One of the beauties of astronomy is that you do not have to be an expert to enjoy it. Anyone can step outside on a clear moonless night, gaze at thousands of stars shining across the vast interstellar spaces, and become intoxicated by a heady mix of grandeur and existential chill. The same questions come to mind time and again: How far away are the stars? How many are there? Are they strewn endlessly through space, or are we part of an island universe of suns that ends abruptly somewhere out there in the black ocean of space? It has been the sometimes heroic and often frustrating task of astronomers since the dawn of science to chart our position in the cosmic ocean. In the twentieth century, significant progress has been made in constructing an accurate map of the cosmos.
We know, for example, that our solar system is part of a much larger system of hundreds of billions of stars. This system is the Milky Way Galaxy, a huge disk of stars and gas. We also know that ours is not the only galaxy in the universe. As far as the largest telescopes in the world can see, there are galaxies in every direction. The nearest galaxies to our own are the Magellanic Clouds, the "crown jewels of the southern skies," to use Bart Bok's description. The Magellanic Clouds are small irregular galaxies, a catch-all term for those somewhat shapeless galaxies that do not fit neatly into a spiral or elliptical category. The Large Cloud is about 160000 light years from the solar system and has a mass about one-tenth that of the Milky Way Galaxy. The Small Cloud is about 200000 light years away; it has a mass about one-thirtieth that of the Large Cloud. Because they are so near, they are an invaluable laboratory for astronomers to study the evolution of stars and galaxies. The nearest large galaxy to the Milky Way Galaxy is the Andromeda Galaxy, at a distance of about 2 million light years. It is a giant spiral galaxy, much like our own in size, shape, numbers of stars, and types of stars. This nearby sister galaxy provides us with an opportunity to get a bird's eye view of a galaxy much like our own, to, in effect, "see ourselves as others do."
Just as stars are bound together by their gravitational forces to form galaxies, so also do galaxies come together to form groups or clusters. Our galaxy and the Andromeda Galaxy, together with at least 19 smaller galaxies, form a complex called the Local Group. The Andromeda Galaxy and the Milky Way Galaxy are at opposite ends of the group. The smaller galaxies are clustered around the larger ones, but on the whole, there is a lot of empty space in the group, and the chance of the large galaxies ever colliding is remote.
 As we look beyond the Local Group to distances of a few tens of millions of light years, we find other similar groups of galaxies. At a distance of about 60 million light years, in the Virgo constellation, we encounter the Virgo cluster of galaxies. The Virgo cluster is a system of several thousand galaxies. Most of these galaxies are large spirals and smaller, elliptically shaped galaxies. In addition, it contains a few giant, spherical galaxies. About 10 percent of all the galaxies in the universe are in rich clusters such as Virgo. One especially rich cluster is located in the constellation Coma Berenices. Called the Coma cluster of galaxies, it is almost directly overhead in the springtime sky, about 400 million light years away.
In the Coma cluster, 1000 galaxies are packed into a space roughly the size of the Local Group, which contains only about 20 galaxies. The crowded conditions in the Coma cluster and other rich clusters of galaxies have undoubtedly played a dominant role in the evolution of the individual galaxies and the cluster as a whole. X-ray observations have revealed that the space between the galaxies in rich clusters is filled with hot gas. As the galaxies move through this gas, they are stripped of gas and dust, adding to the supply of cluster gas. At the same time, this process deprives the galaxies of the raw material from which to form new stars. In a sense, then, galaxies age more quickly in the congested environment of rich clusters, because new generations of stars cannot form.
The galaxies in clusters show other peculiarities. An example is the giant elliptical galaxy known as M87. This remarkable galaxy is located in the center of the Virgo cluster. It is about three times more luminous than our galaxy and is estimated to contain as many as a trillion stars. Emanating from the nucleus, or central region of M87, is a peculiar jet-like feature. This jet is a strong source of radio, optical, and X-ray emission. Evidently, the jet was ejected from the nucleus of M87 a million or so years ago in a violent explosion that may still be occurring today.
In the last 20 years, evidence of explosive activity has been found to be a common feature of many elliptical galaxies and other peculiar objects most notably quasars. In fact, virtually every galactic nucleus is the site of explosive activity at some level. In quasars, these explosions are awesome. An energy output exceeding that of 10 trillion suns is produced in a region approximately the size of our solar system! It is as if a power source the size of a small flashlight could produce as much light as all the lights in metropolitan Los Angeles.
What is the source of the violent activity in the galactic nuclei and quasars? Many intriguing ideas - multiple supernova explosions, supermassive stars, huge spinning magnetized disks of gas - have been suggested, but the one that is being taken most seriously nowadays involves black holes having the mass of hundreds of millions of suns. The basic idea is the same as that discussed in the previous chapter. As matter falls toward a black hole, it gains a tremendous amount of energy. If only a small fraction  of this energy, say 10 percent, is radiated away before the matter falls into the black hole, and if there is a steady supply of infalling matter, then a large amount of energy can be radiated from a very small region of space. For the most powerful quasars, roughly one solar mass of material would have to fall into the black hole every year. In this picture, a quasar is a more or less ordinary galaxy with an extraordinarily active black hole in its nucleus.
Because of their high luminosity, quasars can be seen at great distances - 10 billion light years or more. That is, the light, radio waves, and X-rays we now see from the most distant quasars left those objects more than 10 billion years ago. That is about the time that most astronomers believe the galaxies were forming. The observations indicate that quasars were considerably more numerous then. This raises the suspicion that the quasar phenomenon may be intimately related to the process of galaxy formation. In any event, quasars are proving to be extremely valuable beacons with which to illuminate the frontiers of the cosmic ocean. The questions raised by the observations of clusters of galaxies and the explosive galactic nuclei are at the frontier of modern astrophysical research on the structure and evolution of galaxies and the universe. Observations made in the high energy band of the spectrum, especially those made with the sophisticated HEAO instruments, have played and are still playing a vital role in pushing back this frontier and enlarging our perspective of the universe in which we live. In the next few chapters, I touch on some of the results from the HEAO experiments and how they relate to some of the outstanding cosmologic questions of the day: How do galaxies evolve, both inside and outside of rich clusters of galaxies? What is the nature of the hidden mass that holds clusters of galaxies together? What causes the explosive activity in galactic nuclei and quasars? Is there matter between the clusters of galaxies? Is the universe finite or infinite?