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