WHAT ARE BLACK HOLES? By Andrew Fraknoi and Sherwood Harrington JUST TWO DECADES ago, black holes were an interesting footnote to our astronomical theories that few non-specialists had heard about. Today, black holes have "arrived" - one hears about them in Hollywood thrillers, in cartoon strips, and more and more on the science pages of your local newspaper. What exactly are these intriguing cosmic objects and why have they so captured the imagination of astronomers and the public? A black hole is what remains after the death of a very massive star. Although stars seem reasonably permanent on human time scales, we know that over the eons all stars will run out of fuel and eventually die. When smaller stars like our own Sun burn out, they simply shrink under there own weight until they become so compact they cannot be compressed any further. (This will not happen to the Sun for billions of years, so there is no reason to add a rider to your home owners policy at this time!) When the largest (most massive) stars have no more fuel left, they have a much more dramatic demise in store for them. These stars have so much material that they just cannot support themselves once their nuclear fires go out. Current theories predict that nothing can stop the collapse of these huge stars. Once they begin to die, whatever remains of them will collapse FOREVER. As the collapsing star falls in on itself, pull of gravity near its surface will increase. Eventually its pull will become so great that nothing - not even light - can escape, the star will look BLACK to an outside observer. And anything you throw into it will never return. Hence astronomers have dubbed these collapsed stellar corpses "black holes." Alert readers will quickly note that this expanation of black holes does not bode well for finding one. How do we detect something that cannot give off any light (or other form of radiation)? You might suggest that we can spot a black hole as it blocks the light of stars that happens to lie behind it. That might work if the black hole hovered near the Earth, but for any black holes that are a respectful distance away in space, the part of the sky it would cover would be so small as to be invisible. To make matters worse, the sort of black hole that forms from a single collapsing star would be only 10 or 20 miles across - totally insignificant in size compared to most objects astronomers study and much too small to help a distant black hole hunter on Earth. The size of a black hole, by the way, is not the size of the collapsing star remnant. The stuff of the former star does continue to collapse forever inside the black hole. What gives the hole its "size" is a special zone around the star's collapsing core, called the "event horizon." If you are outside this zone, and you have a powerfull rocket, you still have a chance to get away. Once you passed inside this zone, the gravitational pull of the collapsing stuff is so great, nothing you can do can help you from being pulled inexorably to your doom. The name "event horizon" comes from the fact that once objects are inside the zone, events that happen to them can no longer be communicated to the outside world. It is as if a tight "horizon" has been wrapped around the star. How then could we detect these bizzare objects and verify the strange things predicted about them? It turns out that far away from a black hole the only way to detect it is to "watch it eating." If a black hole forms in a single star system, there is very little material close to the collapsed remnant for its enormous gravity to pull in. But we believe that more than half of the stars form in double, triple or multiple systems. When two stars orbit each other in proximity, and one becomes a black hole, the other one may have some difficult times ahead. Under the right circumstances, material from the outer regions of the normal star will begin to flow toward its black hole companion. As particles of this stolen material are pulled into a twisting, whirling stream around the black hole's event horizon, they are heated to enormous temperatures. They quickly become so hot that they glow - not just with visable light, but with far more energetic X-rays. (Of course, all this can be seen only above the event horizon; once the material falls into the horizon, we have no way of ever seeing it again.) Astronomers began searching in the 1970s for the tell-tale X-rays that indicate that a black hole is consuming a part of its neighbor star. Since cosmic X-rays are blocked by the Earth's atmosphere, these observations became possible only when we could launch sensitive X-ray telescopes into space. But in the last decade and a half, at least three excellent candidates for a "feeding" black hole have been identified. Probably the best-known case is called Cygnus X-1, a system in the constellation of Cygnus the swan, in which we see a normal star that appears to be going around a region of space with nothing visable in it. Smack dab from the middle of that region, we see just the sort of X-rays that reveal the stream of material being sucked into the hole. While this sort of indirect evidence is not quite as satisfying as seeing a black hole "up close," for now (and perhaps fortunately) it will have to do. What is intriguing astronomers these days is the posibility that enormous black holes may have formed in crowded regions of space. These may not just eat part of a companion star, but may actually consume many of their neighbor stars eventually. What we would then have is an even larger black hole, able to eat even more of the material in its immediate neighborhood. In the most populated areas of a galaxy - for example, its center - black holes may ultimately form that contain the material of a million or billion stars. In recent years, astronomers have begun to see tantalizing evidence from the center of our own galaxy and from violent galaxies in the distant reaches of space indicating that such supermassive black holes may be more common than we ever imagined. If this evidence is further confirmed, we may find that the strange black hole plays an important role not only in the death of a few stars but even in the way entire galaxies of stars evolve. 