II. The Hidden Lives of Galaxies
A. Hidden Objects
Observations of galaxies at wavelengths other than optical light reveal other objects and components. Some are also seen in optical wavelengths, but are brighter in other parts of the spectrum.
For example, at radio wavelengths astronomers can detect much of the hydrogen that lies between the stars. These hydrogen atoms emit radio waves having a frequency of 1420 MHz (= 1420 x 106 Hertz), or as it is more commonly referred to, a wavelength of 21 cm. Astronomers use the detection of this gas to map out the location of hydrogen in our Galaxy. Astronomers can also determine the velocity at which the gas is moving, and whether it is moving toward or away from us. In this manner, the general motion of gas, and presumably the stars formed from the gas, can be determined.
In X-ray wavelengths, we see individual stars, supernova remnants, binary star systems, and globular clusters. All of these occur in our own Galaxy, and we can see other galaxies which also contain these objects.
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.
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. 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.
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. Often, this central part of the galaxy is very bright in x-rays gamma rays and radio, because of the large amount of material interacting near the very massive black hole. Such galaxies are said to have an Active Galactic Nucleus, and are often referred to as AGNs. The central black hole can have a mass of millions (or even billions) times the mass of our sun.