Our Sun moves alone across our daytime sky, but when we look up at the stars, there are a great many of them that seem to have partners. Especially when we look in a small telescope, many stars seem to be located very close to other stars. One has to wonder - are they really near each other in space, or are they just near each other from our viewing angle?
The answer is: some of both. Many of the stars that appear near each other in the sky are just that - optical doubles. They are near each other only from our viewing angle, and actually are great distances from each other in outer space.
The other possibility is that the stars are indeed physically associated - these are called binary star systems. (It is also possible to have more than two stars - the Centauri star system, containing the closest stars to the Earth, is actually a triple star system) Sometimes, we can actually see the two stars in telescopes - these are called visual binaries, and StarCalc denotes it as a "binary". Sometimes, the two stars are too close together and the only way we can tell them apart is from their spectra - this is known as a spectroscopic binary, or "spec-binary" in StarCalc. Most visual binaries can also be detected spectroscopically.
The stars in a binary star system are gravitationally bound together, the same as the Earth and the Sun, and they orbit each other, as we can see in the sketch above. Binary star systems are where most of our data about stellar masses come from, because Newton's Law of Gravitation allows us to figure out the masses from the orbital period.
