A black hole is "black" or invisible because according to theory, its intense gravity pulls everything into itself. Its gravity is so strong that even light cannot escape. Without light, we can't see an object or take a picture of it. In fact, any object whose gravity is so tremendous that its escape velocity exceeds the speed of light is a black hole. (Escape velocity is the velocity required for an object to leave the surface of a body and avoid being drawn back down to it. For example, the escape velocity for Earth is about 39,600 kilometers per hour. Thus, if you threw a baseball up into the sky, you would have to throw it at least at 39,600 km/hr for the ball to keep going out into space and not fall back down. Our Sun's escape velocity is 2,160,000 km/hr. By contrast, the escape velocity of a black hole must begreater than the speed of light, which is about 1,079,252,848.8 kilometers per hour.) If some force could squeeze our Sun into a ball just 6 kilometers in diameter, it would be a black hole. The more massive an object is, the more gravity it has. But it takes a very great amount of mass compressed into a small area to create a gravitational field strong enough to keep light from escaping. The escape velocity for an object, keeping its mass constant, will be greater if the object's radius is smaller. Thus, this same mass squeezed into a smaller radius has a stronger gravity than it did before, and thus a higher escape velocity.

The strength of gravity between two objects is related to their mass and the distance between them. The more massive the objects are, the stronger the gravity; the closer the masses are to each other, the stronger the gravity. Consider what would happen if our Sun were to be 'magically' compressed so tightly that it became a black hole. What would happen to the orbit of the Earth? Actually, the answer is: practically nothing. In our imaginary example, the mass of the Sun has not changed, nor has its distance from the Earth. Thus the Earth's orbit is uneffected. However, the Earth gets very cold since the "black hole" Sun is no longer sending out any light or energy.

One way a black hole may form is from the collapse of giant stars. Stars have lifespans just as people do, but they shine for such long times compared to the scale of human life that to most of us stars seem to exist forever. Astronomers believe that stars form from nebulae: huge clouds of dust and gas (mostly hydrogen) in space. Gravity between particles pulls bits of the nebula together. Slowly, over millions of years, the nebula shrinks, and matter swirls together until enough mass is accumulated to begin nuclear fusion and a star is "born." Some stars are smaller than our Sun and fuse their hydrogen slowly, shining for billions of years. Others are giants (Rigel in the constellation Orion, for example) that burn hot and squander their fuel in just a few million years. Because of their different masses, stars end their lives differently. Some astronomers believe that the most massive stars, those with ten or more times the mass of our Sun, have the most spectacular deaths. When such a star has used up most of its fuel, it may die in a violent explosion; a supernova that rips off the star's outer layers of gas, sending matter flying into space for millions of miles. At the same time, the dense, inner core of the star implodes, collapsing in on itself so rapidly that it becomes too dense for any force in nature to stop it from crushing itself down to a black hole.

Another type of black hole may be giant black holes at the centers of galaxies. M87 (also called NGC 4486), located in the constellation Virgo the Maiden, is a galaxy thought to contain a giant black hole. Astronomers also suspect a massive black hole may lurk at the center of our own Milky Way Galaxy.

Equations show that as matter falls into the gravitational field of a black hole, it accelerates. The matter heats up and gives off X-rays. Earth-orbiting telescopes are looking for sources of those X-rays that might indicate the presence of black holes. One possibility is a star in the constellation Cygnus (the Swan) called Cygnus X-1. Cygnus X-1 shows up as a bright X-ray source as well as a visible star. Some astronomers believe that Cygnus X-1 is a huge blue supergiant star with a massive black hole for a companion. The black hole could be pulling a million tons of stellar matter into itself each second. As the star's gasses spiral down into the black hole, the X-rays it would emit may be the X-rays we are detecting.

Recently, astronomers have detected two very strong candidates for black holes inside our Milky Way Galaxy. The first is GS2023+33, known as V404 Cygni and discovered in 1992. The second, GS 2000+25, was first recognized as an X-ray nova in 1988, and at that time was first suspected to be a black hole in a binary star system. Later, in 1995, astronomers using the Keck Telescope calculated GS 2000+25 to be at least five times more massive than the Sun, giving strong evidence that this object is probably a black hole.

What happens to matter pulled into a black hole? No one knows for sure. It could be that the matter just disappears into the core of the black hole, which is called a singularity. Another fascinating possibility is that black holes may be connected to other parts of our own universe, like the cosmic shortcuts called "wormholes" that science fiction writers have imagined. For now, we know nothing for certain about what black holes are, or even if they are, where they come from, or whence they lead. But the search is on.

More information about black holes is available in the following:

Magazines

"The Care and Feeding of Black Holes," Astronomy. May 1995, p. 19-20.

"Destination: Galactic Center," Sky & Telescope. June 1995, p. 26-30.

"Black Holes, Ants, and Roller Coasters," Discover. July 1995, p. 54-61.

"Best Black Hole Yet," Sky & Telescope. September 1995, p. 11.

"A Quiet Beast," Astronomy. September 1995, p. 22.

"Death by Black Hole," Natural History. October 1995, p.20-21.

"From Black Holes to Quarks," Astronomy. October 1995, p. 24.

Books

Chaisson, Eric. Relatively Speaking: Relativity, Black Holes, and the Fate of the Universe. New York: Norton, 1988.

Gribbin, John R. Unveiling the Edge of Time: Black Holes, White Holes, Wormholes. Harmony Books, 1992.

Hawking, Stephen W. Hawking on the Big Bang and Black Holes. World Scientific, 1993.

Luminet, Jean-Pierre. Black Holes. Cambridge University Press, 1992.

Thorne, Kip S. Black Holes & Time Warps: Einstein's Outrageous Legacy. New York: W.W. Norton & Co., 1994.

Trefil, James. The Dark Side of the Universe. New York: Charles Scribner's Sons, 1988.

Wald, Robert M. Space, Time, and Gravity: The Theory of the Big Bang and Black Holes. Chicago: University of Chicago Press, 1992.

IS-18
Modified 27 March 1996