The Hidden Lives of Galaxies - The Clustering of Galaxies
Some stars have a hot corona composed of gas at a very high temperature. This gas emits X-rays. In external galaxies, the individual stars must be very bright X-ray emitters for us to see them. Thus, most individual stars we see in other galaxies are "O" type stars, which are very massive and very hot. "O" type stars don't live very long and are, in fact, rarely seen in galaxies.
X-rays are also emitted by supernova remnants. These are shrouds of gas and dust left behind after a massive star has exploded at the end of its life. The hot ejecta from the exploded star runs into the gas and dust lying in the region around the star, emitting X-rays. Some massive stars leave behind a dense neutron star after the supernova. Neutron stars have a strong magnetic field, which can also feed energy into the remnant.
In addition, observations at X-ray wavelengths show that other galaxies contain binary star systems that emit X-rays. These X-ray binary systems consist of a normal star and a "compact object". This compact object may be a black hole, neutron star, or white dwarf. These objects are formed from normal stars which have used up their nuclear fuel. In the binary system, material from the companion star is funneled into the compact object. This material is heated as it spirals in and emits X-rays as it is heated.
X-rays may also come from globular clusters. In these dense clusters of stars, the most massive members quickly exhaust their nuclear fuel and become neutron stars (or sometimes black holes). Through motions and gravitational interactions within the cluster, these neutron stars can join with a normal star to become an X-ray binary system. In our Galaxy, some globular clusters are observed to have a number of individual X-ray sources, all of which are believed to be X-ray binaries. Because other galaxies are far away, we see individual globular clusters as a point-like X-ray source.
Finally, it is common for a galaxy to harbor a massive black hole near its center. Observations by the Chandra X-ray Observatory of the central region of the Andromeda Galaxy reveal more than 100 X-ray sources. Many of them are likely X-ray binaries. One of them was at the previously determined position of a supermassive black hole, which has a mass of 30 million times that of our sun. The Chandra observation also showed a diffused glow surrounding the central region of the galaxy. It is not known whether the glow is from many faint individual sources or from a diffuse, hot gas.
