Binary_stars are pairs of stars at close proximity that revolve around a mutual center.
At first glance, all stars seem to have fixed positions. Closer examination reveals the existence of multiple-star systems, comprising two or more stars.
Binary stars were once thought to be very rare. Today we know that almost half of all stars (within a 30 light-years radius from the Earth) are part of binary systems.
Some stars only appear to be binary stars. Although they seem to be close together, they are very often far apart. These are called optical binary stars.
Real binary stars orbit around each other. They are held together by the interaction of their gravitational_forces. Their orbital path depends on the relation of the mass of the two objects. If their mass is about equal, the orbital paths are circular around a central point. If one star is much bigger than the other, the smaller one moves around the larger or both follow eccentric paths around a mutual center of gravity.
Binary stars sometimes have other stars in their system, which are invisible at first glance. An example is the middle star in the handle of Ursa Major, Mizar. A dimmer star named Alkor lies above it. These two are known to be binary stars. They belong together and probably take about 800,000 years to orbit each other once.
A closer examination of Mizar through a telescope already reveals that it is actually two stars (A and B). A spectroscopic examination of these binary stars reveals even more stars in close proximity. This easily visible binary star is actually a multiple-star system.
