Binary << BY nuhr ee >> star or double star, is a pair of stars that are close together. The stars hold each other captive by the force of gravity. Each one orbits around the other. The individual members of a few binaries can be seen without a telescope. But the stars in most binaries are so close together that they look like single stars. The binary star nearest Earth looks like a single star. It belongs to the triple-star system Alpha Centauri , 4.3 light-years from Earth.
Binaries reveal some traits of stars that cannot be seen in single stars. For example, the orbital motions of binaries indicate the masses of the stars. The greater a star’s mass, the stronger its gravity and the faster it pulls its companion around in orbit. The fastest binaries can complete an orbit in less than six minutes traveling at over 300 miles (480 kilometers) per second. Studies of binaries show that some stars are probably 60 times as massive as the sun. Astronomers have also found stars so small that 15 of them together would barely equal the sun in mass.
The stars in some binaries are so close that they almost touch. In these binaries, the gravity of each star distorts its companion, causing enormous tides to form on the surfaces of both stars. Violent events can occur in such close pairs. One star may become an X-ray pulsar, a star that radiates X rays in precisely timed bursts. Other close pairs radiate powerful radio waves. In still other close pairs, one of the stars pulls matter upon itself from the other star. This accumulating matter may explode in nuclear reactions, causing the star to flare brightly.
Some stars appear to orbit around an invisible companion. The companion may be a black hole, a star whose gravity is so strong that not even light can escape from it.
