You might never guess it from looking at the sky, but estimates indicate that between one-third and one-half of all stars belong to star systems called double stars or binary stars. Double stars come in many different combinations. Some consist of a faint star teamed with a bright star, while others comprise two equal-magnitude suns. Still others have three or more members, and are called multiple stars. Then, there are optical double stars, two stars that only appear closely set from our vantage point. In reality, these celestial imposters are not physically linked, and in fact are nowhere near each other in space.
For amateur astronomers, binary stars offer both charm and challenge. Many pairs display beautiful color and/or magnitude contrasts, while others are so close together that "splitting" them becomes a good visual test of one’s optics and of the night’s seeing conditions.
In a double star system, the brighter star is labeled the primary or "A" star, while the fainter member is called the companion or "B" star. If the system has other members, they are labeled alphabetically C, D, and so on. Their apparent separations are usually expressed in arc-seconds, abbreviated ". (An arc-second is a small fraction of an angular degree. There are 60 arc-minutes in one degree and 60 arc-seconds in one arc-minute.) Seven-power binoculars will resolve, or separate, double stars separated by approximately 30". A 60mm refractor can split equal-magnitude doubles separated by 2" at high power. A high-quality 6-inch telescope can resolve binaries less than 1" apart, given nearly perfect seeing conditions (i.e., a very steady atmosphere at the time of observation).
Check Out These Pretty Pairs
Some of the prettiest doubles in the sky are made up of two color-contrasting suns, where both shine like glittering jewels against a velvety backdrop. One of the most dazzling binary systems is Albireo in the summer constellation Cygnus. Here, a golden primary star radiates in sharp contrast to its fainter companion, which is blue. Another gorgeous double is Eta Cassiopeiae in the constellation Cassiopeia. It features yellow and red members separated by about 13 arc-seconds.
The table below lists some of the sky’s most interesting stellar couples. You’ll need to refer to a star atlas to find them.
|Xi Boötis||Bootes||14 51.4||+19 06||5,7||7"|
|Cor Caroli (alpha)||Canes Venatici||12 56.0||+38 19||3,6||20"|
|Alcor & Mizar
|Ursa Major||13 23.9||+54 56||2,4||12’|
|Ursa Major||13 23.9||+54 56||2,4||14"|
|Albireo (beta)||Cygnus||19 30.7||+27 58||3,5||35"|
|Nu Draconis||Draco||17 32.2||+55 11||5,5||62"|
|Rasalgethi (alpha)||Hercules||17 14.6||+14 23||3,6||4"|
|Epsilon Lyrae||Lyra||18 44.3||+39 40||5,5||208"|
|Algedi (alpha)||Capricornus||20 21.0||-14 47||3,6||3’|
|Almach (gamma)||Andromeda||02 03.9||+42 20||2,5||10"|
|Eta Cassiopeiae||Cassiopeia||00 47||+57 5||3,7||13"|
|Delta Cephei||Cepheus||22 29.2||+58 25||4,6||41"|
|Polaris (alpha)||Ursa Minor||02 31.8||+89 16||2,9||18"|
|Orion||05 14.5||-08 12||0,7||9"|
|Pollux (beta)||Gemini||07 34.6||+31 53||2,3||3"|
|Sigma Orionis||Orion||05 38.7||-02 36||4,8,
|Theta1 Orionis||Orion||05 35.5||-05 23||7,8,
"Mag" is the visual magnitudes of the A and B stars, respectively.
"Sep" is the separation of the component stars, usually expressed in arc-seconds (").
Resolution and the Dawes Limit
While all of the doubles in the listing here should be resolvable through nearly all amateur telescopes, others challenge both our eyes and our telescopes to be seen. Just how close will your telescope be able to resolve a double star? In the 19th century, a British astronomer named William Dawes experimented to find how close he could resolve a pair of 6th-magnitude stars with different apertures. This value, called Dawes’ Limit, can be estimated by dividing 4.54 by the aperture of a telescope in inches. In other words, a 6-inch telescope should be able to resolve a pair of 6th-magnitude stars separated by 0.8 arc-seconds, while an 8-inch telescope can resolve stars to 0.6 arc-seconds.
But this is not set in stone. While Dawes’ Limit is a good guide for testing a telescope’s optical quality, resolving power can be greatly affected by a number of things. Above all, seeing conditions play a tremendous role. "Seeing" is a measure of how steady the Earth’s atmosphere appears. A good way to judge seeing conditions is to check the stars. If they appear to be twinkling, which is caused by a turbulent atmosphere, then Dawes’ Limit will never be reached. Frequently, the steadiest nights appear slightly hazy, when our atmosphere is more tranquil and seeing is enhanced.
Easy on the Power
Equally important is the optical quality of a telescope’s optics as well as those of the eyepiece and the observer’s eye. Refractors are often favored for splitting tight binaries, but reflectors and catadioptric telescopes can be equally adept provided their optics are precisely collimated. The secret to success is not to overpower the telescope. Use moderate powers for the best results, as high magnification will also amplify atmospheric turbulence and optical faults.
To test Dawes’ Limit for yourself, choose a binary star that has two equally bright components, both as close to 6th magnitude as possible. A large disparity in star brightness will render the test null and void. Beside steady seeing, be sure to use a moderate-power eyepiece, wait for the telescope’s optics to cool to the ambient outdoor temperature, and move away from any buildings and other objects that may be radiating the heat of the day. Dawes’ Limit will never be reached if test conditions aren’t just so, but under the right circumstances, some observers can actually exceed it.
Observing double stars is a great project for anyone who is looking for an enjoyable and varied observing program as well as an enjoyable way of testing the acuity of both your telescope and yourself. Dozens, even hundreds of targets are waiting for you in tonight’s sky. Perhaps best of all, double stars can be studied from anywhere any clear night of the year. Unlike other, diffuse deep-sky objects that are badly hampered by light pollution, double stars look striking even from urban or suburban skies, even at Full Moon.