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Chapter 4.9 Binary Systems

Many astronomers believe that up to half of the stars visible to us orbit and are orbited by companion stars. Some white dwarfs, neutron stars, and even suspected black holes also appear to pair up in binary systems.

Here's an optical image of the powerful X-ray source Cygnus X-1.

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Cygnus X-1
Located but 8000 light years from Earth, Cygnus X-1 is a suspected black hole with a mass exceeding 6 suns. Every 5.9 days, the invisible object orbits a hot, blue supergiant companion whose bulk approaches that of 30 suns.

The black hole's powerful gravitational pull sucks gas away from the giant star. As infalling gas strikes an "accretion disk" spinning around the edge of the black hole, the gas becomes superhot and emits short, powerful bursts of X rays. The spiralling motions of this deadly duet could prove to be a strong source of gravity waves, as might other binary systems containing white drawfs or neutron stars.

For example, the orbital motions of the neutron binary pulsar, PSR 1913+16, have been observed to decline over several years. As the paired stars draw closer, they may eventually collide.

General Relativity implies that some of the stars' orbital energy is being dissipated as gravitational radiation; the theory predicts precisely the observed change in orbit. This system is now considered an important test of General Relativity, for which its discoverers, Hulse and Taylor, were awarded the 1983 Nobel Prize in Physics.

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Colliding Stars

Neutron Binary in Simulation

A supercomputer simulation portrays two neutron stars locked together gravitationally. Eventually they may collide as their orbits around each other decay.

A cataclysmic event such as this would likely generate a burst of gravitational waves as the stars' cores spiral inwards and collide.

Elsewhere in the universe, perhaps nearer than we realize, two orbiting black holes may be drawing closer, spiralling towards each other and eventually coalescing to form a larger black hole. Each stage of this process is expected to generate distinct displacements in spacetime. Such interactions and the patterns of gravitational radiation they generate can now be studied computationally.

Once ground-based gravitational wave observatories are built and operational, scientists expect that it will be only a matter of time before the calculated gravitational wave signatures are matched against the "real" thing.

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