Optical Doubles, Binaries, and Spectral Binaries
If you look at some stars, such as alpha Libra, from a dark location, you may notice a dimmer star next to it. In binoculars it is easier to see. This is an example of a double star. Some of these duos are optical doubles. One star is close to us, the other is far away, but they share the same line of sight. Some doubles really do have relationships and are called binaries, such as alpha Libra. They are actually close to each other and are connected by gravity. Some pairs are so close that both cannot be seen even with a large telescope. They can be detected by using an instrument called a spectroscope that analyzes starlight. These are called spectral binaries.
75% of all stars in our galaxy are binaries, but some are even more complex. They are called multiples. Castor in Gemini looks like a single star, yet it is a system of 3 binaries, for a total of 6 stars. Our sun is odd in that it has no solar companion.