Field guide · Stars
Double starDouble star
Two stars that look like one to the eye. Some just line up by chance; most are real pairs orbiting each other.
- How common
- About half of Sun-like stars
- Orbits
- From hours to many thousands of years
- Best seen with
- Anything from the eye to a telescope
About half the Sun-like stars in the galaxy have a companion. When the two are far enough apart, a telescope — sometimes just good eyes — splits them.
Some pairs are only a trick of perspective, one star far behind the other. Most are true binaries, and in 1803 William Herschel proved it: re-measuring pairs he had catalogued twenty years earlier, he found some had moved around each other. It was the first proof that gravity works the same way beyond the Solar System.
Mizar and Alcor, in the Big Dipper’s handle, are a classic eye test. Albireo in the Swan’s beak is the favourite telescope pair: gold and blue, side by side.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.