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- 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.
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