of galaxies gathering in their multitudes. And though they may be hidden to the naked eye, these spectral, stellar gatherings are of enormous interest to astrophysicists. So come with us as we explore the science of some of the more famous inhabitants of Virgo's ghostly galactic realm.

The Sombrero Galaxy

Virgo is a constellation famed for its huge population of distant celestial smudges, one of which is our first object, Messier 104. M104 actually sits close to the border between Virgo and the more southerly constellation Corvus, approximately 11° from the bright star Spica (Alpha Virginis). M104 is more commonly known as the Sombrero Galaxy and it's not difficult to see how it acquired this name when you look at it through a large telescope or see images of it taken by astrophotographers. Its scientific story is every bit as striking as its visual appearance too.

Perhaps its most obvious feature is the dark swathe across the bright mass of stars that make up its glowing oval shape. The swathe is a silhouetted portion of the galaxy's disc of dust and gas, which is viewed edge on from our line of sight. Hubble Space Telescope images have shown this disc in remarkable detail, revealing intricate structures in the dust lanes there. Infrared observations made with the Spitzer Space Telescope meanwhile have revealed that, unusually, M104's disc sits within another, larger elliptical galaxy, only part of which we see in visible light and which only becomes more fully apparent at longer infrared wavelengths.

Abell 1689

Astronomy is full of mind-bending physics — and there's no shortage of weird and wonderful behavior in and around galaxies. Nowhere is this better demonstrated than when distant galaxies swarm together in vast clusters. Abell 1689 is one such galaxy cluster that astronomers have scrutinized intensely in recent decades. It lies at the heart of Virgo, around 7.5° east of the bright star Porrima (Gamma Virginis). At a distance of over two billion lightyears from us, and extremely faint, this cluster is not one you'll be tracking down through the eyepiece of a modest back-garden telescope. But thanks to the powerful orbiting eye of the Hubble Space Telescope, this faraway galactic gathering has been imaged in spectacular detail revealing a lot more than just the individual glowing members of the cluster itself.

Scan your eyes over Hubble's image of Abell 1689 (right) and you might see what makes the cluster so interesting. Scattered throughout it are thin, hair-like arcs of light. These aren't exotic celestial structures, but highly warped visions of other galaxies that sit far beyond the cluster. These arcs appear because the huge combined mass of the cluster galaxies distorts the space surrounding it, causing it to behave like a lens. Though the quality of the image provided by this gravitational lens might raise eyebrows in amateur telescope-making circles, the lens shares one key trait with the telescope lenses we use: it can reveal distant objects that we might otherwise be unable to see. Indeed in 2008 researchers announced that they'd used Hubble, in conjunction with the Abell 1689's gravitational lens, to observe a distant galaxy in the early Universe, some 700 million years after the Big Bang.

M87

Look at any image of the rich fields of galaxies in and around the constellation of Virgo, and among the stars and galactic swirls that fill your view you'll see numerous bright ovals of light. These are elliptical galaxies and although they may not have the beauty or spectacular star-forming regions of their spiral cousins these often vast stellar swarms are some of the most enigmatic intergalactic inhabitants we know of. Foremost among the ellipticals in this part of the sky is the gargantuan M87. It's truly a giant — a recent study by astronomers at the European Southern Observatory was able to determine the size of the halo of stars around the galaxy: the ring of stars spans about 980,000 lightyears, dwarfing the Milky Way's stellar halo, which measures roughly 640,000 lightyears across.

However, M87's most famous feature is not its size but what lies at its heart: a supermassive black hole. Unlike the Milky Way's central black hole M87's is active. Images of the galaxy show an enormous jet emanating from the black hole; the jet is glowing due to light released by high-energy particles that are racing at tremendous speeds along magnetic field lines within it.

Aside from the jet and some globular clusters, though, the rest of M87 might seem rather bland in visible-light. At other wavelengths, however, a hidden maelstrom of activity in and around the enormous galaxy is revealed. Radio telescopes, for example, have observed huge glowing streams of material associated with the black-hole jet, while X-ray images from the orbiting Chandra observatory show immense swirling clouds of superheated gas within the galaxy. Something to consider the next time you set eyes on that seemingly placid, fuzzy patch in your telescope's eyepiece.

The Virgo Galaxy Cluster

Ask a seasoned stargazer to name one of their favorite spring galaxies to observe and chances are it'll be located in Virgo. The constellation boasts an extraordinary array of galactic treasures, including some of the most famous in the sky. It's perhaps no surprise then that many of the galaxies crowding into this part of the heavens are associated, that is they're all part of an enormous — and in cosmic terms relatively nearby — grouping known as the Virgo Galaxy Cluster. Recent surveys suggest that there are some 1,900 galaxies in this cluster, which sits roughly 56 million lightyears from the Milky Way.

The cluster counts within its number many relatively bright galaxies that are familiar to amateur astronomers — for example M87 as well as M86, M84 and the others that make up the sweeping curve of galaxies known as Markarian's Chain. The heart of the cluster itself lies in the region around 6° west of the star Vindemiatrix (Epsilon Virginis). However, modern studies have shown that there are members of the cluster spread all over this patch of sky, with some in neighboring constellations of Coma Berenices and Leo too.

NGC 4488

Looking out into the cosmos, the distances to even the nearest galaxies can seem immense, and yet spiral galaxies collide frequently. As two galaxies approach, their gravitational interactions cause them to distort each other.

You can get a sense of what happens when spiral galaxies engage this way if you look into the constellation of Virgo — specifically within Markarian's Chain. In the chain are two galaxies — known as The Eyes — that lie roughly 50 million lightyears from us. The pair are catalogued as NGC 4438 and NGC 4435, and deep images of NGC 4438 show a contorted jumble of scattered dust lanes and ribbon-like streams of stars around a brighter, central region. Astronomers think that what we're seeing in NGC 4438 is actually a spiral galaxy that's been disrupted by a violent encounter with the elliptical galaxy M86, which now sits less than 0.5° away on the sky.

The Whirlpool Galaxy

This journey across Virgo's ethereal realm of galaxies has taken us across some 60° of the sky and we end our exploration of this extraordinary region with one of the most beautiful galaxies anywhere on the celestial sphere. M51, otherwise known as the Whirlpool Galaxy, has captivated astronomers for centuries and continues to intrigue both amateurs and professionals today. M51 was being scrutinized by astronomers long before its true nature — as a galaxy in its own right and not just another glowing nebula within the Milky Way — was really known.

William Parsons, the third Earl of Rosse, famously sketched M51 in 1845 using the enormous Leviathan of Parsonstown, a 72-inch reflecting telescope housed at Birr Castle in Ireland. His exquisite drawing clearly depicts the sweeping form of the Whirlpool — and its neighbor, the galaxy NGC 519 that's instantly recognizable in the astro images taken with today's photographic equipment.

Our perspective of M51, looking down on the galaxy's disc, affords us a superb view of the physics unfolding there. Within the disc, density waves have formed spiral arms, which are home to vast numbers of hot, relatively young, blue stars. Photographs of the galaxy reveal another striking feature of these arms: numerous crimson patches of light scattered throughout M51's disc. This feature is one that, just like the hot young stars, is testament to the star formation occurring there. These crimson patches are regions where the radiation of infant and newborn stars is exciting their surrounding maternal nebulae, causing the gas clouds to shine with the characteristic ruby hue of glowing hydrogen.

These dramatic flourishes of star formation aren't the only dynamism on display with the Whirlpool Galaxy either. NGC 5195 is interacting with M51 and long-exposure images of the pair show extensive swathes of stars — known as tidal streams — near the galaxies that have been drawn out during this gravitational dance.

See the Galaxies

Although none of the galaxies we've covered here are visible to the naked eye, several, such as M104 (the Sombrero Galaxy) and M51 (the Whirlpool Galaxy), are fine sights through amateur telescopes. If you've never observed a distant galaxy through a telescope before, you'll soon realize why many astronomers affectionately refer to deep-sky objects as faint fuzzies. It's a description that sums up rather well the view of many galaxies through the eyepiece of a modest amateur telescope: a faint, fuzzy blob. That's not to say there aren't brighter examples that show more structure or interesting features, such as M104's dark bar, though. As with many celestial objects the key to seeing more detail is to get away from light pollution and use a larger aperture telescope. If you don't have one then pay a visit to your local astronomical society observing evening or star party during the galaxy seasons of spring and autumn. These events often provide access to large-aperture instruments.

ABOUT THE WRITER
Will Gater is an astronomy journalist, author and presenter. Follow him on Twitter at @willgater or visit willgater.com

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Treasures of Orion

Paul Money takes us on a tour of the best sights the Hunter has to offer.

Image by Ron Adams, M42 the Great Orion Nebula

Orion holds something for everyone, whether you enjoy the naked-eye splendor of its stars, want to tour it with a pair of binoculars, peer deeper with a modest telescope or delve into its deepest and faintest targets with 10- to 14-inch systems. It is easy to enjoy the view of the Orion Nebula alone, but a host of astronomical treasures awaits those willing to look a little closer. In this article we reveal some of Orion's most striking features and the equipment needed to see them.

Naked Eye

Allow 30-40 minutes for your eyes to adapt to the dark before you start observing.

Betelgeuse (Alpha Orionis)
RA: 05h 55m 10s
Dec.: +07° 24' 25"
We begin with the most famous star in Orion, mag. +0.5 Betelgeuse (Alpha Orionis). An unmistakable bright orange star of spectral class M0, Betelgeuse is often cited as the most likely red supergiant to go supernova any time in the next million years.

Rigel (Beta Orionis)
RA: 05h 14m 32s
Dec.: –08° 12' 06"
On the opposite side of the Belt stars to Betelgeuse is mag. +0.2 Rigel (Beta Orionis). In contrast to Betelgeuse, Rigel is a brilliant blue-white star of spectral class B8. It is technically a little brighter than Betelgeuse despite being designated Beta.

Binoculars

Delights await you whether you have a pair of 7x42s, 10x50s or 15x70s.

Orion's Belt
RA: 05h 36m 12s (Alnilam)
Dec.: –01° 12' 07" (Alnilam)
With 10x50 binoculars you will see a little deeper. The 6° field of view allows a stunning view of the three stars that form Orion's Belt: mag. +1.9 Alnitak (Zeta Orionis), mag. +1.7 Alnilam (Epsilon Orionis) and mag. +2.4 Mintaka (Delta Orionis). All three are B0 spectral class.

Sword of Orion
RA: 05h 35m 16s (Theta Orionis)
Dec.: –05° 23' 23" (Theta Orionis)
For now let's sidestep the Orion Nebula, as the sword also contains the wonderful open cluster NGC 1981 at the top. A group of stars including mag. +4.6 42 Orionis and mag. +5.2 45 Orionis sits north of the Orion Nebula (M42) and the adjacent De Mairan's Nebula (M43), which itself is above mag. +2.8 Hatsya (Iota Orionis).

Meissa (Lambda Orionis)
RA: 05h 35m 8s
Dec.: +09° 56' 03"
Mag. +3.5 Meissa (Lambda Orionis) is found in a neglected group of stars known as Collinder 69 or the Lambda Orionis Association. Meissa makes a triangle with mag. +4.4 Pi1 Orionis and mag +4.1 Pi2 Orionis. Meissa and the cluster it resides in are thought to be 1,100 lightyears away and certainly worth looking at with larger binoculars.

Orion's Shield
RA 04h 49m 50s (Tabit)
Dec.: +06° 57' 40" (Tabit)
Another neglected pattern is that of Orion's Shield, formed by the six stars designated Pi Orionis (mag. +4.6 Pi1, mag. +4.4 Pi2, mag. +3.2 Pi3, mag. +3.7 Pi4, mag. +3.7 Pi5, and mag. +4.5 Pi6). They form a curved line best seen with low-power binoculars, such as a pair of 7x42s, as the distance between the two ends of the shield is 8.5°. Pi3 Orionis, also known as Tabit, is a relatively close 26 lightyears away.

Small Telescope

Use a reflector up to 6 inches or refractor up to 4 inches and you'll see more detail.

The Orion Nebula
RA: 05h 35m 16s (Theta Orionis)
Dec.: –05° 23' 23" (Theta Orionis)
The Orion Nebula is the showpiece of the constellation and really comes alive with a small refractor. It has two patches with Messier designations: M42 is the main nebula, its wisps and tendrils stretching out from the central Trapezium Cluster. Just above it is the much smaller M43, also known as De Mairan's Nebula.

M78
RA: 05h 46m 45s
Dec.: +00° 04' 45"
M78 would be the showcase nebula of the constellation were it not for the Orion Nebula. It possesses two stars immersed in nebulosity, shines at mag. +8.0 and from Earth looks like a typical white-sheeted ghost. Look out for nearby NGC 2071: it is smaller than its neighbor but shines at mag. +8.0.

NGC 2112 and Barnard's Loop
RA: 05h 53m 45s (NGC 2112)
Dec.: +00° 24' 39" (NGC 2112)
The emission nebulosity described as Barnard's Loop is well known among astrophotographers, yet part of its section above and slightly east of M78 can be traced with a 6-inch Dobsonian. This faint, 'milky' patch curves and ends close to mag. +9.0 open cluster NGC 2112. Low magnification is best for the loop.

Sigma Orionis
RA: 05h 38m 44s
Dec.: –02° 36' 00"
Close to mag. +1.9 Alnitak (Zeta Orionis) is mag. +4.0 Sigma Orionis, which appears as a stunning multiple star system through small to medium telescopes. There are four splittable stars, the brightest of which is another double ? though this one is too tight to resolve in amateur instruments.

The Flame Nebula (NGC 2024)
RA: 05h 41m 55s
Dec.: –01° 51' 00"
The Flame Nebula needs dark skies and low magnification to see well. Use a 6-inch reflector, making sure you keep nearby Alnitak out of the field of view to improve contrast, and you should be able to see its mottled fan shape. As a bonus, reflection nebula NGC 2023 lies nearby.

NGC 1662
RA: 04h 48m 27s
Dec.: –02° 56' 38"
Now for something different. NGC 1662 is a lovely mag. +6.4 open cluster forming a right-angle triangle with mag. +4.6 Pi1 Orionis and mag. +4.4 Pi2 Orionis, the two stars at the top of Orion's Shield. Pi1 Orionis sits in the right angle. This is another overlooked target, said to resemble a Klingon Bird of Prey from Star Trek.

NGC 2022
RA: 05h 42m 6s
Dec.: +09° 05' 10"
This little planetary nebula can be found just southeast of mag. +3.5 Meissa (Lambda Orionis). The nebula shines at mag. +11.6. In a 6-inch Dobsonian it is small and round, appearing a pale greenish-blue. It can sustain high magnification if conditions permit.

The 37 Cluster
RA: 06h 08m 24s
Dec.: +13° 57' 53"
Also designated NGC 2169, this cluster gets its name because its stars appear to form the numerals three and seven. A lovely little cluster shining at mag. +5.9 and well worth seeking out even under moderately light-polluted skies. This cluster bears higher magnifications well.

Large Telescope

Delve deep into the constellation with a reflector over 6 inches or a refractor over 4 inches.

The Trapezium Cluster
RA 05h 35m 16s (Theta Orionis)
Dec.: –05° 23' 23" (Theta Orionis)
At the heart of the Orion Nebula is the Trapezium Cluster. The main stars can be easily seen through small scopes, but use a large instrument and two more pop easily into view. Two more challenging stars are mag. +16.0.

Jonckheere 320
RA 05h 05m 40s
Dec.: +10° 42' 21"
This is a stunning but neglected planetary nebula shining at mag. +11.8. In smaller telescopes it looks like a green star at low magnification, so larger telescopes really do it justice and bring out its true nature. Through a 14-inch Newtonian it appears as a small green disc.

NGC 1999
RA: 05h 36m 25s
Dec.: –06° 42' 58"
This is another nebula that could have more attention if it were not for the Orion Nebula. NGC 1999 shines at mag. +9.5 and in small telescopes looks like a small misty star, but a 14-inch scope reveals the mag. +10.3 star V380 Orionis surrounded by faint nebulosity.

NGC 1788
RA: 05h 06m 54s
Dec.: –03° 20' 05"
Off the beaten track and roughly north of mag. +2.8 Cursa (Beta Eridani), NGC 1788 is a reflection nebula that deserves to be better known. It glows by reflecting the light of the mag. +10.0 star embedded within it, and using a large scope reveals more stars around it.

NGC 1924
RA: 05h 28m 02s
Dec.: –05° 18' 39"
Orion is home to dozens of galaxies. One of the easier ones to find is NGC 1924, which lies to the west of M42, shines at mag. +13.3 and may be as far as 100 million lightyears away. When viewed through a 14-inch Newtonian at 200x magnification it appears as a pale, oval smudge of light.

IC 421
RA: 05h 32m 08s
Dec.: –07° 55' 06"
This barred face-on spiral galaxy has a stated magnitude range of mag. +14.2 to mag. +16.4 and is a challenging object. See if you can detect it with a 14-inch Newtonian at 200x magnification as a faint roundish smudge of light. It lies 140 million lightyears away.

UGC 3188
RA: 04h 51m 49s
Dec.: –08° 50' 38"
Use mag. +4.4 Pi2 Orionis to home in on this faint galaxy, which rests just 18 arcminutes east of the star and shines at mag. +15.0. This galaxy has a couple of mag. +10.0 stars nearby that help you locate it. Just south of Pi2 Orionis is UGC 3180, another mag. +15.0 galaxy, this time all alone in the night sky.

The Horsehead Nebula (Barnard 33)
RA 05h 41m 01s
Dec.: –02° 27' 14"
To see the famous Horsehead Nebula, you have to be able to pick up faint emission nebula IC 434, which hangs south from mag +1.9 Alnitak (Zeta Orionis). The horse's head appears as a dark notch through a 14-inch Newtonian and requires averted vision — a great, subtle challenge.

ABOUT THE WRITER
Paul Money is the BBC Sky at Night magazine reviews editor and an experienced astronomer who regularly organizes outreach events.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Deep Sky

M42, the Great Orion Nebula, taken by Ron Adams from his backyard in Oakdale, Louisiana.

Galaxies

These concentrations of millions or billions of stars are gravitationally bound together along with gas clouds and pockets of dust. There are probably over 100 billion of them in the Universe. Some of the largest nearby galaxies appear in the night sky as faint smudges of light, but it was only in the early 20th century that astronomer Edwin Hubble proved that they actually exist well beyond our Galaxy — the Milky Way. Before then, they were thought to be spiral-shaped nebulae on its outskirts.

Hubble also established that galaxies vary in shape and size. Two-thirds have distinctive spiral patterns, while the rest range from neat ellipticals to irregular blobs. They can be dwarves containing millions of stars or giants harboring trillions.

Astronomers are still piecing together why this is the case, but collisions and mergers seem to be important in determining how a galaxy evolves. Central black holes also seem to govern how gas is consumed and when stars are formed within these cosmic conurbations.

Galaxies are much more massive than they look. Around 90 percent of their mass is not in luminous stars and gas, but in unseen 'dark matter'. It's arranged in a spherical halo, which governs the motions of the stars within. This invisible cocoon explains why the outskirts of spiral galaxies spin faster than if they were influenced by the quantity of stars and gas alone. Dark matter also governs how galaxies clump together under gravity to form filaments and clusters. Yet dark matter remains an enigma and astronomers are still trying to work out exactly what it is.

Nebulae

These clouds of gas and dust are scattered throughout the Milky Way, mainly in the galactic disc. Nebulae are where stars are created. One idea of how it all starts is that a shockwave from a nearby supernova explosion compresses the cloud. Once the density of the gas passes a critical point, gravity takes over.

Gravity causes clumps of the nebula to pull together. The pressure at the centers of the clumps builds and the temperature rises dramatically. If there is enough gas to fuel the process, the region can become a protostar, an early stage in the making of a star.

If the temperature in the clump reaches 10 million degrees Celsius, the nuclear furnace that powers stars ignites. Over tens of millions of years it settles into normal life and joins what's called the main sequence, like our Sun.

Star Clusters

When you gaze up at the night sky, it looks like a lot of stars are on their own. But a solitary-looking star may be a member of a vast group that's travelling through space as a unit. If we wind the clock back millions of years, we might find these stars forming in the same vast cloud of dust and gas.

Known as open clusters, these families of anywhere from a few dozen to a few thousand stars are created in the dusty spiral arms of the Milky Way. They travel together through space, but gentle tidal forces eventually cause the stars to move apart until they begin to merge into the general starry background.

There is another variety of star cluster out there: the globular cluster. These are much bigger than the open sort, consisting of hundreds of thousands or millions of generally reddish, older stars. Whereas open clusters are found and made within the plane of the Milky Way, globular clusters form a halo around it and their creation is less well understood.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Icy Wanderers

Spectacular comets may be visible only once in a lifetime.

Image courtesy of NASA.

Wandering through the Solar System, comets can be among the most incredible of astronomical sights and, after years of careful observation, astronomers have coaxed out the secrets hidden within their glow.

The heart of a comet is its nucleus, a core of ice laced with rock and dust, a few kilometers wide. Though sometimes called a 'dirty snowball', the ice found on comets is far more exotic than that on Earth.

When the Rosetta spacecraft reached the comet 67P/Churyumov-Gerasimenko in 2014 it performed the first in-situ analysis of the comet's nucleus, finding not only water ice, but also carbon dioxide and monoxide, as well as traces of ammonia, methane and methanol. These highly volatile compounds are usually found as a gas or liquid on Earth, but the frigid depths of space have frozen them to ice as hard as rock.

These snowballs travel in huge elliptical orbits, briefly visiting the inner Solar System at one end before travelling billions of kilometers to the outer regions. 'Long-period comets' travel into deep space, taking thousands of years to complete an orbit, while 'short-period comets' have orbits that take only a few years or decades. Halley's Comet is visible to the naked eye from Earth every 75.3 years.

Comas & Tails

It's thought short-period comets come from the Kuiper Belt, after being knocked out of orbit. Beyond the Kuiper Belt, the Oort Cloud stretches to 3.2 lightyears from the Sun. If a passing star kicks one of its bodies off course, it creates a long-period comet.

For most of these orbits, the nucleus remains an inert lump of ice, but this changes as the comet nears 'perihelion' — its closest approach to the Sun. When close enough, the solar radiation heats the surface, causing the volatile components to boil. As the gas escapes into deep space it lifts off dust, creating a shroud that can stretch out over 50,000km around it — the coma.

As the comet gets closer to the Sun, this envelope begins to feel the solar influence even more acutely, as its wind and magnetic field sweep the dust and gas out into a huge tail, which can extend for millions of kilometers. Some of the tail's debris is left behind in its orbit, forming a meteoroid stream. Several of these cross the Earth's orbit, and when we pass through them every year, we see the debris burning up in the atmosphere as a meteor shower.

Sunlight reflecting off the coma and tail causes these celestial visitors to glow in the night, making them an ever-popular target for astronomers. But, only a handful of comets can be seen every year with the aid of a small telescope. Websites such as www.icq.eps.harvard.edu/cometobs.html or www.ast.cam.ac.uk/~jds will tell you which comets are active and where to find them.

Chasing the Tail

The most alluring part of a comet is surely its huge tail, but it's not always obvious that there are two. The most apparent is the dust tail, swept out in an arc by the solar wind. However, the magnetic field captures the gas, forming a fainter second tail. Sometimes the comet's position relative to Earth means the tails appear to go in two different directions.

Famous Comets

Dominating the sky or the landing site for a probe, these are the best-known comets

Hale-Bopp
Closest approach: 136 million km
Orbit: 2,520-2,533 years
Famed for: Visible to the naked eye for a record 18 months in 1996/97, Hale-Bopp will return around the year 4385.

67P/Churyumov-Gerasimenko
Closest approach: 186 million km
Orbit: 6.4 years
Famed for: Target of the Rosetta mission, which sent the Philae lander to its surface, finding water and organic compounds.

Great Daylight Comet
Closest approach: 19 million km
Orbit: 57,300 years
Famed for: Spotted in January 1910, this comet quickly brightened until it outshone even Venus. Its tail was noticeably curved.

Halley's Comet
Closest approach: 88 million km
Orbit: 75.3 years
Famed for: The only known short-period comet routinely visible to the naked eye, this regular visitor was observed as early as 240 BC.

Ikeya-Seki
Closest approach: 450,000 km
Orbit: 876.7 years
Famed for: Its 1965 close pass of the sun made Ikeya-Seki one of the brightest comets in 1,000 years. It's thought to be a fragment of the Great Comet of 1106.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

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Starting with Binoculars

Telescopes aren't the only option for observing the night sky, a pair of binoculars is ideal for budding astronomers.

Observing the heavens © Ashley Dace, cc-by-sa.

New to astronomy and trying to work out what to buy for your first telescope? There's a simple answer to that question: don't buy one, buy two. Two small scopes joined with a hinge so that the distance between them can be adjusted to exactly match your eyes — binoculars. Binoculars allow you to observe hundreds of astronomical objects. Not only can you see many more objects through binoculars than with the naked eye, but the detail and color you can see will become richer too.

Binoculars are still suitable even if you want to do 'serious' astronomy. There are variable star observing programs designed specifically for binoculars and being lightweight and easy to carry makes them ideal for getting out and about to view a lunar graze or asteroid occultation.

Closer to home, why not simply wrap up warm, lie back on your garden recliner and just enjoy the objects your binoculars let you find as you cast your gaze among the stars. You'll soon be able to find your way around the night sky and navigate better than with the entry-level GoTo telescope you nearly bought instead.

What Size Should You Buy?

You'll notice that binoculars are classified by two numbers, their magnification and aperture. A 10x50 pair of binoculars has a magnification of 10x and each of the front lenses has an aperture of 50mm. These numbers also allow you to work out the size of the circle of light, or 'exit pupil', that emerges from the eyepieces: to do this you divide the aperture by the magnification. This means a 10x50 pair of binoculars has an exit pupil of 5mm. The exit pupil should be no larger than the dark-dilated pupils of your eyes: so a pupil of anywhere between 4-6mm is fine for your first pair of binoculars. Larger apertures can show you more but being heavier you will probably need to use a mount to keep a steady view over a longer period. The most common sizes are:

1. 8x40: almost anyone over the age of 10 can hold these steadily.


2. 10x50: most adults can hold these steadily, so this size is a popular compromise between size and weight.


3. 15x70: this size really needs to be mounted, although they can be held for short periods.

It's also important to check that the distance between the two eyepieces will adjust to your eyes. If you wear glasses, check that the binoculars have enough distance from the eyepiece to your ideal eye position; 18mm or more should be adequate. Finally, there are two basic types of binoculars: Porro-prism and roof-prism. In any price range, roof-prisms are lighter but Porro-prisms tend to have better optical quality. Once you've decided on the size and type that best suits you, go for the best quality you can buy for your budget.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Great American Eclipse

Dan Falk looks forward to the moment when day turns into night across North America.

Image by Luc Viatour, www.Lucnix.be

The date is etched in the brains of eclipse enthusiasts: August 21, 2017. On that Monday, for the first time in nearly 40 years, the path of a total solar eclipse cuts right across the US. For about two and a half minutes, the Moon will completely cover the face of the Sun, turning day into night.

For thousands of years, solar eclipses were seen as shocking, fearful events; our ancestors would witness them and wonder if the world was coming to an end. Today, eclipses no longer take us by surprise: astronomers can calculate when an eclipse will occur hundreds of years in advance. Knowing the physics behind an eclipse, however, doesn't diminish the spectacle. A total solar eclipse is, quite simply, a spellbinding event, one of the most captivating phenomena the natural world has to offer.

People that have never seen a total eclipse might question what all the fuss is about, says astronomer, author and photographer Alan Dyer. "They think it just gets dark, the same way it does every night. No! A total eclipse is unlike anything you've experienced," says Dyer, who's seen 15 total eclipses over the past 40 years. (I've been lucky enough to see four of them, including one that I observed from Easter Island in 2010.) "You see, hear and feel a total solar eclipse," Dyer says. "Experience one and you'll be hooked."

There's another bonus: an eclipse can be enjoyed without any expensive astronomical equipment; you don't need a telescope or even binoculars. A word of caution is in order, though.

During the partial phases of the eclipse, when some portion of the Sun's disc remains visible, it's not safe to look at directly without eclipse glasses or equipment fitted with a certified solar filter. But when the Moon is completely covering the Sun — during the total phase of the eclipse — you can gawk at it safely. You can even use binoculars or take photos with a telephoto lens (again, that's only during totality).

It's been a long wait for the Moon to cast its shadow on US soil again. The last time was in 1991, when it landed in Hawaii but didn't reach the mainland. Prior to that it was 1979, when observers in the contiguous 48 states last saw a total eclipse, and even then it was only visible from the northwestern corner of the country.

Chasing shadows

The situation for eclipse observers will be very different in August. The path of totality — the narrow zone within which the total eclipse will be visible — will be just 110km wide, but will stretch from coast to coast, running from Oregon to South Carolina.

During a solar eclipse, the Moon's shadow (think of it as a very long, narrow cone that points away from the Sun) makes contact with Earth's surface. Since Earth rotates east to west, the Moon's shadow travels along in the opposite direction, running from west to east. After making landfall on Oregon's Pacific coast, the shadow continues east through the Rockies and on into the nation's heartland. It continues its eastward rush, crossing the Appalachian Mountains and finally zipping across the Carolinas and out over the Atlantic, near the historic city of Charleston.

Note that simply being within the path of totality isn't enough: you'll want to be near the middle of the path, known as the centre line. Most locations near the centre line will experience about two and a half minutes of totality. People living just south of Carbondale, Illinois, can brag that they'll get the longest duration of totality, with a little over two minutes and 40 seconds. That duration drops sharply as you move away from the centre line. Meanwhile, anyone viewing from north or south of the path of totality will experience a partial eclipse — far less dramatic than totality.

As the moment of totality approaches, the entire landscape can appear altered. In the half-hour or so before the Sun disappears, the quality of the light changes, shadows get sharper and the temperature drops. Dogs bark and roosters crow in confusion. This is the moment to make sure the batteries in your cameras are fully charged.

Because the eclipse path cuts right through the US, a record number of people are expected to witness the spectacle. More than 10 million Americans live within the path of totality; nearly 30 million live within 100km of the path. Some are already calling on the federal government to declare Monday August 21, 2017 a national holiday.

Location, location, location

With the eclipse's path running some 4,500km across America, where should you go to watch it? The weather, of course, is a big issue. Roughly speaking, the weather prospects improve from east to west; once you're west of the Mississippi, you've got a better than 50/50 chance of having a clear sky on 21 August, based on many years of climate data. Of course, what the local forecast says the day before the eclipse is more important than historical weather data! Some of the driest spots, with the highest chances of clear skies, include the valleys of central Oregon and central Idaho; some locations have a roughly three-in-four chance of cloudless weather. And of course, there's the scenery. No doubt, many visitors will be drawn to places like Grand Teton National Park, in northwest Wyoming, right inside the path of totality. Nearby Yellowstone is just outside the path, but many people will likely drop by for a visit before or afterward.

Another big unknown, apart from the weather, is the size of the crowds. "My guess is that they'll come by the thousands, from all over the US and other parts of the world," says Randy Holst, President of the Boise Astronomical Society in Idaho. Congestion is a real concern: most of the highways in the Northwest, especially those in the mountains, are two-lane, winding roads. And as Holst and others point out, this part of the country is famous for its natural beauty and is often jam-packed with tourists in August, even when there's no eclipse. Not surprisingly, many hotels and campsites are already booked up — but remember, this is an eclipse that you can, at least in theory, drive to; if your hotel is 80km outside the path of totality, you may still be okay — as long as you don't end up stuck in traffic!

Farther east, the population density is greater; millions of Americans will be able to see the eclipse from their backyards. "Every day the momentum is building," says Don Ficken, who heads the Eclipse Task Force for the greater St Louis area, in Missouri. "This is a historic event." In Columbia, Missouri, 50,000 people are expected to gather at a public event at the city's football stadium; the airport in St Joseph, in the northwest of the state, will host up to 60,000 at an eclipse-viewing event. Details for other events, large and small, are likely to be announced in the months ahead.

But what if you miss this particular eclipse? The next total solar eclipse you could go and witness will happen on 2 July 2019 — the path of totality passes through Chile and Argentina. The next one visible from the US comes on 8 April 2024.

Why wait until then, though? As solar eclipses go, this one is relatively accessible and the weather prospects in many locations are reasonably good. As Jay Anderson, a meteorologist and avid eclipse chaser puts it, "You only go around once. So do it while you can."

Top places to view the eclipse

Five of the best locations to see totality from

Grand Teton National Park, Wyoming
The park features some of the most spectacular mountain scenery in the US, and equally majestic Yellowstone, known for its wildlife as well as the Old Faithful geyser, is right next door.

Carbondale, Illinois
If you want the longest possible eclipse, a spot just south of this small university city boasts the maximum duration of totality. Totality is expected to be just over two minutes and 40 seconds here.

Central Nebraska
What's in central Nebraska? A useful 400km stretch of Interstate 80, which happens to run along the path of totality. If may also make an emergency relocation possible if bad weather is forecast on the 21st.

Charleston, South Carolina
Tourists and history buffs flock to this picturesque city on the Atlantic coast even when there's no eclipse. The first shots of the American Civil War rang out over Charleston's harbour on 12 April 1861.

Madras, Oregon
This part of Oregon boasts some of the driest conditions anywhere along the path of totality; statistically, there's about a 65 per cent chance of having a clear sky on Monday 21 August.

Viewing and Imaging the Eclipse

Here's how to get the most out of nature's greatest spectacle

1. Weather forecasts, along with your rental car, may be your best friend. Check the forecast the night before the eclipse and again in the morning. You've come this far, another bit of driving — if it gets you to clearer skies — may be well worth it.


2. Before and after totality, the Sun is far too bright to look at directly — so don't, unless you have a certified solar filter. Your local astronomy club can help you get your hands on one.


3. The partial phases of the eclipse last much longer than the brief moments of totality — so enjoy this slow period. Notice the changing quality of light and shadow as the Sun is reduced to a thin sliver of light.


4. The two and a half minutes of totality will go by very fast. Have a plan for how you want to spend that time. If you want to take photos, be sure that your batteries are fully charged.


5. If you have binoculars, use them during totality. They'll bring out the details in the Sun's pearly-white corona (its tenuous outer atmosphere). They'll also help you see the bright-red solar prominences that flare up from the Sun's surface.


6. During totality, take a few moments to look at the overall scene in the sky. Can you see the bright planet Venus, above and to the right of the hidden Sun?


7. "Pictures or it didn't happen." So the younger generation say. But do you really want to spend those two and a half minutes of totality fiddling with your camera? There's a lot to be said for just looking.


8. If photography is a must, consider taking wide-angle views that include the scenery. Close-up views of the eclipse all look pretty much the same; a wide-angle shot from your location will be more unusual.

ABOUT THE WRITER:
Dan Falk is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time. Find him at @danfalk

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Outer Gas Giants

These distant worlds include the Solar System's largest

By NASA (JPL image) [Public domain], via Wikimedia Commons

Jupiter

Diameter: 143,000km

Moons: 67

Distance from Sun: 778 million km

The largest planet in the Solar System, Jupiter has more mass than all of the other planets put together and is second only to the Sun in terms of gravitational power. In 1994 it enticed comet Shoemaker-Levy 9 to fragment and crash into its swirling clouds — other likely comet crashes were recorded in 2009, 2010 and 2016. Jupiter is mostly gas, its composition of hydrogen and helium similar to that of the Sun.

With a good pair of binoculars the first things you'll notice are its four most famous moons: Io, Europa, Ganymede and Callisto, spied by Galileo Galilei in 1610. With a telescope you'll see a slightly squashed sphere. This is due to its fast spinning 'day' of just under 10 hours, which causes the equator to bulge outwards and the poles to flatten. Jupiter's cloudy atmosphere is revealed as dark bands separated by white zones. The longer you look, the more features appear, so keep an eye out for spots, wisps and kinks. The most famous feature is the Great Red Spot; twice the width of Earth, this is a gigantic storm with winds reaching up to 644km/h.

Saturn

Diameter: 120,500km

Moons: 62

Distance from Sun: 1.43 billion km

Saturn is known for its spectacular rings, made from millions of chunks of water-ice spread out into a thin disc only a few tens of meters thick but stretching 100,000km from the planet's surface. The rings form bands, some broad, some narrow. Scores of moons orbit within the rings, some carving out wide gaps. As with Jupiter, a handful of them are visible to observers.

Saturn's brightness varies due to the way the rings are tilted and how much sunlight they reflect. The planet is not so bright when the rings are edge-on to us, but its brightness increases over 7.5 years as the rings open up to observers on Earth. Then it fades again over the same period. If you're wondering why this takes 7.5 years, it's a quarter of the time that Saturn takes to go around the Sun.

The best way of understanding Saturn's tilting effect is to go out and look at the planet — it really is one of the telescopic marvels of the Solar System. It doesn't matter if you have a small scope — the sight of this tiny, ringed world hanging in a large, inky black field of view is magical. The view of larger scopes will start to show detail in the rings and on the planet itself.

Uranus

Diameter: 51,000km

Moons: 27

Distance from Sun: 2.87 billion km

The first planet to be discovered with a telescope, found by William Herschel in 1781. Its blue-green hue comes from the abundance of methane ices in its hydrogen and helium atmosphere, which also contains water and ammonia ices. Like Venus, Uranus spins from east to west, but its axis of rotation is tilted almost 90° from the plane of its orbit, suggesting that it might have been knocked over by a collision. Five rings were discovered in 1977 — in 1986 the Voyager spacecraft identified a further six, and two more were found by the Hubble Space Telescope in 2005, bringing the total to 13.

Visually, Uranus doesn't have much going for it, whether you use your eyes, a pair of binoculars or a telescope. By simply turning your head upwards, you can just about see this gaseous world as a very faint star at the limits of visibility (around mag. +5.6). You won't see much from anywhere with light pollution, however — the sky has to be very black. The view improves a little through a telescope, showing a greenish speck.

Neptune

Diameter: 49,500km

Moons: 13

Distance from Sun: 4.5 billion km

Neptune's composition is similar to that of Uranus, being mainly hydrogen and helium with methane ices, water ices and ammonia ices mixed in. But unlike featureless Uranus, Neptune is wracked by stormy weather, with giant tempests boiling among the clouds. Its winds are the fastest in the Solar System, reaching an incredible 600m/s (that's 2,200 km/h). Neptune has six known rings. They appear to have bright clumps within them, which may be short-lived collections of debris.

At around mag. +8.0 you need at least binoculars to see Neptune. When looked at through a telescope it looks like a 'star' with a hint of blue, but it is not as spectacular as its larger, closer compatriots. If you have a very large scope you can also catch a glimpse of Neptune's largest moon, Triton, which is mag. +13.5.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Inner Planets

These worlds are mostly made up of metals or silicate rocks.

Released under Creative Commons CC0 on Pixabay.com.

Mercury

Diameter: 4,880km

Moons: 0

Distance from Sun: 58 million km

The closest planet to the Sun, Mercury is a place of extremes. It is the smallest and densest planet in the Solar System, barely larger than our Moon. It takes 59 Earth days to rotate once and 88 to orbit the Sun, meaning its parched surface experiences temperatures hot enough to melt lead on the sunward side, but is sub-Antarctic on the side in shadow.

This small world is a real challenge to observe for a variety of reasons. It's a fast mover, travelling around the Sun four times more quickly than Earth, so don't expect it to hang about in any part of the sky for very long. Mercury's orbit is a fairly eccentric oval shape, and it's on a bit of a tilt too, which means some times are better for viewing it than others: spring evenings and autumn mornings. If that's not tricky enough, you only have a relatively short observation window on any day you choose to look, as Mercury never strays very far from the Sun.

In spring, start looking 30 minutes after sunset, after which you'll have about another 45 minutes to see it. Autumn gives you a longer view, from about an hour and 45 minutes before sunrise, but that does mean getting up exceedingly early.

Venus

Diameter: 12,100km

Moons: 0

Distance from Sun: 108 million km

Venus is sometimes called 'Earth's evil twin'. It is similar in size and composition to our planet, but a dense carbon dioxide atmosphere and sulfuric acid clouds make its surface a hellish 470°C. The planet spins slowly, in the opposite direction to most planets, and takes about the same time to rotate on its axis (243 Earth days) as it does to travel around the Sun (225 days).

As Venus's orbit is slower than Mercury's, it can be visible for months on end, and sometimes for up to three hours after sunset or before sunrise. When Venus is at its brightest, it becomes the third brightest object in the sky, only beaten by the Moon and the Sun. This is caused by sunlight reflecting off its bright white carbon-dioxide clouds, and has led to Venus being called the 'Evening Star' or 'Morning Star' depending on when it appears. Venus can come very close to Earth, plus it's rather big, meaning that it's a good target for binoculars, through which you can easily see its larger phases.

Mars

Diameter: 6,800km

Moons: 2

Distance from Sun: 228 million km

The Red Planet is the most visited extraterrestrial destination in the Solar System. Dozens of missions have ventured there, and they have explored the Martian landscape in incredible detail. Smaller than Earth but with the same land area, Mars is like a cold, rocky desert, littered with canyons and volcanoes. The planet has polar caps and a thin atmosphere of mostly carbon dioxide. Although dry today, Mars's mineral salts and rock formations suggest that it was wet in the past, and could possibly have harbored life.

Mars differs from Mercury and Venus in that its position in the Solar System — on the other side of Earth — means it can be 'up' from sunset until sunrise. A small telescope can reveal lighter, pale-reddish areas, darker patches and the bright white of the ice caps.

The Dwarf Planets

Diameter range: 975km to 2,330km

A dwarf planet is, according to the International Astronomical Union, a body that orbits the Sun and is not a satellite, spherical in shape due to its own gravity and too small to have cleared its orbit of debris and so claim the title of a fully fledged planet. This classification was agreed after the 2005 discovery of Eris, an icy body in the outer Solar System very similar to Pluto, which was then considered a planet. In the fierce debate that followed Pluto was demoted into the newly created class, which also contains outer Solar System bodies Haumea and Makemake, and Ceres in the Asteroid Belt.

Ceres is the largest, but still comparatively small, so you will need binoculars to find it. Pluto is best seen by taking images of the region of sky it is in over consecutive nights and looking for the faint moving dot.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

The Heart of Darkness

At the core of the Milky Way lies an unseen monster. Elizabeth Pearson investigates how astronomers are trying to glimpse the supermassive black hole at the center of our Galaxy.

By Nick Risinger [Public domain], via Wikimedia Commons

At the centre of the Milky Way lies a dark behemoth. One like it is thought to reside in the core of nearly every major galaxy, a hidden heart no one has ever seen — a supermassive black hole.

This body has the mass of over four million Suns but is crammed into a space that's only 20 times our star's diameter. Its gravitational pull is so strong that even light can't escape, meaning that while its influence is felt throughout the Galaxy, we'll never be able to look directly into the black hole.

We aren't completely blind to its mysteries, however. Though the black hole remains shrouded, it's surrounded by a swirling cloud of material known as an accretion disc. As this disc rotates, friction causes the gas and dust in it to heat and glow brightly. The visible part of this glow is shielded by dust, but enough emissions escape at radio wavelengths to create a bright spot 26,000 lightyears away in the constellation of Sagittarius. This radio source is called Sagittarius A* (Sgr A*).

Although enough radio emissions get through the dust for the black hole to be detected, what we can see still seems rather lacklustre compared to what might be expected from looking at the cores of other galaxies.

"The Milky Way's black hole is known to be very inactive. It's radiating several orders of magnitude less than the Eddington luminosity, the maximum luminosity the black hole could have," says Abhijeet Borkar, from the Czech Academy of Sciences in Prague, who was part of a team that spent four years monitoring SgrA* with the Australia Compact Telescope Array.

"Either most of the energy in the accretion disc isn't emitted as radiation, giving it a low luminosity, or there's no stable accretion disc around the black hole. Instead there's a clumpy, discontinuous disc so there is not enough material falling into the inner parts of the accretion disc to maintain its brightness," says Borkar.

Disquiet in the calm

This does not mean the region is a calm and placid place, however. Despite its quiet background emissions, the material around the black hole regularly flares into brightness.

"We see about four instances of flaring each day, when we observe a six-fold increase in the luminosity of the black hole in infrared and X-rays. At radio and submillimetre wavelengths the luminosity increases by 30 per cent," says Borkar.

The flares typically last one or two hours, depending on the wavelength of light observed. They are also likely to originate from material that orbits around the black hole at speeds so fast that the effects of relativity become apparent.

"It's thought that either the light has been Doppler shifted as it goes around the black hole (creating an increase in luminosity when it's coming towards us) or a blob of material gets caught in the base of a jet and is pushed out. And as it's pushed out it expands," says Borkar.

Similar flares have been seen occurring around many other supermassive black holes, but what causes them remains largely unknown. Our proximity to SgrA* gives astronomers a fantastic view of these strange events: by studying the black hole in our Galaxy, astronomers can learn much about these beasts that lie at the centre of every galaxy.

"Black holes are highly significant astronomically," says Frank Eisenhauer from the Max Planck Institute for Extraterrestrial Physics. "They influence the full motion of a galaxy much more than stars do. There is a very strong correlation between the size of black holes and the inner parts of galaxies. They can blow the outer dust about and prevent further gas streaming in, so there's a very strong interplay between the formation and evolution of black holes and the centres of galaxies."

Eisenhauer is the principal investigator for the new Gravity instrument on the VLT, which has been specially made to take advantage of our excellent view of SgrA*. Gravity will image the region around the black hole, not only with an increased resolution compared to previous instruments, but also with a higher precision too. This will allow it to look at the stars that lie around the black hole, which are known as S stars.

"These are stars in the centre of our Galaxy that are a few million years old, with masses around 20 times that of our Sun. They trace the gravitational field and are very good test particles because they are frictionless through the vacuum of space," says Eisenhauer.

The motions of the stars are governed by the black hole they surround. It is by monitoring the motions of these stars that scientists have been able to determine the mass and size of SgrA*. The closer the star, the more accurate the estimation, and the VLT's Gravity instrument will help to refine those measurements.

"Gravity will see fainter stars further in, which are on shorter orbits. We hope to find stars that orbit on the order of a few months or years," says Eisenhauer. Currently the closest known star is S2, which takes 15.5 years to complete one lap around SgrA*. In 2018, it will pass through the point of closest approach — a 'mere' 120 AU from the black hole. During this time, it will be accelerated to 30 million km/h, or 2.5 per cent the speed of light. Travelling this fast means S2 will experience the effects of relativity on its motion, giving the Gravity team a fantastic opportunity to put Einstein's equations through one of their most extreme tests yet.

Little kicks

"Most of the time the star follows Newton's laws, but when it comes very close in 2018 it gets a little kick, and the orientation of its orbital ellipse rotates a bit due to the effects of general relativity," says Eisenhauer. "We know so little about black holes, but they are such a fundamental cornerstone for the understanding of relativity and gravity."

These observations will test Einstein's theories in the one place where they might falter — at the edge of a black hole. "Relativity has passed every test so far, but it hasn't been tested in a scenario where gravity becomes dominant," says Sheperd Doeleman, an astrophysicist from the Smithsonian Astrophysical Observatory. "Gravity is really the weakest force, so it's only near a black hole that it can play with the big boys."

Doeleman is the director of one of the most ambitious astronomical collaborations ever undertaken, the Event Horizon Telescope, which aims to take the first ever image of the shadow cast by the Milky Way's black hole.

"We can't see the black hole directly because it is surrounded by this event horizon that does not permit information to leave the black hole. Because the gravity around the black hole warps light around it, we expect to see the silhouette of the black hole against the backdrop of superheated gas," says Doeleman. "We expect to see a very characteristic strong lensing feature, a ring of light that indicates the last orbit that photons can move through around the black hole before they themselves are sucked in. You end up with an annulus with a relatively dim interior — the silhouette of a black hole."

The size and shape of this silhouette was predicted by Einstein's theories of relativity, which were laid down over 100 years ago. Comparing the shadow that has been forecast with reality could help to solidify our understanding of them. On the other hand, if the observations do not show what is expected, they could throw Einstein's theories into doubt. Within the next year, humankind should have its best look at the dark heart of a galaxy, and with it may even find the key to unlocking the rules that govern our Universe.

Tuning in to a black hole

The center of the Milky Way only became apparent in the age of radio astronomy

In the 1950s, radio astronomy was beginning to come into its own as huge telescopes were built across the world. As radio astronomers surveyed the sky at these newly available wavelengths, a bright new source was discovered in the night sky.

Located at a declination of –29°, the object was best viewed from the southern hemisphere. At the time, one of the largest radio telescopes in the world was the 21.9m 'Hole-in-the-Ground' scope at Dover Heights in Australia, perfectly located to observe this intriguing object. In 1951, the observations of the source, now called Sagittarius A, revealed that it was located at the middle of our Galaxy. In 1958 the International Astronomical Union decided to adopt its position as the centre of the Milky Way.

As radio telescopes increased in size and precision, it became apparent that Sagittarius A was not one single object but something made up of several regions. The eastern section appears to be the remnant of a supernova, while the western part seems to be a three-armed cloud of gas and dust.

In 1974 observations with the National Radio Astronomy Observatory in New Mexico revealed that there was a single bright source embedded within the region, Sagittarius A*. The stars in orbit around it showed the object must have a colossal mass in a tiny area, for which there was only one explanation. The astronomers had found the supermassive black hole at the centre of the Milky Way.

The Event Horizon Telescope

Capturing the heart of our Galaxy requires a telescope the size of the planet

The Event Horizon Telescope (EHT) is one of the biggest projects in the history of astronomy. It aims to combine up to a dozen of the world's premier radio telescopes — from the US, South America, Europe and the south pole — to observe SgrA* in greater detail than ever before.

The telescope works using a technique called very-long baseline interferometry, in which a signal is collected at a number of telescopes. The size of the telescope is not based on the diameter of the dishes, but the distance between them. Each of the scopes has been updated with an atomic clock, the most precise timekeepers on Earth. As well as allowing researchers to make precise timing measurements of the black hole's changing brightness (effectively taking its pulse), the timepieces will make it possible to combine the signals. Using the slight differences in arrival time for the light at each of the scopes, it's possible to reach a much higher precision than can be attained by simply stacking the images together, creating a 'virtual telescope' with the diameter of the Earth.

This huge size is needed as SgrA* is expected to cover a tiny area of sky: around 10 microarcseconds across ? the equivalent of looking for a coin on the surface of the Moon. As the smallest angle resolvable is determined by dividing the wavelength of light by the size of the dish, to further boost the angular resolution of the scope researchers will be observing at 1.3mm. This is the shortest wavelength ever used for the technique and will create an even higher resolution scope.

The EHT's first set of observations will be made on 5-14 April 2017. Once they are completed, the data will be transported to the Massachusetts Institute of Technology, where it will take a supercomputer several months to mix the separate signals into one image. By the beginning of 2018, we should have our first real glimpse of our Galaxy's core.

The Event Horizon Scopes

APEX: Atacama Pathfinder Experiment

ASTE: Atacama Submillimeter Telescope Experiment

ALMA: Atacama Large Millimeter/Submillimeter Array

CARMA: Combined Array for Research in Millimeter-wave Astronomy

CSO: Caltech Submillimeter Observatory

IRAM: Institut de Radioastronomie Millimétrique

JCMT: James Clerk Maxwell Telescope

LMT: Large Millimeter Telescope

MIT: Massachusetts Institute of Technology

NOEMA: Northern Extended Millimeter Array

SMA: Submillimeter Array

SMT: Submillimeter Telescope

SPT: South Pole Telescope

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Buying Your First Telescope

Astronomer Will Gater offers tips and advice on the exciting moment you decide to take the plunge and invest in a telescope.

Image Credit: El Camino College Astronomy Department

One of the things all of us at BBC Sky at Night Magazine love the most about astronomy is that you don't need any fancy or expensive equipment to get started. A warm coat, clear skies and a sense of intrigue about what's up there are all you need to begin your adventure in this wonderful hobby.

But there comes a time, naturally, when your thoughts turn to delving deeper into the heavens and when that happens most people start to think about getting a telescope. If you're at that point now, this article will help to guide you through the process of selecting your first telescope, from the things to look for in an instrument, its mount and the essential, and non-essential, accessories.

What about Binoculars?

The best way to begin is not by diving into the world of apertures and eyepieces, but by taking a step back and asking a question. For the purposes of this guide we're going to assume that you're familiar with the naked-eye night sky and can identify many of the bright stars and constellations, but have never used a telescope before. That's important, because the question, which has become something of a cliché in astronomy circles, is: have you considered getting a decent pair of binoculars first?

There are very good reasons why this question is repeated so often that is become a cliché. First, binoculars can open up a great many more objects to observation than the naked eye, from rich Milky Way star fields to star clusters and the brighter galaxies and nebulae. What's more, a good pair of binoculars will often outperform a cheap, poor-quality starter telescope. But the other reason — and one of the key arguments for considering binoculars before a telescope — is that they offer an easy way to learn crucial observing skills that will be useful later in your astronomy career. For example, with binoculars the experience of moving from a naked-eye view to one seen through an eyepiece is easier, as is learning the essential skill of 'hopping' from one star to another, in a magnified view, to track down a celestial target.

You may have already been using binoculars for a while, though, or perhaps you simply want to jump straight to a telescope. In which case there are some other questions to answer before you begin choosing an instrument. Questions such as what type of telescope do you want; what you intend to do with it; and, of course, how much you're willing to spend. Don't worry if you can't answer these straightaway as there are ways to gather the information you need to make an informed decision for each of them. For example, you could visit your local astronomical society, star party or astronomy trade show before you set foot in a shop. At a society meeting you may be able to see some small telescopes in use and speak to people who have used specific models. At a star party, however, you might even get to look through the telescopes, perhaps even the model you're considering.

What will become immediately obvious when you go to any of these events is the huge array of telescope designs, sizes and mountings that are available. So it's well worth getting to know the different types of telescopes and how to decipher the specifications you may encounter.

Types of Telescopes

Generally speaking, telescopes fall into one of three categories. Firstly, there are reflectors, whose defining feature is an arrangement of mirrors that collect and focus light; then there's refractors, which use glass lenses to do the same things; and finally there are catadioptrics, the telescopes that use a combination of lenses and mirrors to do the job. Within these categories there are numerous permutations and, of course, designs that vary from one manufacturer to another.

Refractors tend to be what most people imagine when they think of a telescope: a lens, or group of lenses, mounted in a long metal tube with the eyepiece (the bit you look through) at the bottom end. Refractors often come with a 'diagonal', an accessory that reflects the view 90° up off the telescope's axis to make your observing easier.

The two most common catadioptric designs are the Maksutov-Cassegrain and the Schmidt-Cassegrain. A Maksutov-Cassegrain telescope uses a curved lens and mirrors mounted in a relatively short, stubby tube while a Schmidt-Cassegrain has a large, glass corrector plate holding a small secondary mirror at one end with the primary mirror mounted at the other end.

When it comes to reflector telescopes, many beginners gravitate towards Newtonian models. In telescopes of this design light is collected by a main mirror housed at the bottom of a long tube. Once it hits this primary mirror the light is reflected back to a smaller secondary mirror that bounces it out at a right angle through the eyepiece. For this reason you look through a Newtonian by standing by the top end of the telescope, rather than the bottom end, and peering through an eyepiece that's mounted on the side of the tube.

The Newtonian design uses the same optical configuration as the other popular type of reflector, the Dobsonian. The difference with a Dobsonian is that the telescope's tube is mounted on a simple rotating base and not the more complex type of mount that Newtonians are typically found on? Which brings us onto the matter of mounts.

Mount up

If a telescope's lens is its eyes, then the mount is its neck. When you look at something, you use your neck to tilt your head up or down and turn it left or right to point your eyes in the right direction. Telescope mounts do exactly the same — they allow the telescope to be moved up and down and turned to the left or right. Astronomers call that upward or downward tilt altitude, and left or right rotation azimuth. So, for instance, Dobsonians sit on a basic altitude-azimuth (more commonly known as altaz) mount, while many Maksutov-Cassegrain and Schmidt-Cassegrain telescopes are attached to one or two computer-controlled arms that are essentially just advanced versions of an altaz mount.

With an altaz mount you need to adjust both the altitude and azimuth settings in order to track an object across the sky and keep it in view. This is because Earth's rotation makes it appear as if the sky is moving, so frequent manual adjustments are required to keep your target centred. That is unless you have an advanced (often expensive) motorised and computer-controlled altaz mount that will take care of the adjustments for you.

But there's another type of mount that gets around this issue in a simple way. The equatorial, or EQ, mount also moves on two axes, but instead of having altitude and azimuth axes, equatorial mounts have a right ascension (RA or polar) axis and a declination axis. These two axes refer to an astronomical coordinate system for navigating the sky that's similar to the latitude and longitude system that's used for navigating on Earth. An equatorial mount is built in such a way that when the RA axis is aligned with Earth's rotational axis, changes need only be made to that one axis to match the sky's movement.

To align an equatorial mount to Earth's rotational axis, the mount's RA axis needs to be pointing precisely towards the north celestial pole — the same point in the sky that Earth's rotational axis points towards. In the northern hemisphere this point is very close to the star Polaris. It's for this reason that many mid-range equatorial mounts come with a small 'polar scope' within the RA axis that has a reticle (targeting crosshair) for precise polar alignment.

Manual or automatic?
Whatever type of mount you end up with it has to be rock-solid. If there's any instability in the tripod's legs or play in the mount's fixings and controls then you'll find observing can become a frustrating affair — another reason to try any telescope before you buy.

While you can track the moving sky manually, it's far more efficient to use an electronically driven mount. These come in several forms from simple, motor-driven equatorial mounts, all the way up to Go-To mounts that, with the aid of a small computer handset (and sometimes GPS), take care of all tracking. Go-To mounts allow you to point your telescope towards a target simply by typing in the object's name or New General Catalogue (NGC) number.

Modern Go-To systems are a superb tool for observing but can often add a lot of money to the price tag of a beginner telescope; money that might be better spent on larger optics sitting on a simpler mount. After all, it's the telescope's aperture — the size of its main mirror or lens — that is perhaps the most important specification. The larger the aperture, the more light can be gathered. Hence most beginners are usually better off going for a good reflector, such as a Newtonian, since refractor telescopes tend to be more expensive than reflectors of the same aperture.

Eyepieces and Finderscopes
After the telescope's main body and the mount, the other key component of any stargazing setup is the eyepiece. The eyepiece is the glass lens that you look through to see whichever celestial body you're observing.

On the side of the eyepiece you'll find a measurement given in millimetres. This is the focal length of the eyepiece and it's by using eyepieces with different focal lengths that you change the magnification of the view through the telescope. The longer the focal length of the eyepiece, the less magnified the view through it will be. It's a good idea to have one or two good-quality eyepieces — one with a short focal length, perhaps around 10mm, and another maybe in the region of 25-30mm — rather than a whole range of cheap ones. And you can completely ignore any marketing hype boasting that a scope can provide hundreds of times magnification — this isn't the measure of a good instrument or a guarantee of superior views. A poor-quality telescope can still magnify many hundreds of times.

Finally, on top of the telescope you'll often find a miniature, low-magnification refractor telescope. This is a finderscope and it's used for centering a celestial object in the main scope's eyepiece. Finder scopes — or their cousins the illuminated red dot/reticule finders — are extremely useful when it comes to tracking down celestial targets.

Needless to say, there's certainly a lot to consider when you buy a telescope. But, then again, a good first telescope will last you many years and be a joy to observe with throughout that time. Choose wisely and your scope will take you on a thrilling journey of discovery that no other pastime can offer.

Which Telescope is best for you?

Answering these questions should help you choose the right first telescope for your needs

What do you want to look at?
Sounds obvious right? The night sky! If you're keen to focus primarily on observing faint galaxies, clusters and nebulae, then it makes sense to go for something like a Dobsonian with as big a mirror as you can afford. A smaller aperture Newtonian on a fancier mount will be more suited to closer, brighter objects.

Which telescope has the right size, weight, portability for you?
A good large-aperture Dobsonian will provide superb views but it may hardly ever be used if you've got to lug it downstairs to observe with. Trade shows are useful for examining many different telescopes in detail to gauge their true size, portability and, to some extent, their build quality.

Is the telescope right for your future aims?
It's worth considering early on where you think your interests may develop in the future. If you think you might eventually want to do more imaging, for example, you may want to choose a telescope with a mount that you can easily upgrade to a more advanced model at a later stage.

Do you need 'feature X' when you could get a better 'feature Y'?
The allure of electronic gadgetry or a computerized mount can be strong when buying your first telescope, but do you really need all that extra tech? You may find that your budget would go much further on a simpler setup with larger optics that'll probably give you better views.

ABOUT THE WRITER
Will Gater is an astronomy journalist and presenter. Follow him on Twitter at @willgater.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

How to Use a Planisphere

When you need to find your bearings in the night sky a planisphere is indispensable, even in our digital age.

This hand colored celestial map of the Stars and Constellations is a steel plate engraving, dating to 1878 by the well regarded French cartographer Migeon. By http://www.geographicus.com/mm5/cartographers/migeon.txt [Public domain], via Wikimedia Commons

They don't look like much — usually a planisphere is simply two discs of cardboard or plastic fastened together with a central pin — but as a new stargazer, you'll soon discover that this tool is one of the greatest aids to helping you navigate the night sky. In fact, this deceptively simple design will allow you to work out which bright stars are in the night sky on any date and at any time throughout the year.

Although the two discs are pinned together, they can still be rotated independently of each other. Printed over most of the lower disc are the stars, constellations and brighter deep-sky objects that you can see from a given latitude. Marked around the outside of this lower disc are the days and months.

Latitude Matters

The upper disc will be slightly smaller than the lower one or will have a clear rim, so you can still see the day and month markings underneath. It also has an oval window in it, revealing part of the star chart on the lower disc. The edge of this window represents the horizon with appropriate north, south, east and west markings and everything within it is the visible sky. Just like the lower disc, the upper disc has markings around its edge. In this case, they indicate the time of day. By lining up the date and time, the stars visible in the window will match the ones in the night sky at that time.

The crucial point to keep in mind when using a planisphere is that they are designed to work at specific latitudes. If you try using one too far north or south of the location it has been intended for, you'll find that the stars don't appear in the right positions.

Getting Started

Follow these simple steps and you'll soon be navigating the night sky like a pro

1. Find Your Bearings
   Before you can start using your planisphere you need to know the cardinal points from where you live. If you don't have a compass, use the Sun. It rises roughly in the east and sets roughly in the west. You can also download a free compass app for most smartphones.


2. Set the Time and Date
   Let's imagine that you are heading out to observe at 9pm on October 15th. Spin the upper disc to align the 9pm marker on this disc with the October 15th marker on the lower disc. The stars in the oval window should now match the stars in the night sky above you.


3. Look to the North
   Look north to begin with, holding the planisphere up so that the word 'north' is at the bottom. If you change the direction you're facing, you need to move the planisphere round so that the compass point sitting at the bottom corresponds with the direction you're facing.


4. Start at the Big Dipper
   Helpfully, the central pin in your planisphere represents Polaris and the north celestial pole. Just to its lower right will be the seven bright stars of the Big Dipper asterism. Use the Big Dipper and the five stars forming the distinct 'W' shape of Cassiopeia to get to know the constellations.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

A Time of Tumult

Elizabeth Pearson investigates how the Solar System was transformed 3.9 billion years ago.

This digital collage contains a highly stylized rendition of our solar system and points beyond. Image Credit: NASA/JPL

When the Solar System first formed 4.5 billion years ago it was a violent place. But quickly, by 4.4 billion years or so ago, the planets had calmed into a familiar configuration — several rocky inner worlds surrounded by gas giants ringed by icy objects. Then, four billion years ago, something catastrophic happened. The planets were thrown once more into chaos, and the face of the Solar System changed forever. This period of upheaval reveals startling truths about the evolution of our home planetary system, and perhaps the origins of life itself.

In the Solar System's first hundred million years or so, the beginnings of planets clumped together from the dust of a boiling protoplanetary disc around our young star. Frequently the young planets would collide and grow larger, though sometimes they would be destroyed entirely. To start, this early Solar System was much like it is today, but there were several key differences.

"Today, we have giant planets from Jupiter at about 5 AU from the Sun to Neptune at about 30 AU," says William Bottke from the Southwest Research Institute in Boulder, Colorado; 1 AU is the distance between the Earth and the Sun. "Modelling work shows the ice giants Neptune and Uranus would never have reached their current sizes if they had to form in the current configuration of the planets. Studies suggested instead that all these bodies formed between about 5 and 20 AU. Beyond that lies the Kuiper Belt. What's interesting about the Kuiper Belt now is that a lot of objects have very special orbits, called resonances. It's very hard to get them into these resonances."

Synced Surprise

A resonant orbit occurs when the ratio between the orbits of two bodies is two whole numbers. Pluto and Neptune share a resonance: for every two orbits Pluto makes, Neptune completes three. To explain what might have caused these resonances, as well as the change in the ice giants' positions, a group of researchers in Nice, France, came up with what is now known as the Nice model in 2005.

"They suggested that you had a gigantic Kuiper Belt with maybe 10 Earth masses in it," says Bottke. "This lets you form Neptune and Uranus on reasonable timescales and you can make lots of Pluto-like objects in the primordial disc."

Using this setup, the team created a computer model of the early Solar System. It's thought that a few hundred years after the formation of Jupiter, leftover gas dragged on the planet and caused it to drift deeper into the Solar System. In time this caused the giant planets to fall into a resonance and their combined gravity acted on the surrounding Kuiper Belt objects, pulling them inwards. In turn, these bodies pulled on the orbits of the gas giants. Though only a small effect compared to that of the planets, little by little the icy rocks began to upset the precarious gravitational balance.

"They found that when the system becomes unstable, Uranus and Neptune move into the primordial disc and actually migrate across it. The giant planets end up with almost identical orbits to what we see today," says Bottke.

This would have thrown the Solar System into disarray. According to the model, Jupiter moved inward while the other gas giants moved out. In turn, the inner planets were jostled and shuffled, pulling some of them into highly eccentric orbits which might have flung some of our siblings out into the Galaxy.

"There's pretty compelling evidence that we didn't start with four giant planets, but five. We had an extra Neptune and then lost it in this process. Jupiter is so massive that anything that encounters it has a good chance of being thrown out of the Solar System," says Bottke.

The addition of a fifth planet to the Nice model also helps to explain observations of the small bodies around Jupiter, as well as certain aspects of the asteroid belt. Could it be that our long-lost sibling is currently floating between the distant stars?

Mass Migration

But planets are not the only things that the Nice model predicts being relocated during this time. The Kuiper Belt currently contains around the same mass as Mars, meaning that during this planetary reshuffle, 99 percent of its mass would have been redistributed. Many Kuiper Belt bodies have been sent hurtling towards the inner Solar System. And it's the scars left behind by these impacts that could help explain a lunar mystery that has been around since the first moonrock samples were brought back by the Apollo missions.

"Many of the Apollo samples, more than you would expect, had been melted in impacts that took place around 3.9 billion years ago," says Barbara Cohen, a planetary scientist at NASA's Marshall Space Flight Center. "You would think that there would be a lot of impact craters from when the Moon formed 4.5 billion years ago, which would fall off as the impactors got used up, but we didn't see any. Instead we saw a lot at 3.9 billion years ago, which was a strange and unusual result."

Apollo scientists hypothesized that 3.9 billion years ago the inner Solar System was pelted with comet-like objects at an impact rate 100 times larger than what's seen today — an era now known as the late heavy bombardment (LHB). However, all the Apollo samples came from a limited area of the Moon, close to the large Imbrium Basin. This 1,150km-wide crater is the result of a huge impact 3.85-3.9 billion years ago, which then flooded with lava. While this is one of the largest examples of the effect the LHB had on the lunar landscape, there is also the chance that all the Apollo samples were simply the ejecta of this one event. To create a bigger picture Cohen had to look towards our only other samples from the Moon — lunar meteorites.

"The meteorites I've been looking at are from places that the astronauts didn't go, they have different geochemical signatures so we think they are from faraway places," she says. "I didn't find any of them to be very old. We see a big pile up at 3.9 billion years, with a long tail off. This tells us there was a prolonged impact rate on the Moon, and then over time the impacts got smaller and smaller."

The Same Old Story

Other meteorites from the asteroid Vesta tell much the same story, indicating a lack of impacts between four and 4.5 billion years ago. But relying on meteorites means that researchers investigating the bombardment history of the Solar System are constrained, as they can only study what happens to arrive on Earth. It's currently impossible to identify meteorites from Mercury and Venus, and all known Martian rocks are volcanic in origin. This leaves large holes in the impact timeline, ones that are unlikely to be filled until we can test the craters directly. Luckily that day could come relatively soon.

"I'm developing an instrument that we could take to Mars to find impact craters and get geochronology on them," says Cohen. "It would have a precision of around 100 million years, a few per cent the age of the crater. That's good enough to distinguish major geologic events in the planet's history."

The recent renaissance in lunar missions means we could soon understand the Moon's history a little better. China's Chang'e program aims to return the first sample from the Moon in over 40 years in late 2017, and then to land the first ever probe on the far side of the Moon in 2019.

A full impact history will prove useful in solving one of the main debates around the LHB. "The question is whether the LHB was a unique event," says Herbert Frey, chief of NASA Goddard Space Flight Center's planetary geology, geophysics and geochemistry lab. "Astronomers have always been keen to know if this is an impact rate spike or whether it is the tail end of a bombardment that had been going on for a long time and we're just seeing the ones that managed to survive because they came in last."

Understanding the precise timing of the bombardment is key to those considering the Nice model. The length of the delay between the Solar System's formation and the bombardment is central to working out what our Universe looked like before the migration.

Though there are still many mysteries surrounding this era this one is certain — our Solar System became a very different place four billion years ago.

The Lasting Effect

The effects of the bombardment can be seen in more than just the craters left behind

Titan

Saturn's largest moon, Titan, is now covered in a thick nitrogen atmosphere but it's unclear when this was created — during the moon's formation or at a later date. If it formed with the moon, then it must have been much thicker as the bombardment would have blasted much of the gas away. Alternately, it could be that the bombardment provided the energy needed to liberate the gas from ammonia ice on the moon's surface.

Mercury

The Caloris Basin, the largest impact basin on Mercury, was formed during the late heavy bombardment. The collision that created the 1,550km-wide site is thought to have been strong enough that it sent shockwaves through the entire planet, creating an undulating terrain on Mercury's other side. It's also been supposed that the impact could have kickstarted volcanic activity on the planet, which created its smooth plains.

Ganymede

Despite being similar in size and composition, Jovian moon Ganymede is very different from its sister, Callisto. While the former has a tectonically evolved surface and a differentiated core, the latter does not. One explanation is that a giant impact struck Ganymede, but not Callisto. The energy from this collision enhanced geologic processes, causing a fully separated iron core and subsurface ocean to form.

Mars

Bombardment would have melted Mars's subsurface ice and produced enough heat to create a temporary climate that might have had the right conditions for life to start. Unfortunately, such conditions would have only lasted a few million years, before the constant shelling began to erode away more atmosphere than it created. By the end of the era, Mars was the cold and dry planet that we recognize today.

The Bombardment of Earth

Despite the extreme conditions of the bombardment, could life have survived the LHB?

On Earth the end of the late heavy bombardment (LHB) coincides with another important epoch for our planet — the emergence of life.

When the LHB was first postulated it was thought that comet-like impactors may have brought the ingredients necessary for life, most notably water. However, findings from missions such as Rosetta show that it's unlikely comets brought any appreciable about of water to the Earth, though they may have brought other prebiotic compounds such as hydrocarbons.

Another theory is that life emerged long before the bombardment, but that all evidence was eradicated by the barrage. If this was so, then only the hardiest of life would have survived. An impact large enough to affect the global environment would have struck every century or so. And around every 10 million years there would have been an impact large enough to melt up to 10 per cent of the surface.

But even such colossal collisions would not have destroyed all havens for life across the planet. While the top few kilometers of ocean might boil away, there could still be enough water left behind for life to survive.

"It's quite possible that life started before, and found ways to protect itself," says Herbert Frey of NASA's Goddard Space Flight Center. "I think life is pretty hardy once it gets started, and it may have found a way to survive through that."

ABOUT THE WRITER
Dr Elizabeth Pearson is BBC Sky at Night Magazine's news editor. She gained her PhD in extragalactic astronomy at Cardiff University.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Cassini the Ring Grazer

As the Cassini spacecraft prepares to fly between Saturn and its rings, Will Gater looks at the latest results from the mission.

Image Credit: NASA

As most of us were getting up to go to work on 16 January this year, NASA's Cassini spacecraft was making a spectacular dive towards the rings of Saturn, some 1.6 billion km away. From high above the planet's pastel-yellow globe, Cassini's trajectory brought it racing down past the outer edge of the planet's main rings, in what the mission team are calling a 'ring-grazing' orbit. These thrilling close swoops, which draw to a close in April, in some ways mark the penultimate phase of Cassini's time at Saturn — a paradigm-shifting exploration that began over a decade ago and which will end in September this year when the spacecraft will be crashed into the gas giant's atmosphere. But the ring-grazing orbits are also evidence of how the Cassini team intends to squeeze every last drop of science from the veteran spacecraft, all the while capturing imagery of breathtaking detail.

One such image, of the tiny moon Daphnis, was captured by Cassini's cameras during the probe's close pass of the rings on 16 January. Daphnis is just 8km wide and looks, like many small moons throughout the Solar System do, like a pockmarked potato. Unlike Saturn's larger moons — Titan, Rhea and Dione, for example — it actually orbits within the planet's main rings, close to the outer periphery of the so-called A ring. And its presence there profoundly influences its surroundings.

Daphnis's most obvious impact on the rings is a thin parting that it has created in the icy material. The 'Keeler Gap', as it's known, spans a mere 42km and extends all the way around the A ring. But to see Daphnis's most striking creations one needs to look a little closer to the moon itself.

Either side of Daphnis, on the diagonally opposing edges of the Keeler Gap, the ring material has been swept into exquisite wave-like structures. Cassini's scientists have been scrutinising these extraordinary features from afar for years, but the spacecraft's ring-grazing orbit on 16 January afforded them their finest — and closest — sighting of the mission so far.

Many rings, one disc

To understand what's going on in the new image we first need to briefly explore the physics of the rings themselves — a system that is composed of countless individual objects. "The particles in Saturn's rings range from marble-sized to house size," says Matthew Tiscareno, a Cassini scientist based at the SETI Institute in California, US.

And while the major sections of Saturn's famous ring system certainly have their own designations — the 'A ring', the 'B ring' and so on — it's best not to think of them as a collection of rings.

"This is one of my pet peeves. It's a very common misconception," explains Tiscareno. "There are very few gaps that would separate one ring from another. Instead you should think of it more as a broad disc. But each part of the disc is orbiting Saturn at a different rate."

This motion plays an important role in the creation of the Daphnis wave structures seen in Cassini's recent image. In fact, researchers use modified equations relating to fluids to examine the physics of Saturn's rings, says Jeff Cuzzi, a Cassini scientist and ring expert at NASA's Ames Research Center in California. "[At] the top of the image the ring particles are moving towards the right the fastest. Daphnis is going a little slower. And then the material at the bottom is going the slowest," he says. "You can think of this material at the bottom as flowing past Daphnis from right to left".

It's when the ring particles drift by Daphnis that the gravity of the small moon leaves its mark. "As it goes by, it experiences a gravitational pull towards Daphnis," explains Cuzzi. "That distorts the orbits [of the ring particles] pulling them up towards Daphnis."

The end result is a series of beautiful wave-like peaks trailing the moon, two of which are seen in remarkable detail in the Cassini image shown left. Multiple waves are created — and there are even more out of shot — because, as Daphnis goes around Saturn, the slower orbiting material it has disrupted at the outer edge of the Keeler Gap lags behind the moon in its orbit; Daphnis thus has a constant stream of unperturbed edge material parading past it that it can repeat the 'rippling' process on. "So in that second wave to the left of Daphnis [are] the particles that had encountered Daphnis, just like in that first wave one orbit ago," says Cuzzi.

Daphnis's gravity also creates waves on the inner edge of the Keeler Gap. But, because the material there is orbiting faster than the moon, the waves extend in the opposite direction to those on the outer edge. In the same image you can see that, to the right of Daphnis, the ring particles on the inner edge have been subtly deflected. This is the onset of one of the inner-edge waves.

Cuzzi says there's an Earthly analogy for this remarkable interaction between Daphnis and the edge of the Keeler Gap. "Think about a river going by and there's a rock in the river. As the river goes by the rock, the water flows up and down and you get this ripple downstream of the rock. This is exactly what we're seeing here," he explains. "In the river the ripple is always fixed to the rock, that is there's always a ripple sitting right behind that rock, but the actual water molecules are moving right through that ripple."

Although it's tricky to get a sense of it in Cassini's latest picture, the wavy ripples that Daphnis creates are in fact three-dimensional features. "Daphnis actually has an orbit that's slightly inclined so it kind of slowly moves up and down relative to the rings," explains Cuzzi. "As it does this these perturbations that it causes on the edges are actually flipped up vertically." Indeed previous long-range images of Daphnis taken by Cassini — when the ring system was lit nearly side-on by the Sun — have shown the waves throwing shadows across the icy material below.

Tiny Moon, Huge Influence

What's abundantly clear from Cassini's new image is that even a diminutive moon like Daphnis can have a dramatic effect on the rings. Yet there are even smaller inhabitants of the rings that Cassini's recent orbits have been revealing in exceptional detail. And though these objects may be tiny, and their interactions with the ring system less obvious, they still could have an important story to tell us.

As Cassini was making another one of its ring-grazing orbits on 18 December last year, it turned its wide-angle camera towards a section of the A ring. The image it captured revealed a blizzard of artefacts from radiation and cosmic rays striking the camera's sensor. But it was the subtle features that the picture also revealed embedded within the immense, striated, swathe of icy material that were of interest to Cassini's scientists. Across much of the frame were numerous small, bright streaks within the rings — features known as 'propellers'. Cassini has been scrutinising propeller features in the rings ever since it first spotted them during the early phases of its time at Saturn, says Tiscareno. They come in two types, essentially large ones and small ones. "These are the smaller ones," he says. "We call this part of the ring the 'propeller belts'. They're just swarming here."

The bright streak of the propeller itself is caused by the gravity of a tiny, icy, moonlet disturbing the material around it. "You should probably think of the moonlet as a snowball about the size of a football pitch [roughly 100m]," says Tiscareno. There's even something of a connection between the propellers and their fellow ring-inhabitant Daphnis. "The propellers here and the gap that Daphnis is orbiting in are fundamentally the same thing," explains Tiscareno. "The only thing is that with these propellers [the moonlet] tries to start excavating a gap in the ring, but the ring is so massive that it fills the gap back in before it is able to extend all the way around."

Tell-tale Blades

The December 18th image represents Cassini's finest view yet of the smaller propellers. But Tiscareno and his colleagues have also been using the close ring-grazing orbits to capture spectacular pictures of some of the larger propellers — those that are thought to be created by slightly more substantial icy moonlets. On February 21st the spacecraft imaged one such example informally dubbed 'Santos-Dumont' by the mission team. The image is shown above; although it does not reveal the moonlet itself, it shows fine detail in the 'blades' of the propeller structure that the moonlet has made within the rings.

"This propeller is one of about a half-dozen whose orbits we know well enough that we had the ability to target them with flyby imaging, and it is one that turned out to be passing relatively close by during this particular flyby of the Cassini spacecraft," says Tiscareno. "The central moonlet is the size of a city block [around 500-1,000m], and the disturbance it creates in the rings can stretch for a few thousand kilometers, though it's only a few kilometers wide."

What is it, then, that studying the detailed nature of these features can tell researchers? Why might the Cassini team be using this time in the mission's final months to capture images like those of Santos-Dumont and the propeller belts? Part of the answer is that the propellers could illuminate our understanding of an enigmatic process that we have much to learn about. "It opens a window onto how planetary systems form because, when you have a baby planet forming itself out of the disc around [a] nascent star, it's a very similar situation to this moonlet that's embedded in the disc of Saturn's rings," explains Tiscareno.

It's not just with the propellers that Cassini's ring-grazing orbits are offering broader insights either. One recent high-resolution image reveals what Cassini scientists call 'straw' — conglomerations of icy material that have gathered to form huge clumpy structures within the rings. Examining this 'straw' in detail could shed light on how icy rubble 'sticks' together, which in turn could tell us something about planet formation says Cuzzi. "There are lots of things, big-picture problems, that we are understanding better by looking at the rings." he adds.

Edging towards the end

On April 22nd, once Cassini has completed its ring-grazing orbits, it will switch to a final set of trajectories that the mission team have dubbed the 'Grand Finale' orbits. These will take the spacecraft between the inner edge of the ring system and Saturn itself, with the last orbit hurtling the spacecraft into the planet's atmosphere. As the probe loops around the planet, Cassini will still be gathering data of immense interest to researchers back on Earth. "We're going to be directly measuring the mass of the rings," says Tiscareno. "That will help us distinguish between different models that we have for the origin and operation of the rings and might give us more clarity on how old the whole ring system is."

Cassini will also acquire unprecedented radar observations of the ring material. And its dust instrument will analyse the particles' chemical composition says Cuzzi. "So, finally, we'll be able to answer the big question that we've always had: why are the rings red," he says. "They're actually not white, like pure ice should be, they're actually a little red and we really don't know why that is."

Cassini's grand finale promises to be a period of intense excitement tinged with inevitable sadness then. But perhaps it's Cuzzi who best sums up the spirit for the weeks and months ahead: "We're definitely not done yet," he says.

Cassini's Orbital Timeline

October 1997 — Cassini launches from Cape Canaveral in Florida, US.

July 2004 — The spacecraft enters into orbit around Saturn.

June 2008 — The probe finishes its primary mission. It moves into a new set of orbits for Saturn's equinox in summer 2009.

September 2010 — The Equinox Mission complete, Cassini starts its Solstice Mission orbits, many of which take it far from Saturn.

November 2016 — Cassini starts its series of close 'ring-grazing' orbits. 22 April 2017 - The 'Grand Finale' trajectories will begin; Cassini will dive between the inner edge of the rings and Saturn itself.

15 September 2017 — The mission will come to an end as Cassini enters Saturn's atmosphere.

ABOUT THE WRITER
Will Gater is an astronomy journalist and presenter. Follow him on Twitter at @willgater.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Our Sun is an oddball: most stars don't exist alone. But sociability has consequences, writes Mark A. Garlick.

By ESO/L. Calçada (http://www.eso.org/public/images/eso1540a/) [CC BY 4.0], via Wikimedia Commons

Like living organisms, stars are a social bunch. Of the 20 star systems closest to the Sun, only 12 are solitary. Stellar multiplicity is very common, and our seemingly lonesome Sun drew a short straw.

If the Sun were reduced to the size of a pea, the closest stars would still be hundreds of kilometres away. So when we consider the truly enormous scale of our Galaxy, it is obvious that stellar multiplicity cannot be the result of neighbours just passing by and becoming entangled by gravity. Though this does happen, it's phenomenally rare. The inescapable conclusion is that most stars have companions because that is how they are formed.

Stars in binaries are commonly quite far apart from each other. They might be separated by less than an astronomical unit (approximately 150 billion km, the Earth-Sun distance) or they could be hundreds of times further apart, taking centuries to complete their orbits around each other. We call this latter group detached binaries. This is where both stars occupy their own Roche lobes — the region where matter is gravitationally bound to the particular star — without spilling over and influencing its companion.

Yet often the situation is very different. If two stars are sufficiently close together when one evolves into a red giant, the atmosphere of this evolving star can stretch beyond its Roche lobe and actually reach the companion. This creates drag, which brings the stars spiralling together and makes for a much more dynamic system — what is called an interacting or semi-detached binary — with two or more stars engaged in a frenetic gravitational tug-of-war. This situation can lead to some extreme behavior.

A distorted demon

Algol (Beta Persei) is the first example of an entire class of semi-detached star systems, hence collectively known as Algol binaries. The two stars are huddled close together, with a separation comparable to their radii and orbital periods ranging from hours to (more typically) days. This proximity means that one of the stars, usually one that has entered the late stages of its life causing it to greatly expand, is distorted by the gravity of its companion, which is usually more massive, hotter, brighter and still in the prime of life. The larger star is so expanded it is said to fill its Roche lobe. Gas from the secondary component flows towards the primary and strikes its surface on the equator. The impacting gas is then flung off, somewhat like water bouncing off a spinning ball, and spreads out around the primary to form a tenuous, messy outflow around it.

Algol binaries are variable stars, and we know of thousands of them. The variation comes from the fact they are eclipsing binaries as seen from Earth: when the secondary star passes in front of its smaller but brighter companion, the brightness dips significantly making the star flicker.

However, in cataclysmic variables we find a much more extreme semi-detached binary. Cataclysmic variables are a very broad class that includes novae and dwarf novae. The two components are almost always the same: a red dwarf and a white dwarf. These two stars are so compact that typically the entire system would fit within the bounds of our Sun. They swing around each other in a matter of hours. As in Algol binaries, one of the stars, the red dwarf, fills its Roche lobe. Gas from this star flows towards the voracious white dwarf — 10 billion tonnes of it every second — forming what is called an 'accretion disc' around it.

It is the accretion disc that gives these systems their dynamic properties and puts the cataclysm into their name. As material in the disc spirals towards the white dwarf, friction heats it to extreme temperatures, making these systems very bright in the ultraviolet and sometimes X-ray regions of the electromagnetic spectrum. Sometimes the accretion disc become unstable, suddenly dumping more gas than usual onto the white dwarf. Alternatively, there might be a momentary, rapid increase in the amount of gas being fed into the disc from the secondary. Both situations can cause a huge influx of extra material accreted by the white dwarf, which then flares up to produce a dwarf nova outburst. The amount of time between such events can vary from days to years. In even more extreme cases, the gas accumulated by the white dwarf becomes so hot and dense that the outer layers undergo thermonuclear burning, as in the centre of a star. This is a classical nova. The event is sudden and explosive, and typically destroys the accretion disc but neither star. In time, the disc will grow again and the 'cataclysm' will repeat.

Not all cataclysmic variables have accretion discs, however. Sometimes the white dwarf is highly magnetic — around 10,000 times more so than a typical bar magnet. This is strong enough to disrupt the formation of the accretion disc, resulting in a ring instead. These systems are known as 'intermediate polars'. In 'polars', where the white dwarf's magnetic field is stronger still, even a ring cannot form. In this instance, gas escaping from the secondary latches onto the white dwarf's magnetic field lines and flows towards it in great auroral arcs.

From powerful to phenomenal

X-ray binaries are more powerful still. As the name suggests, these systems are bright X-ray sources. It's a slight misnomer in that cataclysmic variables are also often X-ray powerhouses, but X-ray binaries tend to be much more luminous at these wavelengths, for reasons that will become clear.

In appearance cataclysmic variables and X-ray binaries are quite similar: a large star sometimes filling its Roche lobe, losing gas to a compact companion, often &mdsah; but not always — through a disc. The distinction is that in X-ray binaries the compact primary is a neutron star or a black hole, rather than the white dwarf found in cataclysmic variables. This extra degree of compactness is what gives them their awesome power.

In cataclysmic variables, the gas in the accretion disc reaches very high temperatures as it spirals around in the disc towards the white dwarf. But neutron stars are hundreds of times smaller than white dwarfs, and black holes are tinier still. This means that the gas orbiting in X-ray binaries travels a lot closer in towards the compact star, picks up much more speed and so more kinetic energy, and experiences much greater frictional heating. The energy released when this gas accretes onto the neutron star or black hole is phenomenal. Some X-ray binaries emit as many X-rays as thousands of Sun-like stars combined.

X-ray binaries exist in two flavours. If the secondary star is lightweight, such as a red dwarf, astronomers call the system a low-mass X-ray binary. But in some cases the mass-donating star is a massive giant, in which case they are called high-mass X-ray binaries; black hole system Cygnus X-1 is an example. The class as a whole resembles cataclysmic variables in that one of the stars fills its Roche lobe. But what happens when both stars fill their respective lobes?

The two members touch at the inner Lagrangian point, causing the system to resemble a gargantuan dumbbell. Because each star is in contact with the other, astronomers refer to these as contact binaries. A famous example is W Ursae Majoris. Binary systems such as this one exchange atmospheric gases and share a sheath of gas, or envelope, and eventually they settle into a configuration where each star has the same temperature. Usually the stars are yellow to orange in color (spectral types F to K) with orbital periods ranging from five to 20 hours. Often one or both of the stars are highly magnetic, and large star spots may be present as a result. These systems are true cannibals. In time, the larger star will completely consume the envelope of its smaller, luckless cohort, laying bare its core. Eventually even this is swallowed in a final merger, until only a single star exists where previously there were two.

Roche Lobes

The delicate balance of gravity is key to understanding binary stars

The Roche lobe is the teardrop-shaped region around a star in a binary system, within which gas is gravitationally bound to the star. But at the point between the two stars where the lobes meet — the inner Lagrangian point (L1) — gravity and centrifugal forces cancel out and the gas feels no net force. That is unless one of the stars has expanded to the point that it fills its Roche lobe. This happens in cataclysmic variable and many X-ray binaries. In these cases, the atmosphere of the lobe-filling star is pushed beyond the L1 point and flows towards and around the other star. As the escaping gas stream encircles the star it eventually loops around and collides with itself. This causes it to lose energy, spread out and form an accretion disc around the companion star.

Accretion Discs

Many powerful phenomena come not from stars, but from the dusty discs around them

There are many kinds of astronomical objects that grow or evolve by gravitationally attracting and harvesting nearby material, a process termed 'accretion'. The accretion disc is simply what we call the gathered material. It is the machine that surrounds an object and allows it to grow larger and more massive. For example, stars are born at the centre of protoplanetary discs, illustrated above. As dust and gas orbit in the disc, it loses momentum and spirals into the central regions, where it accumulates.

In binary stars, accretion discs are created when a secondary star fills its Roche lobe and spills material into the lobe surrounding an adjacent primary star. The gas leaves the inner Lagrangian point and runs in a narrow stream towards and around the primary star, creating a ring-like flow. Friction in this ring causes the gas to heat up, converting potential energy into kinetic energy. The gas also loses angular momentum, so it drops down to lower orbits and spreads slowly inwards, forming a fully fledged disc. The inner regions of an accretion disc around a stellar mass black hole can have temperatures measuring millions of Kelvin — hot enough that they emit most of their energy as X-rays. Astronomers find these emissions useful because they can detect the disc easily and thus infer the presence of a black hole, even if the latter emits no radiation. Supermassive black holes also exhibit accretion discs, although they are orders of magnitude larger and cooler.

ABOUT THE WRITER

Mark A Garlick is a writer and illustrator and animator specializing in astronomy. His latest book is Cosmic Menagerie: A Visual Journey Through the Universe.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Top Tips For Stargazing

Follow this practical advice and enjoy a good first night under the stars.

Image released under Creative Commons CC0 public domain, courtesy of Pixabay.com.

1. The eyes have it

Forget the myth that to be a 'proper' astronomer you need to have a telescope — this is complete rubbish! There are myriad things you can see with the naked eye alone — from the constellations to meteor showers, the band of the Milky Way and even the occasional galaxy. If you want to take things further, consider investing in a pair of binoculars before a telescope — you'll be able to see more of the night sky without dealing with the practicalities of setting up.

2. Keep out the cold

We know it's not rocket science (if you'll excuse the pun), but astronomy involves a lot of time outdoors being still, so it's important to stay warm. Several layers of thin clothing are recommended, as are waterproof shoes, a hat and gloves. If you have pages to turn or equipment (especially touchscreens) to operate, fingerless gloves are ideal.

3. Give your neck a rest

If you stand still staring up at the sky you'll soon find that you get neck ache. So avoid the pain entirely by finding something that you can lie back on. A reclining garden chair or an old-fashioned deck chair are ideal, but your spine will thank you even if all you have on hand is a camping groundsheet, a yoga mat or a waterproof picnic blanket to spread over the grass.

4. Accustom your eyes

If you go outside from a brightly lit room you'll probably only see a handful of stars so it's vital to wait and let your eyes adjust to the darkness — ideally for 30 minutes — and you'll notice an incredible difference. Doing so should allow you to see much fainter stars.

5. Use a star chart

These are a great way to learn your way around the night sky. Astronomy magazines publish star charts every month or you can buy a book. You can begin by identifying patterns of bright stars. From there you can gradually learn your way around the constellations, and before too long they'll become familiar and you'll be able to navigate your way around the night sky without reference to a book or chart.

6. Bring a red flashlight and a compass

A red-light flashlight is a must when you've given your eyes time to adjust to the dark but still want to see your star charts. This is because dark-adapted eyes are much less sensitive to red light than they are to white. You can buy a red-light night vision flashlight, or make one by taking a regular flashlight and sticking a piece of red acetate over the front. A compass will help you find north, which is essential not only when using star charts but also in setting up your telescope mount.

7. Stay away from streetlights

If you can, head out to the countryside to take advantage of properly dark skies. But if you are observing in an urban area, shield yourself from any artificial light sources, as they will prevent your eyes from acclimatizing to the dark.

8. Slow and steady

No one has ever looked at the night sky and instantly understood how to find their way around; there really is a lot to see up there! Not even the legendary Sir Patrick Moore was immune to this — he honed his knowledge by memorizing one new constellation each night.

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Shooting Stars

Spotting a meteor streaking across the sky is a truly captivating sight to behold.

By C m handler (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

You may know of meteors as 'shooting stars', but the truth is there is nothing stellar about them. The dramatic, bright trails that streak across the sky come from a much more harmless source — a dust particle the size of a grain of sand colliding with Earth's atmosphere, making it glow.

You will see several random, or 'sporadic', meteors an hour on any clear night, but a better way to catch them is during one of the annual meteor showers. These occur when Earth passes through the debris trail of a long-gone comet — a path of dust waiting to burn up in our planet's atmosphere.

Meteor showers are named after the constellation they appear to come from — and, sometimes, the closest star. Most major showers will be active over a period of at least a few days — and some for a few weeks. But they have what's known as a 'peak' — the night when you can expect to see the greatest number of meteors. The rates can vary quite a lot, but prominent displays, such as the Perseids, can produce an average of one meteor a minute under clear, moonless skies at their peak. There is also the chance a particularly dense patch of dust could lie along the path of debris, creating a surge in meteor numbers.

How to view

The best time to observe is shortly after midnight on the date when peak activity is predicted, when the sky is darkest and Earth's rotation faces the direction of the planet's motion in space, so the oncoming meteors seem to travel even faster.

As with any other form of observing, the best place to view is away from light pollution, so find as remote a location as possible and give your eyes at least 20 minutes to get really used to the darkness.

Don't look directly at the constellation that the meteor tracks appear to come from, concentrate your gaze high in the direction of the darkest portion of the sky that's free from obscuring trees and buildings. If the Moon is in the sky, try to make sure it's not in your field of vision or reflecting off walls or windows, as this will seriously degrade your night vision.

If you need to look at star charts or books to find your bearings, use a dim red light rather than a white one so that you don't lose your night vision. If you use a smartphone app, place a red cellophane filter over the screen.

Making meteors

By the time a comet approaches Earth, the Sun's heat has evaporated ice in its nucleus. This releases dust that follows the comet and, over time, can be spread out along all of the comet's orbit. When Earth intercepts this dusty path, lots of particles collide with the atmosphere and we see a meteor shower.

Meteor diary

Quadrantids

Peak: Around 3 January

Max possible activity: 120 meteors per hour

Activity window: Early January

Eta Aquariids

Peak: Around 6 May

Max possible activity: 60 meteors per hour

Activity window: Early May

Perseids

Peak: Around 12 August

Max possible activity: 80 meteors per hour

Activity window: Mid July to mid August

Orionids

Peak: Around 21 October

Max possible activity: 26 meteors per hour

Activity window: Mid to late October

Leonids

Peak: Around 18 November

Max possible activity: Usually 15 meteors per hour but can be higher

Activity window: Mid to late November

Geminids

Peak: Around 13 December

Max possible activity: 110 meteors per hour

Activity window: Mid to late December

Copyright © Immediate Media. All rights reserved. No part of this article may be reproduced or transmitted in any form or by any means, electronic or mechanical without permission from the publisher.

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Amanda Doyle looks at the evidence for life amid Europa's apparent plumes and examines the missions that could deliver proof

Europa by NASA's Planetary Photojournal

Europa is the smallest of Jupiter's Galilean moons, its strange icy surface riddled with fractures. But it is what lies beneath the surface that is of most interest to scientists: in September 2016, Hubble captured images of finger-like projections coming from Europa's limb.

One possible explanation for these findings is plumes of water vapour bursting out from Europa, a theory that is supported by earlier Hubble observations in 2012, when its spectroscope identified water vapour in the moon's south polar region. Both observations are strong evidence that Europa boasts a subsurface liquid ocean, making the moon one of the best places in the Solar System to search for alien life.

The first evidence for Europa's subsurface ocean came from the Galileo spacecraft. Jupiter's immense gravity causes a tidal bulge to be raised on Europa, and this tidal heating is sufficient to cause some of the ice to melt below the surface. Magnetometer readings from Galileo showed that Europa has an induced magnetic field, which can only occur if there is a medium in which a current can travel, such as salty water.

Water in the aurora

In 2013, a team led by Lorenz Roth from the Southwest Research Institute in Texas announced the detection of what appeared to be a plume rising into space in data. They used the spectrograph on Hubble to determine that an ultraviolet auroral glow from Europa's south pole — observed in 2012 — was possibly caused by water molecules being broken apart by Jupiter's powerful magnetic field.

Artist's Concept of Europa Water Vapor Plume
by NASA/ESA/K. Retherford/SWRI

The latest observation was announced in September 2016, when further evidence for these plumes was revealed. Initially searching for a tenuous atmosphere surrounding Europa by viewing the moon transiting in front of Jupiter, William Sparks from the Space Telescope Science Institute was surprised to find traces of a water plume.

"By an interesting coincidence, Roth and the team announced their discovery of evidence for plumes using [Hubble] STIS spectroscopy within a couple of weeks of our transit program beginning," said Sparks. "Our approach is independent, but that changed the landscape and people started looking right away for plumes."

Sparks' latest observation of the plumes adds weight to evidence for an active water cycle on Europa, but is this an environment that could support life? There are three key ingredients for life as we know it, and water is only one of them. The correct 'biogenic elements' need to be present in order to provide the building blocks for life and an energy source is also considered essential.

If life on Europa were to avail itself of photosynthesis as an energy source, it would have to be situated near the surface, where ice is potentially thin enough for sunlight to filter through. However, living near the surface has its pitfalls, as radiation from Jupiter would likely exterminate anything unprotected by thick ice.

As on Earth, so on Europa?

In the pitch-black depths of Earth's oceans, hydrothermal vents spew out enough hot material for ecosystems to thrive despite never seeing sunlight. If similar vents were to exist on Europa they could provide a safe haven for life. It is unknown how the tidal heating occurs on the icy moon but if the heating were to penetrate to the core, then the flow of heat up from the ocean floor could create vents. However, if the heating is restricted to the upper layers of ice, then venting would not occur.

It may seem far-fetched that life could exist in freezing conditions on Europa, but we know that there is microbial life on Earth that is extremely resilient to such hostile environments. Microorganisms are known to survive in Antarctic ice by producing their own antifreeze, and lakes situated far below the ice also have microbial life.

If some form of life exists on Europa, then could it be detected? "If the biomass in the plumes were high enough, it may be possible to find biosignatures," Sparks explains. "A more likely approach — and plumes are very relevant — is that presumably most of the plume material gets deposited back on to the surface. Along with all the other places on Europa where material appears to have seeped out onto the surface, that would certainly be a place you'd want to look."

The best way to explore Europa and the tantalising possibility of life would be to send a lander with a powerful drill, which would ultimately drop a probe into the ocean below. Such an ambitious project is still many decades away, but there are missions planned which will take the first steps in revealing the moon's secrets.

ESA's Jupiter Icy Moons Explorer (JUICE) is not a life-finding mission, but it does have the ability to finally confirm the existence of the subsurface ocean on Europa, as well as measure the thickness of the ice shell. JUICE will also explore the chemistry of the moon to ascertain if it has the right chemical soup needed for life. Meanwhile, NASA is planning the Europa Multiple-Flyby Mission, also known as the Europa Clipper. While the goals of this mission and JUICE are similar, the NASA mission will spend more time focused on Europa. There is also the possibility that this mission will include a small lander.

Both missions will further our knowledge of the surface of Europa and what lies beneath, thus paving the way for a mission dedicated to finding life.

Drilling Deep

The technology for drilling through Europa's thick ice crust has been tested

Drilling through the Antarctic ice to the lakes below is an excellent testbed for studying the type of life that might exist beneath Europa's surface, as such organisms are cut off from the atmosphere and from sunlight. These Antarctic lakes are kept in liquid form due to the immense pressure from the ice above.

There have been numerous drilling expeditions to such subglacial lakes, and the first successful breach of the overlying ice occurred in February 2012 when a Russian team reached the waters of Lake Vostok, some 4km below the ice. DNA analysis of the surrounding ice has shown that microbial life likely exists in the lake, although this has yet to be confirmed from the lake water itself.

Not all expeditions are so lucky. On Christmas Day in 2012, a UK-led project to explore Lake Ellsworth failed when they were unable to drill through the 3km of ice above the lake.

In 2013, an American team had more success when they broke through 800m of ice to reach Lake Whillans. There is also an extensive network of streams, and the entire area covers around 60 square kilometres. Analysis of the lake water revealed nearly 4,000 species of microbes.

Examining Europa

Several planned missions will further our knowledge of whether the icy Jovian moon's environment is right for life

James Webb Space Telescope
The James Webb Space Telescope (JWST), due to launch in 2018, will have the capability of confirming the existence of plumes of water emanating from Europa. It will also be able to observe Europa in regions of infrared light that are invisible to Hubble. If water plumes do indeed exist, JWST will detect the water signatures in the infrared. These observations are impossible from Earth, as the water vapour in the atmosphere blocks the view. However, as the plumes appear to be intermittent, it may be difficult to time JWST observations just right in order to detect the plumes.

Europa Multiple-Flyby Mission
NASA's mission to Europa was approved in 2015 and a suite of nine scientific instruments has been announced for the orbiter. These include high-resolution cameras, spectrometers, an ice-penetrating radar and a magnetometer. The latter will be used to determine the depth and salinity of the ocean by measuring the direction and strength of Europa's magnetic field. Thermal mapping of the surface will also reveal any recent eruptions of warmer water from below the ice. The mission is due to launch in the 2020s, and will perform 45 flybys of Europa over three years, with the orbits varying from a height of 25km to 2,700km.

Jupiter Icy Moons Explorer
ESA's Jupiter Icy Moons Explorer (JUICE) is due to launch in 2022 and reach the Jovian system in 2030, spending three and a half years studying the moons of Jupiter. It will perform two flybys of Europa before moving on to Callisto and then eventually settling into an orbit around Ganymede, the main focus of the mission. JUICE will study surface features on Europa to ascertain how they formed. By thoroughly analysing the Jovian system — including Jupiter itself — JUICE will shed light on planet formation and the conditions needed for life to emerge on icy moons.

About the Writer
Amanda Doyle is a postdoctoral researcher at the University of Warwick and editor of the SPA's quarterly magazine.

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For hundreds of years, astronomers have pondered whether there is any truth to testimonies of a glow illuminating the dark side of Venus. Paul Abel explores this centuries-old astronomical anomaly

Cloud structure in the Venusian atmosphere in 1979, revealed by observations in the ultraviolet band by Pioneer Venus Orbiter - NASA Photo Gallery Venus

Over the past decade, our space missions have revealed some spectacular things about the worlds in our Solar System. We've seen water fountains on Enceladus, methane lakes on Titan and vast icy mountain ranges on Pluto. We know more about the other worlds that orbit our Sun than at any other point in human history.

Yet in spite of that, one of our nearest neighbours continues to tease us with a 400-year-old mystery. The ashen light of Venus is rather like an astronomical ghost story: it would be easy to dismiss the phenomenon as a romantic relic of a bygone era were it not for a small number of consistent observations made by seasoned planetary astronomers well into the 21st century. As we see Venus return to our evening skies, it brings this ancient puzzle, and now is a good time to ask: will we ever solve the riddle of the ashen light?

The story starts in the 17th century. On the evening of January 9, 1643, Italian astronomer Giovanni Riccioli turned his telescope towards Venus. On that date Venus would have appeared as a crescent, with a phase of about 29 per cent. As Riccioli looked, he noticed that the dark side of the planet — which is normally invisible — appeared to be glowing with a faint greyish light that he called 'The ashen light of Venus'.

The next reported sighting came in 1714 when William Derham, who was a Canon of Windsor, observed it with his telescope and described the ashen light as a 'dull rusty colour'. Sir William Herschel also observed the phenomenon on a number of occasions. The British astronomer Thomas William Webb caught sight of the light on January 31, 1878 with his 9.4-inch reflector. Using magnifications of 90x and 212x, he noticed that the light had a slight brown-ish cast. Webb may well have been the first person to recommend using an eyepiece with an occulting bar — a device that hides the brilliant crescent to reduce glare.

Many sightings, little proof

There were many sightings of the ashen light in the 20th century: in 1940, 1953, 1956 and 1957 a number of observers reported sightings on consecutive nights to the British Astronomical Association. Dale Cruikshank, a planetary scientist at the NASA Ames Research Center together with William K. Hartmann, also a planetary scientist, made an interesting observation of Venus in 1962. On November 12, 1962 at 7pm, when Venus was at inferior conjunction, both Cruikshank and Hartmann observed the night side of the planet enclosed within a thin ring of light (this would have been the extended cusps of Venus). The night side seemed to be glowing with a brownish colour, quite different from the surrounding blue sky. The effect was not uniform and appeared to be strongest closest to the thin crescent.

Sir Patrick Moore was another veteran planetary observer who recorded the ashen light. Although he sighted it numerous times during his long observing career, the event that convinced him of the reality of the light occurred on May 27th, 1980. Using his 15-inch reflector at 300x magnification, Patrick described the effect as 'striking', with the ashen light strongly resembling the effect of earthshine on the Moon.

One of the great problems with the ashen light is that it has never been photographed or imaged; all observations are visual and so there is no tangible proof that the phenomenon is real. Yet not all visual observers have been able to view it. Edward Emerson Barnard, for example, never managed to see it. I have been observing Venus regularly for over 18 years and I have never managed to see the ashen light.

A number of amateur astronomers now believe that it is merely an illusion. It is reasonable to suppose that under certain conditions the brilliant crescent of Venus combined with poor seeing tricks the human eye into thinking it can see the night side of Venus, when in reality it is not visible.

Those who believe in the reality of the ashen light have suggested a number of ideas as to its cause. We can probably dismiss the suggestion of 18th-century German astronomer Franz von Paula Gruithuisen, however. He believed the light to be caused by fireworks of the Venusians celebrating the ascension of a new emperor.

More theories, more problems

A more reasonable idea has been advanced that the thick atmosphere occasionally thins in places, allowing the hot surface to be seen. The problem is that this would only be visible in the infrared part of the spectrum, well beyond the threshold of the human eye. The idea that the ashen light is the result of multiple rapid lightning strikes in the upper atmosphere of Venus can likewise be dismissed, since the flashes would be too faint to be seen from Earth.

The only viable idea left is the oxygen emission theory. This suggests that when oxygen atoms combine in the planet's upper atmosphere on the night side of Venus, they emit light. This has been observed by two Soviet spacecraft, Venera 9 and 10. Moreover, the variability of oxygen emission might explain why the ashen light is not always observed.

It seems likely that the enduring mystery of the ashen light will not be settled until the phenomenon is imaged. Only then will we be able to say with any real confidence whether it is really a product of Venusian metrology or an artefact of the human visual system. As Venus becomes well placed in the evening skies at the start of 2017, now might be your chance to catch a glimpse of it — and decide.

Seeing is Believing

How to maximize your chances of catching the elusive ashen light

No one can say when or if the ashen light will next appear, but looking at the observational records a number of interesting things stand out. First, it seems that the light is more frequently observed when Venus is an evening planet (eastern elongations), but even then it is not sighted during every elongation and there can be many years between reports.

The phase of Venus has to be below 30 per cent, so mid February onwards would be the time to start looking. The ashen light can only be viewed in a dark sky, which means the seeing conditions are likely to be less than ideal. Don't use a really high magnification unless the seeing conditions allow for it. Personally I find about 150x quite suitable. It might be worth trying some filters, too. Observers who have seen the light report that orange and green filters may enhance the effect if it is present. It is important to realize that the brilliant crescent will give rise to all manner of spurious optical effects. Some observers get round this by using an eyepiece that contains an occulting bar. Hiding the crescent behind the bar can reduce the glow, but even then you need to be cautious.

Most reports indicate that the ashen light takes the form of a coppery brown glow on the night side of Venus. The glow may cover all of the night side, or just a part of it. If you suspect the light is present, try taking an image of it and of course, alert other astronomers so that other independent images can be taken. If you have more than one telescope, try imaging it with one and observing it with another, and make a drawing so you can compare what you have seen with what you have imaged. Finally, send your observations to the Mercury and Venus section at the British Astronomical Association (https://britastro.org/sections) so that they can be studied and analyzed by professional astronomers.

Imaging the Crescent Venus

Pete Lawrence reveals how capture the planet's crescent on camera

Imaging Venus against a dark sky through a telescope produces tricky imaging conditions, with multiple reflections and unwanted aberrations. Catching it with the Sun up, or immediately after sunset is a good way to tame the planet's brightness, the lighter sky reducing contrast.

A monochrome high frame rate camera with a red or infrared-pass filter is a good choice for this photo, as it makes the blue sky appear dark. These longer wavelengths also less affected by poor atmospheric seeing. Detail in the planet's clouds is tricky to record, normally achieved using either an ultraviolet-pass filter (around 350nm) or an infrared-pass filter (1,000nm plus). Be aware that some telescope coatings are quite effective at blocking ultraviolet light, and so produce a blank disc.

The basic imaging procedure is to centre the planet, focus accurately and capture a high frame rate recording. As Venus is bright, keep the frame rate high and the gain low, recording several thousand frames. Process the capture with a registration-stacking program such as AutoStakkert!.

The bright crescent can be captured using a DSLR camera attached to a telescope or by using the afocal technique of pointing a camera down the telescope's eyepiece.

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