What is a black hole, and what are its features? How is one created? Do black holes really exist, or are they just science fiction? Our Galaxy has existed for approximately 10 billion years, while only 5 billion years ago was our Sun born, and only about 5,000 years ago did the first signs of civilization arise (Shipman, 15; Taylor, 37). Yet, within the last few decades, astronomers and astrophysicists have uncovered marvellous wisdoms of the universe; just one is black holes. To fully examine a black hole, it is first necessary to unravel its ancestry from its beginning to the eventual death of the star it once was. After that, we observe the effect that the super-strong gravitational pull around a black hole has on light, time, and space.

The evolution of a black hole is as important as its features. A black hole was once a star, and even the star was once just interstellar gas and dust. A long, long time ago (about 10 billion years), a cloud of interstellar gas and dust drifted through space very slowly, its width trillions of kilometers long, and its temperature about 100K (-173°C). It contained mostly hydrogen and helium atoms, spaced so that about ten filled a cubic centimeter. To put this in perspective, air contains 30 quadrillion atoms per cubic centimeter (Kaufmann, 4). This cloud had nothing to do but wait for a galaxy to pass by and stir it up. (Taylor, 46; Moore 16)

Eventually, our very own spiral galaxy, called the Milky Way Galaxy, came along and tumbled the atoms closer together. This more compact form is known as a nebula; it is a dark cloud since starlight is not able to penetrate it, much like a dense fog. Without starlight heating the atoms, the temperature in the cloud fell to near absolute zero (-273°C), and the atoms almost stopped moving. (Kaufmann, 5)

Previously, the atoms had enough velocity to withstand the pull of attraction between them. Now, however, with no energy supplied by light, gravitational attraction became the dominant field, and the atoms in the cloud grouped together forming clumps, or globules. As the globules got bigger, the pull of gravity increased. More and more atoms fell into these globules, until finally each globule had a diameter of several billion kilometers, and a mass of several solar-masses. (By definition, 1 solar-mass equals the mass of our Sun.) At such sizes, globules are unstable. (Kaufmann, 5-6; Taylor, 46)

The tremendous pressure of the gases on a globule caused it to contract, squashing the gases inside it and compressing them to high densities. Pressure and temperature are directly proportional; therefore, if the pressure rises, so does temperature. As the temperature rose, the gases began to glow a visible dull red. The globule had turned into a protostar, but still it was unstable and needed to undergo further contraction; the temperature continued to climb. (Kaufmann, 6)

When the temperature at the core of a protostar rises to 10 million degrees, the hydrogen atoms get so excited that if they run into one another, they fuse, turning into helium. This process is known as hydrogen burning, even though the hydrogen atoms do not really burn - they just fuse together. The total mass of the helium is slightly less than the mass of the hydrogen; since energy is conserved, the mass must have been converted to pure energy. The pure energy is a strong enough force to withstand the pull of gravity, and contraction stops. This stable, burning sphere is now a very bright new star. (Kaufmann, 6)

In our Galaxy there are an estimated 100 billion stars (Begelman, 16). Although most stars have masses around 1 solar mass, like our Sun, they range in size from one-tenth to 50 solar masses (Kaufmann, 25). These stars stay in equilibrium for a long time, and may even seem to be unchanging; however, they are decaying. After billions of years of steady hydrogen burning, the first dramatic change in a star begins. When the supply of hydrogen at a star's core is depleted, the star is no longer able to counteract the tremendous pressure of its mass by burning hydrogen, and contracts. Thus, the temperature rises until a shell of hydrogen between the core and the surface becomes hot enough (10 million degrees) to burn. This is called shell hydrogen burning; it creates more pure energy, but not enough to stabilize the star. As shell hydrogen burning proceeds, it builds up pressure in the star's middle, causing it to swell outward, while at the same time heating the core. (Kaufmann, 14)

At the star's core, the huge supply of helium that has been building up for billions of years finally reaches a high enough temperature, 100 million degrees, to begin burning. Helium atoms fuse together to form carbon and oxygen; this is helium burning, and it causes the star to expand more than a million times its previous size. With two sources of energy, one from shell hydrogen burning and the other from helium burning, the star is stable under new conditions. It is called a red giant; it glows red, much like a protostar. (Kaufmann, 14; Shipman, 30)

Of course, after billions of years as a red giant, the star's supply of helium at its core will be depleted. It will again become unstable, yet what happens now? There are three possibilities; it may turn into a white dwarf, a neutron star, or a black hole, depending on its mass. If the star is smaller than about 3 solar masses, it cannot undergo further burning of carbon and oxygen when its supply of helium is used up. Instead, the star begins to pulsate. As it contracts, the pressure builds up, increasing the temperature until it is hot enough to briefly start shell hydrogen burning and helium burning, causing the star to expand. Then, when the star has cooled down, it contracts again, building up the pressure. This cycle happens over a long time, and eventually creates a ringed nebula around the star which can have up to more than half its original mass. This nebula is made of shells of the outermost layers of the star; each time it pulsates, another layer drifts away from its core. This process cannot happen forever, since the electrons inside the star resist being pushed too close together. The Chandrasekhar limit is 1.4 solar masses; the star cannot become any smaller (Kaufmann, 36). It is called a white dwarf, and is extremely compact. As Shipman puts it, "white dwarf stuff is very dense; a cupful would outweigh two dozen elephants" (37). White dwarfs simply radiate away their mass slowly. (Shipman, 36; Moore, 41; Taylor, 48)

White dwarfs are not the only way for a star to die. If the star is larger, say around 15 solar masses, it has enough mass for carbon burning to occur at 700 million degrees, and oxygen burning at 1 billion degrees. After the star's supply of oxygen is depleted, it has shells of hydrogen, helium, carbon, and oxygen burning, and at its core there is a huge supply of iron. However, iron does not burn. The pressure of the gases becomes so great that the nuclei of the iron get ripped apart, and protons and electrons fuse to make neutrons. This reaction causes the star to implode (not explode, but implode) violently, releasing more pure energy in a few days than it has throughout its entire life. A shock wave rips the star apart, turning it into a supernova. (Kaufmann 16; Moore 42-3)

A supernova is unstable, and becomes either a neutron star or a black hole. If the star has less than 2.5 solar masses, it contracts until degenerate neutron pressure, much like degenerate electron pressure, resists further contraction. It is called a neutron star, and is much denser than a white dwarf. To emphasize just how dense it is, Kaufmann points out: "So much matter is confined to such a small volume that a single tablespoon of neutron star material would weigh 40 billion tons" (51).

If the star's mass is greater than 2.5 solar masses, the pressure is so great that the star cannot withstand its own gravitation. Without any fuel left to burn, the star is crushed in on all sides, contracting smaller and smaller unable to stop itself, until it reaches its gravitational radius, which may be a few hundred kilometers. After that, it is crushed out of existence. What remains is a black hole. (Kaufmann, 63)

The evolution of a black hole is indeed terrific, but its structure is out of this world, literally. The most obvious characteristic of a black hole is the immense gravitation that surrounds it. To understand why a black hole is black, we need to understand just what gravity does. "Gravity is the one truly universal force. No substance, no kind of particle, not even light itself, is free of its grasp" (Begelman, 2). We shall look at its effect on light, time, and space.

Light is affected by gravity. Thus when an observer looks at a high-mass neutron star, for example, the very strong gravitational pull causes the light to bend slightly, and the star appears to be in a different location. However, if the observer looks at a black hole, he sees nothing, since the gravitational pull is so strong that light cannot escape. While the escape velocity, the velocity an object must have to be able to leave a planet's gravitational field, for earth is 11 km/s, for a black hole it is the speed of light, 300 000 km/s (Novikov, 9; Moore, 15). Since nothing can travel faster than light, nothing at all can leave a black hole.

Time is also affected by gravity. To indicate that time is not absolute, Kaufmann writes eloquently:

Time does "take time" to reach earth from far away. If the Sun were to stop emitting light at this very instant, it would take about 8 minutes before we realized what had happened 8 minutes ago. Now that we know time is not absolute, we observe it in a gravitational field. As the field increases, time slows down. Actually, time approaches an infinitely slow pace at the surface of a black hole. If this is hard to believe, note that instruments have recently been developed that are able to detect the differences in rates of clocks on the lower and upper levels of a tall building. Since the clock lower down is experiencing a greater gravitational field, it ticks slower (by about one billionth) than the one on the upper level (Kaufmann, 78).

Even space is affected by gravity. "A gravitating body stretches and distorts the space around it, much like a lead weight resting on a rubber membrane" (Begelman, 6). For a black hole, the space is distorted to immense disproportions. If a person were to approach a black hole, he would be stretched lengthwise, becoming quite flat, while being compressed horizontally. Yet as the person reached the very point of the black hole, he would become undistinguishable as all the atoms in his body squished together, and disappeared. (Shipman, 70)

Light, time, and space are affected by gravity. Einstein's general theory of relativity is the best theory so far in explaining this phenomena of warped space and time. Kaufmann explains how interesting Einstein's theory is: "Specifically, his theory tells us that moving clocks run slow, moving rulers shrink, and the masses of moving objects increase without bound as the speed of light is neared" (67). Understanding the importance of the gravitational field is important, yet what really is the best description of a black hole? "A black hole is a region of space (not a solid body) into which matter has fallen, and from which nothing, whether material objects or even light itself, can escape" (Moore, 7). A more emphatic description of the black hole is that "... the black hole is empty. There is absolutely nothing there! No atoms, no rocks, no gas, no dust. Nothing! " (Kaufmann, 86).

The search for black holes has been going on for years, and though scientists suggest that millions of them must be in our universe, they have found only a few candidates; the most likely one is in the system Cygnus X-1, where a bluish star rotates around the black hole. To narrow the hunt for black holes, astronomers look at such binary systems, where two stars rotate around each other in orbit. If one very large star is rotating around seemingly nothing, that nothing may be a black hole. (Hawking, 95; Novikov, 61-2)

Although humans have existed for only a brief second in relation to 10 billion years, scientists have already been able to postulate astounding theories on the laws of nature. We know that stars evolve from clumps of gas and dust, that white dwarfs and supernovas arise from stars, and that supernovas may turn into neutron stars or black holes. And due to super strong gravitation around black holes we find space and time fold over themselves. One may even wonder about the inside of a black hole; does it lead to another universe, or back to ours at a different time? This has been theorized, and someday we may find out. At the rate scientific knowledge is progressing, black holes are only the wisp of a beginning of that which shall come to be known as basic and obvious.

Begelman, Mitchell, Martin Rees. Gravity's Fatal Attraction: Black Holes in the Universe. Scientific American Library. New York: W. H. Freeman, 1995.

Hawking, Stephen W. A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books, 1988.

Kaufmann, William J. Black Holes and Warped Spacetime. San Francisco: W. H. Freeman, 1979

Moore, Patrick, Iain Nicolson. Black Holes in Space. New York: W. W. Norton, 1974.

Novikov, Igor. Black Holes and the Universe. Cambridge: Cambridge University Press, 1990.

Shipman, Harry L. Black Holes, Quasars, and the Universe. 2nd ed. Boston: Houghton Mifflin, 1980.

Taylor, John. Black Holes: The End of the Universe?. New York: Random House, 1973.