Black Holes:
The Invisible Vampires

To many people the term Black Hole conjures up images of massive monsters devouring planets and whole solar systems as it relentlessly lumbers through space. This view has been popularized by science fiction writers and by films, such as Disney's~ "The Black Hole". The idea of a black hole and its nature is somewhat less ominous. As early as the late 1700's, a French mathematician, named Laplace, hypothesized their existence. And in 1915, Albert Einstein's celebrated theory of general relativity predicted and described such an object. But regretfully, the ideas of Laplace and Einstein were not taken very seriously until around the 1960's, when scientist's understanding of the evolution of stars was sufficient to warrant serious consideration of black holes.

What is a Black Hole?

A black hole, in simple terms, is an object that has an incredible amount of mass in a very small (considered as a point or to have no dimensions) volume. This huge mass will have a gravitational influence in its near vicinity so strong that even light cannot escape, thus the term black (no light escaping) hole (no dimensions). The title The Invisible Vampires, is referring to the fact that one way to see something that does not emit any light is to look for signs of material, that happens to be very near, falling into the hole. Kind of like water going into a drain. As the water gets near the drain it spins faster and faster. In stellar terms this would mean material from, say, a nearby companion star, spiraling into the black hole. This material might be hot enough and be going fast enough to be able to be seen.

How are Black Holes Formed?

Stars (like our Sun) are the way they are because of two competing processes. The thermonuclear fusion in the center, or core, of the star causes it to shine and to want to blow itself apart. While the mass of the star acts to hold it together in its spherical shape. It is very difficult to have a good feel for how tremendous these two opposing forces are because they are so much stronger than anything we, on Earth, will ever encounter.

In the course of the life time of a star it will eventually "use up" all the hydrogen in its core via nuclear fusion. At that time the star will not be able to oppose the crushing gravity that is acting inward. When this happens the core of the star starts to collapse in on itself. As the core collapses, the outer layers of the star expand to form a red supergiant. The collapsing causes the core to heat to tens of millions of degrees. At this temperature the "ash" from the first round of nuclear reactions begin to undergo nuclear fusion itself. The temperature eventually gets high enough for another round of nuclear fusion to begin. This cycle continues until iron is produced in the star's core. At this point fusion reactions can no longer take place. Since there is no longer any fusion reaction pushing outward, the core and interior region of the star collapse. As the material outside the core rushes inward, it reaches a high enough temperature to explode and rip the star apart. This explosion is called a supernova. If the collapsing core has enough mass, it will continue to collapse down to zero radius. This infinitely small space is called a singularity. Although the singularity has no size, it still has its mass and therefore it still has its gravitational influence. Around this singularity would be a spherical boundary called an event horizon, inside of which the gravitational force would be too strong for anything, including light, to escape. The radius of the event horizon is determined by the mass of the singularity. The Sun, for example, would have an event horizon of a little over three (3) miles in diameter. If Earth were compressed into a singularity, its event horizon would be about the size of a small marble. Anything inside the event horizon is considered to be part of the black hole.

Characteristics and Properties of Black Holes.

It has been said that of all the characteristics of a star--its brightness, color, mass, charge, size, rotation, composition--only three of these would survive the transition to a black hole: mass, charge, and rotation. The result of these three would give rise to more exotic black hole behavior, but the essentials of the black hole would still be the same.

One very interesting consequence of Einstein's theory of relativity is that time slows down for someone approaching the black hole. If you were to throw something at the black hole or launch a probe into it, the object would accelerate towards the black hole, then appear to slow down and eventually stop just above the event horizon - suspended in space and time forever. Any hope of getting a signal out of the black hole would be in vain since nothing can escape the tremendous pull of gravity at the event horizon.

"Seeing" a Black Hole.

At the present time there are several possible candidates for observing (indirectly) black holes. The most notable is Cygnus X-1, in the constellation Cygnus the Swan. The trick is to try to find a massive star that is emitting large amounts of x-rays while orbiting about a very massive, invisible companion. The x-rays would indicate material being "sucked" off the visible star and spiraling into the black hole. The only problem with this scenario is the fact that many scientists believe that this type of x-ray binary system will only emit the necessary x-rays for a few hundred thousand years--barely the blink of an eye in astronomical terms.

Suggested References

Astronomy Magazine, Dec. '91, "What Makes Quasars
Shine, Oct. '89, "Creating Universes".
The Cosmic Frontiers of General Relativity,
William J. Kaufmann, III, 1977.
Time-Life Books, Voyage Through the Universe,
"Galaxies" 1988.
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