binary star

We tend to picture stars as lonely points scattered across the dark, but in reality a great many stars come in pairs. A binary star is two stars bound by gravity, endlessly circling their common center of mass like two dancers holding hands and spinning. More than half of all Sun-like stars have at least one companion, so binaries are not the exception — they are close to the rule.

There are several kinds, distinguished by how we detect the partnership. In a visual binary you can actually see both stars and watch them orbit over years. In a spectroscopic binary the stars are too close to separate by eye, but their light's spectral lines shift back and forth (by the Doppler effect) as the stars swing toward and away from us, betraying the orbit. In an eclipsing binary the two happen to orbit edge-on from our view, so they take turns passing in front of each other, dimming the combined light. Each type, through Kepler's and Newton's laws, lets astronomers measure the stars' orbital period and separation.

Binary stars matter enormously because they are the only direct way to weigh a star. A lone star gives no handle on its mass, but two stars orbiting each other obey Newton's law of gravity, so timing the orbit and measuring its size yields the total mass — and often the individual masses. Nearly everything we know about stellar masses, and hence the all-important mass-luminosity relation, comes from binaries. They are also the settings for some of the most dramatic events in the cosmos, from novae to certain supernovae.