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 Cygn