X-Ray Astronomy

A wide variety of x-ray sources have been seen since the first extrasolar identification in 1962. A few of the most interesting types of sources are:

The Sun. A number of x-ray satellites have monitored the Sun. Solar flares produce enhancements in its x-ray output.

Stars. Many stars, particularly those with coronae or rapid stellar winds, emit x rays from their outer layers.

Comets. Astronomers have detected x-ray emission from 10 different comets since the phenomenon was first discovered in 1996 with Comet Hyakutake.

Scientists believe that x rays are generated by some sort of interaction between the solar wind and the comet's atmosphere, ionosphere, or atoms within the nucleus.

Groups of galaxies in hot clouds. Bright x-radiation is seen emanating from clusters of galaxies, which, due to their enormous gravitational pull, trap gas in the region. This gas is very hot, and there is a large amount of it. It thus glows in the x-ray region.

X-ray background. The sky is not dark in the x-ray region of the sky like it is in the visible. The diffuse background which was detected in the rocket flight described above is still not understood, although some believe it may be the result of many individual, unresolved sources.

X-ray binaries. These are close binary stars in which gas from one star falls onto its companion, heats up, and gives off x rays. This is especially bright when the companion is a compact stellar remnant such as a neutron star or black hole, because the enormous gravitational field compresses and heats the incoming gas, causing it to glow at x-ray wavelengths.

Supernova remnants. Explosions of stars, or supernovae, show traces of the heavy elements that are formed there when their x-ray spectra are examined.

Quasars and active galactic nuclei. These are among the most energetic objects in the universe, and they emit enormous quantities of radiation at x-ray wavelengths. It is thought that the ultimate source of this energy is a supermassive black hole, surrounded by an accretion disk of in-falling gas that is heated to many millions of degrees.