Binary star
From Simple English Wikipedia, the free encyclopedia
A binary star is a pair of two stars which orbit around each other. For each star, the other is its companion star. Many stars are part of a system of two or more stars. Binary stars are important in astrophysics because looking at their orbits allows scientists to find out their masses. This information can be used to learn the masses of individual stars.
Binary stars are not the same as double stars, which look close together but are not connected by gravity, and may actually be far apart in space. Some binary stars can be seen as two separate stars by someone with good eyesight—Mizar and Alcor in the handle of the Big Dipper (called "The Plough" in some countries) are an example of what is called a visual binary. Other binary stars are so close together that instruments must be used to tell them apart. One way to tell binary stars and double stars apart is to watch them for a long period of time. Over a few decades of observation, true binary stars can be seen to move around each other, but double stars always keep their same position.
Some stars appear to be in orbit around empty space, and appear not to have a companion. In this case, the companion star is either very small and faint, or it is a neutron star or even a black hole. The best-known example of a star with an invisible companion is Cygnus X-1, in which the visible star's companion appears to be a black hole.
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[edit] Observation
[edit] Visual binaries
A visual binary star is one for which the separation of the two stars can be seen with a telescope. The brighter star is the primary and the fainter star is the secondary. Visual binaries take a long time to orbit one another, in the area of hundreds or even thousands of years
[edit] Spectroscopic binaries
A spectroscopic binary is one in which the two stars cannot be seen separately even with a telescope. They are very close together and move around each other very quickly, over a period of a few weeks or even a few days. However, they can be seen to be two separate stars by using a spectroscope, which is able to record the Doppler change in the color of the light sent out by stars moving quickly toward or away from the Earth.
[edit] Eclipsing binaries
Some spectroscopic binaries have an orbit that is edge-on to Earth. When this happens, the stars will take turns passing in front of and eclipsing the partner star, in what is called an eclipsing binary. In this case, the amount of light we see from the double dims slightly during the time one star is in front of the other.
[edit] Astrometric binaries
An astrometric binary is one where only one companion can be seen. For astrometric binaries fairly near the Earth (up to about 10 parsecs), it may be possible to see the visible companion "wobble" as it moves around its invisible companion. By making measurements over a long period of time, it may be possible to calculate the mass of the visible star and how long its orbit takes. This method is also used to detect the presence of large planets orbiting a star; as of 2007, over two hundred planets have been discovered in this way.
[edit] System properties
[edit] Detached binaries
Most binaries are detached binaries. Except for their gravitational pull on one another, they have no effect on each other.
[edit] Semidetached and contact binaries
Some binaries are so close to one another that one or both stars is able to pull material from the other. They may share the same stellar atmosphere, and as friction slows them down over a long period, they may merge into one star.
[edit] Formation
Though it could be possible that binary stars may form when one star passes very close to another, it is highly unlikely (since it would actually take three stars close to one another before two could join), and would occur only in places where stars are densely packed together. Our present understanding is that almost all binaries are formed together in the dense gas clouds where stars are born.
[edit] Runaways and novae
It is possible (though not likely) that a passing star will disrupt a binary system and provide enough gravitational force to split the binary. Such separated stars go on to live lives as ordinary single stars. Sometimes, though, enough gravitational force is involved that the two companions speed away from each other at great speeds, resulting in what is known as runaway stars.
Sometimes a star is in orbit around a white dwarf star. If it is large enough and close enough to the white dwarf, the dwarf may suck gasses from its companion's atmosphere. Over a period of time, a great deal of gas may collect on the white dwarf. As this gas is compacted by the white dwarf's gravity, it will eventually undergo nuclear fusion, resulting in a very bright outburst of light, known as a nova. In some cases, the white dwarf may gather so much gas that the explosion completely destroys it, in what is called a supernova. Such an event may also result in runaway stars, as the larger star no longer has a heavy companion which keeps it in orbit.
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