The stars in a visual binary are widely enough separated that their images can be distinguished. They usually have a long orbital period. If the distance to the binary is known, the masses of the stars can be calculated.
The stars in a spectroscopic binary usually are close together and orbit each other with large velocities. As the stars orbit, their radial velocities change. These changes cause the Doppler shifts of their spectral lines to change in a periodic manner.
The stars in an eclipsing binary periodically eclipse each other, causing the binary to dim and then brighten at regular intervals. The light curve of the binary can be used to determine the sizes and temperatures of the stars.
At least 85% of all stars are members of binary or multiple star systems. Multiple star systems consist of close pairs orbited at great distances by single stars or other close pairs.
Mechanisms have been proposed to account for the formation of both close and wide binary systems. None of these mechanisms is understood well enough for astronomers to be sure just how important it is in the formation of binary stars.
Equipotentials can be used to determine the directions in which gas will flow in a binary system and to find the shapes of its stars. The Roche lobes of a binary system mark the gravitational domains of each of its stars. Gas outside of the Roche lobes is common to both stars.
Angular momentum is conserved as matter flows from one star to the other in a binary star system. This causes the stars to grow closer together as their masses become more equal and to draw apart as their masses become more different. Rapid transfer of mass occurs if the more massive star sheds mass. Slow transfer occurs if the less massive star sheds its mass.
The more massive star in a massive binary system evolves first and sheds its mass to its companion and to a common envelope. When the envelope is ejected, the stars are left in a close binary in which the evolved star is now a neutron star or black hole and is less massive than its companion. A wind from the companion causes matter to fall into the compact object. The evolution of the companion results in a merger of the two stars or a system with two compact objects.
The initial evolution of a low-mass binary leads to a white dwarf and a less massive companion. A long period of slow mass exchange begins as the stars spiral toward one another.
Matter accreting onto a compact object becomes hot enough to emit X rays and ultraviolet radiation. In many cases, the accreting gas falls into orbit about the compact object, forming an accretion disk. Friction in the accretion disk causes gas to spiral inward until it reaches the compact object.
Binaries that contain accreting white dwarfs show temporary increases in brightness caused by runaway nuclear burning of the accreted gas on the white dwarf. Nova explosions result in the ejection of the accreted matter.
In cases in which the rate of accretion onto a white dwarf is large, the accreted matter remains too hot to become degenerate. The white dwarf gradually becomes more massive. Eventually, the fusion of carbon occurs explosively in the core of the white dwarf, producing a type Ia supernova. The explosion completely destroys the white dwarf. Type Ia supernovae are used to find the distances of remote galaxies.
X-ray bursts are produced by runaway fusion in the accreted gas on a neutron star. X-ray pulsars produce pulses of X rays when the X-ray-emitting magnetic polar regions of a neutron star sweep past the Earth.
Some X-ray binaries may contain black holes rather than neutron stars. A black hole is most likely in those cases where the mass of the compact object is too great to be a neutron star.
Binary pulsars have neutron star or white dwarf companions. Some binary pulsars have periods of only a few milliseconds. The rotation rate of these pulsars increased when their companions, before they became compact objects, transferred mass to the pulsars. Single millisecond pulsars probably had companions that they destroyed by bombarding them with high-energy particles and radiation.
