One of the fascinations of astronomy is its continual push against the boundaries of the unknown. This can take the form of peering out across billions of light years, studying ghostly images produced by instruments strained to the limits, or it can involve pressing deep into the gravitational whirlpools surrounding stars that have collapsed to form white dwarfs, neutron stars, and black holes. High energy astronomy has led the advance here, because matter drawn into these whirlpools gives off large amounts of high energy radiation.
In collapsed stars, matter has been pushed to the limit. Internal pressures produced by nuclear power production in the centers of stars are no longer important, because the nuclear fuel has been exhausted.
As discussed in Chapter 9, a star is in a way similar to those inflatable "bubbles" that are used for indoor tennis courts or temporary structures. As long as the internal pressure is sufficient, there is no problem, but if the pressure drops drastically, the structure will collapse into a heap. The size of the heap will depend on the material that the structure is made of and how massive it is. If it is made of stiff rubbery material, it will make a fairly large heap, unless, of course, the weight of the material or material piled on top of it is so large that it squashes the material into a smaller heap.
A star, like an inflatable bubble, is held up by a balance of internal pressure against gravity. In the normal course of its life, this pressure is provided by the energy produced in nuclear reactions deep in the center of the star. When those nuclear reactions stop producing energy, the pressure drops and the star falls in on itself.
How large a heap will a star make when it collapses? The answer depends on the size of the star. A star about the size of the Sun will collapse into a heap about the diameter of Earth, or about one-hundredth the original diameter of the star. Such stars are called white dwarf stars because of their small size and because the heat generated by the collapse has made them white hot. A sphere of white dwarf material with a diameter of the size of this would weigh about two pounds, or about a hundred thousand times more than a lead sphere of the same size.
A star about five times as massive as the Sun will undergo a much more violent collapse. The outer layers of the star will be ejected into space in a supernova explosion, leaving behind a collapsed star called a neutron star.
Ordinary matter, the kind that we and everything around us is made of, is mostly empty. It is made up of atoms, which are made of electrons,  protons, and neutrons. The protons and neutrons contain more than 99.9 percent of the matter, yet they are contained in the nucleus, which has a diameter of only 1/100000 that of the cloud of electrons around the nucleus. The electrons themselves take up little space, but the pattern of their motions, or orbits, defines a size that is the size of the atom. In a white dwarf star, the atoms are crushed to a diameter about 1/100 that of ordinary matter. Still, the matter is mostly empty. The distance between nuclei is 100 times the size of the nuclei themselves. In a neutron star, the atoms are crushed completely. The protons capture the electron clouds and are transformed into neutrons, so that most of the matter in a neutron star is neutrons, as you might have guessed.
The collapse of the core of a star to a neutron star has no analogy on Earth. It is as if a structure the size of the Empire State building were to collapse to a heap of one centimeter high! A sphere of neutron star material the size of this would weigh about 20 million pounds, or about a trillion times more than a lead sphere of the same size.
Neutron stars represent the last stand of matter against gravity. The nuclear forces have been pressed to their limit. Stellar cores having a mass less than about three times that of the Sun can be stabilized this way. What about larger stars? Will the extra mass be thrown off in a supernova explosion, or will an unstable core be formed?
A combination of theoretical research and observation of Cygnus X-1, an X-ray star in the constellation of Cygnus, has led most scientists to conclude that the core becomes unstable.
The properties of such a core are very strange. Since nuclear forces cannot prevent gravitational collapse, the core collapses indefinitely, forming a warp in space. Nothing, not even light, can escape from this gravitational maelstrom, so the name black hole is used to describe these bizarre objects.
White dwarfs have been known to astronomers for some time. The white dwarf companion to Sirius was first observed in 1862 and explained in 1933. Neutron stars and black holes, however, are a product of modern astrophysics. Neutron stars were not discovered until 1968, and the first compelling evidence for a black hole was not obtained until 1971. Now, after a decade of highly successful high energy astronomy experiments, over 100 star-like X-ray sources have been discovered. All of these are thought to be a collapsed star of one type or another. The HEAO experiments have given us a comprehensive look at these objects. The experiments have fixed the positions of the sources, monitored the variation of the intensity of their radiation, analyzed this radiation with spectrometers, and discovered new sources.
In the following chapters we take a closer look into the strange world of collapsed stars.