The Sun is a single star, but stars that go it alone are rare. Many of the stars in our galaxy are members of binary systems -- two stars that are bound to each other by their mutual gravitational pull. By tracking the stars' orbits around each other, astronomers can measure the masses of both stars.
But you don't need to be an astronomer to see another key property of double stars: their stunning beauty. And if the two stars have different colors, the contrast can be especially striking.
A good example is Albireo, a beautiful double star that's visible tonight. Albireo's two stars are orange and blue.
The lovely contrast in color arises because the surfaces of the two stars have different temperatures. The blue star is more than twice as hot as the orange one.
Albireo is about 400 light-years from Earth, which means the light you see from the pair tonight actually left the stars around the time Galileo was turning his first telescope on the heavens.
You can see Albireo with just the unaided eye. It's in Cygnus, the swan, which looks like a large cross high overhead this evening. Albireo is at the base of the cross -- the head of the swan.
You can see the individual stars -- the orange one next to the blue one -- using a good pair of binoculars. But you need to hold the binoculars rock steady by propping them against a fence or tree. A small telescope also shows the pair, providing an excellent example of the beauty of double stars.
Script by Ken Croswell, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.