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Back Garden Astronomy - Nebulae
Back Garden Astronomy - Nebulae

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Whether they glow on their own or reflect the light of nearby stars, these clouds of gas and dust are popular targets

Flame Nebula by Doug Hubbell

Flame Nebula by Doug Hubbell

Nebulae are clouds of gas and dust that are scattered throughout the Milky Way, mainly in the galactic disc, and it's here that stars are born. The word is Latin for 'little mists' — long ago, we considered all deep-sky objects to be nebulae, galaxies included, because they were faint fuzzy patches in the otherwise black night. These days, not only can we differentiate between nebulae and galaxies, but we know that several types of nebula exist.

The most famous nebula of them all, M42 in Orion, is what's known as an emission nebula. Nebulae of this type have a glow of their own, a result of stars within or nearby ionizing the gas cloud. On the other hand reflection nebulae, like the one around the Pleiades star cluster in Taurus, are only visible because there are some stars nearby that light up the gas and dust, just as the Sun lights up a cloud in an otherwise blue sky.

Dark nebulae, such as the Horsehead Nebula, don't glow at all, as they are so dense they absorb light. They are only visible because they are in front of a bright nebula or field of stars. We effectively see a silhouette of the cloud, but no detail in it.

You might think that planetary nebulae, such as the Ring Nebula in Lyra, have something to do with planets, but you'd be wrong. They get their name because, through a telescope, many have the appearance of a faint, small, fuzzy disc and can look a lot like a planet. These nebulae are formed during the death of a star of similar mass to the Sun. As it grows unstable, the star puffs off its gaseous atmosphere to form clouds around it. Stars larger than the Sun end their days explosively in a supernova, leaving a spectacular remnant in their wake.

Astro images will reveal that many nebulae have vivid colors — typically red in emission nebulae from ionized hydrogen atoms and hues of blue stars in reflection nebulae — but the view through binoculars or a telescope will be quite different. Visually, nebulae appear in shades of grey.

Stellar Nurseries

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 center of the clumps builds and the temperature rises dramatically. If there is enough gas to fuel the process, the region can become a protostar. 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 own Sun.

What To See: Deep Sky — Amazing Nebulae

The Orion Nebula, M42
Constellation: Orion
RA 05h 35m 17s, dec. -05 23' 28"
M42 is the brightest nebula in the night sky and the only one that can be seen with the naked eye. With a casual glance below the three belt stars of Orion in a dark, light-pollution free sky, you'll see this emission nebula as a small misty smudge. A pair of binoculars will begin to reveal its curving shape. With a small telescope, you will start to see some structure. In the heart of the Orion Nebula are four stars. These are part of the Trapezium open cluster, named because of the shape the four stars form. It's the radiation from these stars that is energizing the entire nebula and causing it to glow.

The Crab Nebula, M1
Constellation: Taurus
RA 05h 34m 32s, dec. +22 00' 52"
M1 is what remains of a cataclysmic stellar explosion witnessed from Earth in 1054. It can be spotted with a small telescope, but it's best seen through a really large aperture instrument — only then does its texture start to emerge.

The Lagoon Nebula, M8
Constellation: Sagittarius
RA 18h 03m 37s, dec. -24 23' 12"
This easily noticeable emission nebula can be seen as a brighter patch with the beginnings of a core in 10x50 binoculars, even sitting where it does within the constellation of Sagittarius — a busy and star-rich area of the Milky Way.

The North America Nebula, NGC 7000
Constellation: Cygnus
RA 20h 59m 17s, dec. +44 31' 44"
It takes a bit of practice to see emission nebula NGC 7000, also known as the North America Nebula, as it's such a large object. It's close to the bright star Deneb in Cygnus, and the surrounding area contains many targets for binoculars.

The Omega Nebula, M17
Constellation: Sagittarius
RA 18h 20m 26s, dec. -16 10' 36"
This glowing emission nebula and star-forming region sits among the star fields of Sagittarius. It has a curved shape that can be likened to the Greek capital letter omega, Ω, hence its name, though it is sometimes called the Swan Nebula.

The Dumbbell Nebula, M27
Constellation: Vulpecula
RA 19h 59m 36s, dec. +22 43' 16"
This fascinating and relatively bright planetary nebula appears as a misty oval in small telescope, with the Milky Way providing a marvelous backdrop. The 'dumbbell' shape only becomes apparent through large instruments.

The Horsehead Nebula, Barnard 33
Constellation: Orion
RA 5h 40m 59s, dec. -02 27' 30"
The Horsehead Nebula, to the south of Orion's Belt in the Orion Molecular Cloud Complex, is a dark nebula that appears silhouetted against a brighter background of nebulosity. You will need a large aperture instrument and dark skies to make it out.

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.

Date Taken: 09/19/2016
Author: BBC Sky at Night Magazine
Category: Astronomy

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