Binary star

A binary star system consists of two stars both orbiting around their barycenter. For each star, the other is its "companion star".

Related Topics:
Stars - Orbiting - Barycenter

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The term "binary star" was apparently first coined by Sir William Herschel in 1802 to designate "a real double star — the union of two stars that are formed together in one system by the laws of attraction".

Related Topics:
William Herschel - 1802 - Double star

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Any two stars seen close to one another form a double star, the most famous being Mizar and Alcor in the Big Dipper. Odds are, though, that a double star is probably a foreground and background star pair that only looks like a binary system —the two stars are, in reality, widely separated in space but just happen to lie in roughly the same direction as seen from our vantage point. Such "false binaries" are termed optical binaries. With the invention of the telescope, many such pairs were found. Herschel, in 1780, measured the separation and orientations of over 700 pairs that appeared to be binary systems and found that about 50 pairs changed orientation over two decades of observation.

Related Topics:
Mizar - Alcor - Big Dipper - Optical binaries - Telescope - 1780

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A binary star is thus a pair of stars that are held together by the force of gravity. Systems in which the individual stars that compose a binary star can be resolved (distinguished) with a powerful enough telescope (including by interferometric methods) are known as visual binaries. In other cases, the only indication of binarity is obtained from the Doppler shift of the spectral lines. These systems, known as spectroscopic binaries, consist of relatively close pairs of stars whose orbital plane is substantially inclined with respect to the plane of the celestial sphere, such that the spectral lines of both stars are seen to shift regularly to the blue and then to the red, as they orbit towards and away from us. If the orbital plane is very nearly perpendicular to the plane of the celestial sphere, such that the two stars actually occult each other regularly, one has an eclipsing binary.

Related Topics:
Gravity - Interferometric - Doppler shift - Spectral line - Occult - Eclipsing binary

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Binary stars that are simultaneously visual and spectroscopic binaries are rare, and they are a precious source of valuable information when found. Visual binary stars, unless they are relatively close to Earth, have a large true separation, and consequently their orbital speeds are usually too small to be measured spectroscopically. Conversely, spectroscopic binary stars move fast in their orbits, and this is because they are close together — usually too close to be detected as visual binaries. Binaries that are both visual and spectroscopic are thus usually relatively close to us.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Scientists have discovered some stars that seem to orbit around an empty space. Astrometric binaries, for example, are relatively nearby stars which can be seen to wobble around a middle point, with no visible companion. With some spectroscopic binaries, there is only one set of lines shifting back and forth. The same arguments for ordinary binaries can be used to infer the mass of the missing companion. The companion could be very dim, such that it is currently undetectable or lost in the glare of its primary, or it could be an object that doesn't shine in visible light, like a neutron star. In some instances, one can make a very strong case that the missing companion is in fact a black hole —a star with such strong gravitational force that no light is able to get out. Perhaps the best example of such a system is Cygnus X-1, where the mass of the unseen companion is about nine times the mass of our sun —far exceeding the maximum mass of a neutron star, the other likely candidate for the companion.

Related Topics:
Astrometric binaries - Neutron star - Black hole - Cygnus X-1

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Binaries are particularly crucial as one of the primary methods by which astronomers can directly measure the mass of a distant star. The gravitational pull between the individual stars of a binary causes each to orbit around the other. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can therefore be determined.

Related Topics:
Astronomer - Mass

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Because a majority of stars exist in binary systems, binaries are particularly important to our understanding of the processes by which stars form. In particular, the period and masses of the binary tell us about the amount of angular momentum in the system. Because angular momentum is a conserved quantity in physics, binaries give us important clues about the conditions in which the stars were themselves formed.

Related Topics:
Angular momentum - Conserved quantity

~ ~ ~ ~ ~ ~ ~ ~ ~ ~