A binary star system is a pair of stars that orbit each other. They are very widespread, and in fact the majority of stars in the Milky Way are members of binary or multiple systems (with three or more stars): loners like our Sun are in the minority. The distance between stars in a binary system can vary hugely – some take millions of years to orbit each other, while others do it in just a few days. Astronomers find binaries very useful – the stars in these systems have spent their entire lives together, and were born at the same time from a shared cloud of gas/dust. What’s more, the way in which each star orbits can give away its relative mass. So it’s easy to compare how factors such as the mass of each star have affected their evolution and appearance as we see them today.
Answered by Giles Sparrow