The most captivating object yet to be discovered in high energy astrophysics is not a white dwarf or even a neutron star. It is the black hole. Black holes have gripped the imagination of scientist and layman alike. The reason is clear. In a world where absolutes are few and far between, the black hole gives us an absolute. It represents gravitational collapse without limit. According to physics as we know it, the matter that falls inside a black hole never escapes. It collapses forever. Because of their extreme nature, many scientists have rebelled against the notion of black holes. Other, as yet unknown effects must intervene to prevent the formation of black holes, they maintain. Nature would somehow not allow such an extreme situation to develop.
Yet nature apparently does allow it. Combined radio, optical, and X-ray observations have produced an object that looks the way we think a black hole should look. This object is in the constellation of Cygnus. It is called Cygnus X-1 because it is the strongest X-ray source in Cygnus. Before discussing the evidence for believing that Cygnus X-1 is a black hole, consider briefly what a black hole is expected to look like. The theory of stellar evolution predicts that at a certain critical time, when the core, or central parts, of a star has used up its nuclear fuel, the core will collapse. If the star is about the size of the Sun, it will turn into a degenerate dwarf star. If it is somewhat larger, it may undergo a supernova explosion that leaves behind a neutron star, that is, a star in which gravitational forces are held in check by nuclear forces. But if the stellar core has a mass greater than about three solar masses, gravitational forces overwhelm nuclear forces and the core collapses. Since nuclear forces are the strongest repulsive forces known, nothing can stop the continued collapse of the star. A black hole in space is formed. We would expect, then, that a black hole would have a mass greater than three times the mass of the Sun.
The next bit of information comes from Einstein's gravitational theory, which tells us what the conditions must be like around a black hole. First of all, the theory tells us that black holes give off no detectable light or radiation of any kind. Because of the intense gravitational forces near the black hole, nothing can escape from it, not even light, which moves at 300 000 km per second. If we were to send a probe toward an isolated black hole, the probe would detect no radiation from the black hole. It would, however, sense a gravitational field, because the black hole has mass. As long as the....
.....probe were a safe distance away, say a few million kilometers, the gravitational field it sensed would be no different from the gravitational field produced by a normal star of the same mass. The only difference would be that no star would be visible, even though the probe could sense the presence of a large concentration of matter through the gravitational forces. At this point the probe could still escape from the gravitational pull of the black hole, if we could give it a boost of energy from a rocket, for example. Suppose, however, that we chose to send the probe closer.
As the probe approached the black hole, the gravitational forces would increase inexorably. At a distance of a few thousand kilometers from the black hole, the gravitational forces on the probe would be so great that the side of the probe closest to the black hole would literally be torn away from the side furthest away from the black hole. Eventually, at a distance of a few kilometers from the black hole, the particles that made up the probe would pass the point of no return. No matter how much energy we gave these particles, they could not escape. The gravitational forces are simply too strong. The particles would be lost forever down the black hole. This point of no return is called the gravitational radius of the black hole. For a black hole containing about 10 times the mass of the Sun, the gravitational radius is about 10 km. An observed concentration of more than three solar  masses inside a region having a diameter of a few tens of kilometers would be very strong evidence for a black hole. But how can we hope to observe such an object, since black holes give off no detectable radiation? By sending in probes, or test particles.
As in the case of neutron stars and degenerate dwarfs, nature could provide us with a probe of a black hole. All that would be required is a nearby star that is losing mass into the black hole. As this matter swirls into the gravitational maelstrom produced by the black hole, it will send back information, in the form of radiation, about conditions near the gravitational radius of the black hole. Of course, it cannot send back information about conditions inside the gravitational radius; the radiation cannot escape from inside the radius. As the matter of the companion star spirals toward the black hole, it will move faster and faster. This increased energy of motion will be changed into heat energy by dissipative processes-a sort of friction -in the stream or disk of infalling matter. Near the gravitational radius the matter will be swirling around at speeds close to the speed of light, and temperatures ranging from tens of millions to perhaps as much as a billion degrees are expected. At these temperatures, X and gamma radiation are produced. A black hole in a double star system would therefore be expected to be a strong source of X and possibly gamma rays. Furthermore, since a blob of matter near the gravitational radius would be orbiting the black hole about once every millisecond, the X-radiation might be expected to show erratic, short-term variability. The variability in the X-radiation is not expected to show any regular or periodic behavior such as is often observed with neutron stars and degenerate dwarfs. Unlike these objects, the black hole has no strong magnetic field to guide the matter in a preferred direction.
To summarize, then, a black hole that is part of a close binary system should show evidence of a mass greater than three solar masses concentrated into a small area, and it should be a strong source of X-radiation. The X-ray source Cygnus X-1 meets all these requirements. It is part of a binary star system in which a blue supergiant star is orbiting an invisible companion star. This invisible companion star has a mass greater than about nine times the mass of the Sun. It is a strong X-ray source that shows rapid time variations in the intensity of its X-ray flux. Because of the good fit between what is expected and what is observed, and more importantly, because they can think of no other object that could meet the requirements described above, most astronomers believe that Cygnus X-1 is a black hole. This belief is tempered with a dose of caution, however. In most scientific papers describing Cygnus X-l, it is referred to as a black hole "candidate" rather than simply as a black hole; somehow, the concept of a black hole is still a little difficult to swallow.
The HEAO instruments have been used to perform an exhaustive study of the spectrum and time variability of the X-radiation from Cygnus X-1. The goal is to establish in detail the X-ray properties of Cygnus X-1 and to  use these results to better understand the state of matter near the gravitational radius of a black hole. The HEAO I results indicate that there is a distribution of temperatures in the hot gas around the black hole; these temperatures range from 30 million degrees to about a billion degrees. A study of the variability of the X-radiation indicates that a variation in the high energy photons typically occurs about 10 milliseconds later than a variation at a slightly lower energy. This could mean that the higher energy photons take slightly longer to get out of the source. Measurements such as these, when coupled with theoretical analysis, will help greatly to determine the physical state of the gas around the black hole.
They should also help to provide a definite X-ray signature for a black hole. This would be of great help in the search for other black holes in our galaxy. Recently, another strong black hole candidate has been discovered. A team of Canadian and American astronomers have used optical observations and results from HEAO 1 and the Einstein Observatory to show that the strong, highly variable X-ray source LMC X-3 in the Large Magellanic Cloud (a nearby galaxy) is associated with a dark star with a mass greater than nine solar masses. Since this mass far exceeds the upper mass limit of three solar masses for neutron stars, this object is also a strong candidate for a black hole. Two other sources, known as Circinus X-1 and GX 339-4, show rapid and erratic time variations that are similar to those observed for Cygnus X-1. However, unlike Cygnus X-1, these sources show no evidence of very high temperature (hundreds of millions of degrees) gas. Both the sources are known to be members of a close binary system, but it has not been possible to determine the mass of the X-ray source in either case. Both objects could be neutron stars.
Another location in the galaxy at which a black hole has been suggested to be lurking is the center of the galaxy. This black hole would have been formed, not from a single massive star, but from hundreds of thousands of massive stars, or from a supermassive star. It would have a mass of a million times or more solar masses. Some scientists believe that a supermassive black hole is at the center of virtually every galaxy and is responsible for violent activity that is so characteristic of the central regions of galaxies.