Binary Star
From Slackerpedia Galactica
A stellar system composed of 2 stars orbiting around one another, which sometimes looks like one star in a telescope and other times can't be seen at all. Just as binary digits are called bits, binary stars are called bars.
There are several different types of binary star system:
Visual Binaries are systems where you can see that there are two stars just by looking at them (either with your own eyes, or more likely with a telescope). Physical Binary systems are visual systems where the two stars are actually orbiting each other. But there are also Optical Binary systems where the two stars in question have absolutely nothing to do with each other - they just happen to be close in the sky.
Spectroscopic Binaries are systems where if you look with a telescope you only see one star, but if you take series of spectra of the star some of the lines move periodically. This is because the two stars have their own spectral lines, and as they move relative to each other the combination of their spectra changes.
Eclipsing Binaries are systems where the one star passes in front of the other along our line of sight. We can tell they are binaries because the magnitude of the system changes regularly over time as we see mainly the dimmer star, both stars together, and mainly the brighter star.
Binary star systems are a good thing because by observing things like how far apart the stars orbit each other, how long they take to complete an orbit and other things like that we can work out much more about the stars involved than we could if each star was all by itself.
