It is one thing to know that bizarre objects such as neutron stars and black holes exist. Quite an impressive thing, in fact, when you consider that until less that 20 years ago, both objects existed only on paper, the products of the fertile minds of a few theoretical physicists. Now we have convincing proof that neutron stars exist and very strong evidence for black holes. Furthermore, thanks to space observatories such as the HEAOs, we can study the properties of individual neutron stars and black holes in detail. We can also take the next step, that is, to study how these objects fit into the overall scheme of things. To do this, we must take a galactic view, because the galaxy is the stage on which the drama of stellar evolution is acted out.
The difficulty of this approach is that we are part of the galaxy; we are on stage, too, so it is not easy to get an overall view of things. Nevertheless, thanks to the HEAOs, we can construct a broad-brush X-ray picture of our Milky Way Galaxy. It contains at least one black hole candidate, Cygnus X-1, and about 30 other high luminosity sources that are most likely associated with neutron stars. These sources radiate a thousand or more times as much energy in X-rays as the Sun radiates at all wavelengths. About half of these high luminosity sources are associated with young massive stars that supply the stream of gas that falls onto the neutron star to produce the X-rays. Since massive stars evolve much more rapidly than low mass stars, these sources should be associated with regions containing young stars, such as spiral arms. This appears to be the case. The other high luminosity sources are associated with older stars that have masses similar to that of the Sun or less. These sources are associated with an older galactic population. They are found mostly in the central bulge of the galaxy and in tight clusters of old stars called globular clusters. Also prominent in the X-ray picture of the galaxy are the remnants of supernova explosions. Two dozen or more of these sources are scattered throughout the galaxy. They show up primarily as bubbles of hot gas, although at least in four cases, a bright neutron star pulsar shines in the center of the remnant. At much lower luminosities, hundreds of thousands of times dimmer than the bright sources, the degenerate dwarfs show up. Still fainter are the X-rays produced by billions of normal stars. Finally, there is an X-ray and gamma ray source that is associated with the nucleus of the galaxy. The luminosity of this source is comparable to an average bright source of the type discussed above, but it must be of an entirely different nature.
The source at the galactic center has attracted attention out of proportion to its intensity because it may provide evidence for the existence of a massive black hole in the nucleus in our galaxy. Infrared and radio observations suggest the presence there of a concentration of dark matter. The gamma ray observations made by the HEAO 3 Gamma Ray Spectrometer indicate that the galactic nucleus emits both a continuous spectrum of gamma rays and a bright, sharply defined line at the specific energy of 511000 electron volts. The 511000 electron volt line is exactly the energy of gamma rays produced when an electron and an anti-electron, or positron, interact and annihilate. Since positrons are quickly destroyed when they encounter electrons, the emission line must be produced quite near a source of positrons. Very high energy particles or electromagnetic fields are needed to produce positrons. A likely place to find such high energies is in the vicinity of a black hole.
This interpretation is supported by the observation that both the continuum and the line gamma ray brightness vary greatly over a period of about six months. This means that the source of the radiation can be no larger than the distance the gamma rays can travel in six months. Otherwise, time variations on opposite sides of the source would tend to cancel each other out, unless they were carefully coordinated, an unlikely situation. Since gamma rays travel at the speed of light, the gamma ray source must be  less than about half a light year in diameter. This means that the source cannot be produced by a distribution of supernovas or other phenomena occurring in a cluster of stars, which typically has a diameter of several hundred light years. Thus we have yet another indication of a large concentration of matter and energy into a small region. A theoretical analysis suggests that the nucleus of our galaxy contains a black hole with a mass ranging from 100 to a million times that of the Sun.
In summary, an X-ray image of the Milky Way shows the following features: (1) bright sources associated with the young stars in the spiral arms of the galaxy; (2) bright sources associated with the old stars in globular clusters and the central bulge of the galaxy; (3) bright sources associated with the remnants of supernova explosions; (4) faint sources associated with degenerate dwarfs; (5) faint sources associated with normal stars; (6) a fairly bright source associated with the nucleus of the galaxy.
Before the launch of HEAO 2 (the Einstein X-ray Observatory), only three other normal galaxies had been detected in X-rays. For these three galaxies, the Large and Small Magellanic Clouds and the Andromeda Galaxy, we had no real X-ray images, so we could not know whether our galaxy was typical or not. Now over 50 normal galaxies have been imaged in X-rays, and the Magellanic Clouds and Andromeda have been studies in much more detail.
In general, these galaxies look much the way we would expect them to look, based on the picture that has emerged from the study of our galaxy. They have some bright sources associated with young stars and others associated with the older stars in the central bulges and globular clusters. The galaxies that are larger than the Milky Way have more bright sources, and the ones that are smaller have fewer bright sources than our galaxy. They also have sources that can be identified fairly confidently with the remnants of supernova explosions. The faint sources cannot be detected, but presumably they are there, too. Finally, most of the galaxies apparently have an X-ray source associated with the galactic nucleus.
Interesting anomalies exist, however. For example, at least half a dozen extremely bright sources have been detected in nearby galaxies. These sources are about 10 times brighter than the brightest galactic X-ray sources. They cannot be explained in terms of the conventional theory of accretion onto neutron stars. Therefore, either they represent new examples of black holes or an entirely new class of object, or we have overestimated the distance to these galaxies and hence the luminosity of the sources. Any of these possibilities has exciting implications. In the last case, it would mean that the distance scale to nearby galaxies would have to be revised downward. This would have far-reaching consequences, since our estimates of the size of the universe are based to a large extent on our knowledge of the distance to nearby galaxies, which serve as calibrators for measuring the much larger distances to distant galaxies and clusters of galaxies.
 Another interesting development has been the detection of excessive X-ray emission from "starburst" galaxies. These peculiar galaxies are apparently the sites of recent bursts of star formation. This follows from infrared, optical, and ultraviolet observations that show that the colors and spectral features of these galaxies are best explained by assuming that a large number of very young and massive stars are present. The reason for this activity is unknown. It could be that the close passage of another galaxy has created a shock wave, or dumped a large amount of gas into the starburst galaxy, precipitating the formation of new stars from a cloud of dust and gas. Or it could be that an explosion from the nucleus of the galaxy has shocked a gas cloud, causing stars to form.
X-ray observations provide a new approach to these issues, since both explosive nuclei and rapid star formation should be associated with enhanced X-ray emission. An X-ray survey of about three dozen of these peculiar galaxies shows that they typically are a few times brighter in X-rays than normal galaxies. In the cases that have been studied in detail, the excess X-ray emission can be attributed to an extended region around the....
 .....nucleus of the galaxy. The most likely explanation for this excess would appear to be that, since the galaxy has more massive young stars, it also has more of the high luminosity neutron star and black hole X-ray sources of the type that are associated with massive young stars. An analysis indicates that the burst of star formation would have had to occur less than about 10 million years ago. The sources appear to be concentrated near the nucleus of the galaxy. This would seem to indicate that violent activity in the nucleus has produced the burst of star formation and consequently the formation of bright X-ray sources. On the other hand, the correlation between nuclear activity and starburst activity is weak. It seems more likely that the stars form there because that is where the gas is. Many of the starburst galaxies are observed to have nearby companion galaxies, which suggests that a near collision between galaxies has precipitated the burst of star formation. Obviously there are more questions than answers to this problem for the present, and future observations covering the entire wavelength range will be needed before the conundrum of the starburst galaxies can be cracked.
Einstein Observatory X-ray image of the
Andromeda Galaxy. Many bright X-ray sources are apparent. They are
concentrated in the spiral arms of the galaxy and around the nucleus
of the galaxy. (Courtesy of Leon Van Speybroeck)