Double stars are two stars that appear close together in the sky, and they may be physically related or they may only just appear to lie together along our line of sight. Double stars that are not physical pairs are called optical doubles. Double stars that are gravitationally bound and orbit around a common center are a binary system.
In order to see double stars, the atmosphere should be still. A lot of motion in different layers of our atmosphere makes a star waver and dance, which makes it very difficult for an observer to be able to split the pair. When you are looking toward the horizon, you are looking through a thicker layer of atmosphere, therefore it is better for your target to be located near the zenith.
A Double Star for the Unaided Eye
For those new to double stars and observing, one double star that is a cinch to find is the pair of Mizar and Alcor in the Big Dipper. Because the Big Dipper is visible year-round and is an easily identifiable constellation, the Mizar and Alcor pair is a great place to start. There are seven stars that make up the Big Dipper - four in the bowl and three in the handle. The middle of the three handle stars is the Mizar/Alcor pair. The stars appear about 12 arcminutes apart, which anyone with fair eyesight should be able to see. Mizar is the brighter of the two, and Alcor is the dimmer one to the upper left. Mizar shines at magnitude 2.2 from 78 light-years away, and Alcor is magnitude 4 at 81 light-years distant.
Double Stars for Binoculars
If you are unable to split Mizar and Alcor with your eyes alone, go ahead and turn a pair of binoculars on them. Once you split those two, try for a double star system such as Theta Tauri. This pair may also be seen without optical aid, but a steadily held pair of binoculars will make it easier. Theta Tauri is found in the V-shape of Taurus's head, to the right of the bright, reddish Aldebaran. Theta 1 and Theta 2 lie about 5 1/2 arcminutes apart and shine at magnitude 3.8.
Telescopic Double Stars
Because a lot of double stars are binary and orbit around a common center, the distance between the two are frequently changing: growing smaller or larger. The following are some of the more popular doubles:
- Albireo - Also known as Beta Cygni, this blue and yellow pair is the most colorful and photogenic of any stellar pair. The pair is 35 arcseconds apart and shines at magnitudes 3.1 and 5.1.
- Alpha Centauri - The brightest star in Centaurus, found in the Southern Hemisphere, is not only a multiple star system but the closest star to Earth at 4.37 light-years.
- Mizar - You already know this star as being a double star with Alcor, but through a telescope you'll find that Mizar itself is a double star. The two parts of Mizar are 14 arcseconds apart. Fun fact: each of the two parts of Mizar are ALSO double stars, making it a quadruple star system, not counting Alcor.
- The Double Double - Epsilon Lyrae, a system of two pairs of double stars in the summer constellation Lyra, are around magnitude 5. The double double is simple to find, just to the left of brilliant Vega. Epsilon 1 and Epsilon 2 are about 3 arcminutes apart, with the two parts of Epsilon 1 and Epsilon 2 each being about 2 arcseconds apart.
Once you get used to spotting double stars, you'll find them in constellations everywhere. Good luck, and clear skies!
Source: CyberSky 4 and STARS website by Jim Kaler, accessed 1/7/2011.
Kelly Whitt - I have been a professional writer for more than a decade, covering many topics from science-based articles to do-it-yourself home ...
