A binary star is a
stellar system consisting of two
stars orbiting around their
center of mass. For each star, the other is its companion star. Recent research suggests that a large percentage of stars are part of systems with at least two stars. Binary star systems are very important in
astrophysics, because observing their mutual orbits allows their
mass to be determined. The masses of many single stars can then be determined by extrapolations made from the observation of binaries.
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A stellar system which is composed of two stars orbiting one another about their common centre of mass. The two stars are held together by the force of their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud. Since they may have formed with different masses to one another, however, they will evolve at different rates. As stars grow to become red giants, once nuclear fusion of helium begins in their cores, they can fill their
Roche lobes. This means that matter can pass through the inner
Lagrangian point and form an accretion disc around the companion star. This increases the mass of the companion and, thus, causes it to evolve faster. Roughly one half of the stars in the sky are binaries or multiple systems. In some cases, the stars are far enough apart so that they can be clearly seen as having two components. These are known as visual binaries. Other binary stars are too close together for the separation between them to be detected directly. A star, which displays a periodic 'wobble' in its passage through space, is being acted upon by a force of gravity. That gravity is caused by the mass of a companion star. So, although the star cannot be seen its presence can be inferred by the visible star's motion. These are known as
astrometric binaries . If the motion caused by the companion star is too small to be detected by astrometry, spectroscopy can reveal its presence. The spectra of stars sometimes reveal the presence of two stars, either by containing incompatible spectral lines or displaying movement of the lines caused by the
Doppler effect as the stars orbit one another. These are known as spectroscopic binaries (see Figure). Some binary stars appear to be variable stars. These are the eclipsing binaries and have an orbital plane which is inclined to the Earth so that the stars pass in front and behind of one another, causing eclipses which dim the light output. Another, more violent, variable, known as a nova, is also thought to be caused within binary star systems in which one member is a white dwarf. See also
supernova .
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