Supernovae are fairly rare; in a galaxy the size of the Milky Way, one happens every few hundred years. The last two (that were seen) in our galaxy were observed by Tycho Brahe and Johannes Kepler. (There may have been one in the late 1600's, but it was not observed. Possibly, it was the wrong season (i.e., the Sun was up when it was up.)

During a supernova explosion, most, but not all, of the star is destroyed. A remnant will remain in which even the strength of electron degeneracy wont be enough to hold up against the pull of gravity. The electrons in this object will literally be crushed out of their lowest orbitals into the nucleus of the atom. When this happens, the electrons combine with the protons to make neutrons, and it will be as if the star is composed of one great big atomic nucleus, consisting entirely of neutrons. In contrast to a white dwarf, which is a solar-mass star crushed down to the size of the planet earth, this neutron star is a star crushed down into the size of State College.

Neutron stars have an interesting property, which involves their rotation rate. Consider an ice skater, spinning slowly with her arms extended. When her arms are brought in towards the body (so that she is made more compact), she spins faster. This is due to conservation of angular momentum. Stars also conserve angular momentum. The Sun rotates on its axis once every 26 days, but if the Sun were reduced down into the size of State College, it would rotate once per second! This is what a neutron star does.

Because neutron stars are so small, their total thermal emission is negligible. However, neutron stars do produce light via synchrotron emission. This light does not shine in all directions, but only out the north and south magnetic poles of the star. As a result, the star acts light a rotating searchlight: when (if) the magnetic pole points at you, you see a flash of light. A neutron star whose flashing synchrotron emission is seen from earth is called a pulsar.

(It is somewhat interesting to note that most of a pulsar's synchrotron emission comes out in the radio portion of the spectrum. When the first pulsar was detected, astronomers didn't know what to make of it -- a radio signal coming from somewhere in the cosmos that was turning on and off every second, with a timing that was more accurate than any clock on earth. Therefore, the first name for these objects was LGM's -- Little Green Men".)

If the supernova remnant is less than about 3 solar masses, a neutron star will be the result. Neutron degeneracy will hold up the star, just as electron degeneracy holds up a white dwarf. Gravity wants to crush the neutrons out of existence, but neutrons are fairly solid, and push back. However, if the mass of the supernova remnant is greater than about 3 solar masses, the greater gravity will crush even the neutrons out of of existence. If this were to happen, the star would collapse further. In fact, if neutrons are crushed out of existence, then we know of no other force that can resist the crush of gravity. The star will, in theory, collapse all the way down to a point. It will be a black hole.

To understand what a black hole really is, we must first consider the subject of relativity. There are two flavors of relativity, special and general. Special relativity says that movement at a constant velocity in a straight line is equivalent to standing still. In other words, you can always say, "I'm standing still, it's the other guy who is moving."

From this premise, and the observed fact that the speed of light is always the same, it is straightforward to prove that nothing can go faster than the speed of light. No matter how fast you travel, you MUST see light traveling away from you at the speed of light. (After all, relativitity says that everything behaves as if you are standing still, and we know how light works when you're standing still.) So light must be traveling faster than you are. Light is the fastest thing there is, and nothing else travels as fast as light.

Similarly, it is straightforward to prove that time itself must change at high velocity. Suppose I build a rocket that is so large, that it takes one full second for light to travel from one end to the other. (In other words, the rocket ship is 300,000 kilometers long.) Now let's have the rocket ship move. Special relativity says that the rocket ship really isn't moving -- it is standing still, while the whole universe moves past it. So it still takes 1 second for light to travel from the back of the rocket ship to the front of the ship. On the other hand, an outsider observer will notice that in the time it takes light to go from the back to the front of the ship, the rocket will have moved. So, to the outside observer, light has traveled a bit further than 1 rocket ship length. Since the observer knows that it takes light 1 second to travel 1 rocket ship length, the observer measures the elapsed time for the event to be more than 1 second. He says that the clock in the rocket ship is running slow.

Special relativity states that there is no experiment which can tell whether you are undergoing straight-line, constant velocity motion, or you are standing still (and everyone else is moving). General relativity states that there is no experiment that can tell whether you are standing in a gravitational field, or undergoing a constant, straight line acceleration. This latter statement has interesting consequences for the behavior of space. This latter statement has interesting consequences for the behavior of space. In effect, is says that even the path of a light ray can get curved by a gravitational field.

Now lets apply these statements to black holes. On earth, (and any other body) if a ball is thrown up in the air, gravity tries to bring it down to earth. The faster you throw the ball, the higher the ball will go. If you throw a ball fast enough, gravity will not be strong enough to pull it back to earth. You will have given it escape velocity. The escape velocity, obviously depends on the amount of gravity, which depends on mass and the distance. If the earth were alot less massive, there would be less gravity, and the escape velocity would be lower; if you started out further from the earth (out in space), the escape velocity would be less. Conversely, if the earth were more massive, or were more compact (so that the radius of the earth were smaller), the escape velocity would be greater.

Now imagine a scenario where an object is so massive and/or so small that the escape velocity is greater than the speed of light. If light can't go fast enough to escape, then nothing else can. You have a black hole.

Obviously, a black hole's effect on you depends on where you are. If you are far from a black hole, its gravity will be like any other body. If the Moon were suddenly changed into a black hole, it would not affect the earth. However, the closer you get to a black hole, the greater the gravity, and the greater the escape velocity. Eventually, you reach the event horizon, beyond which even light cannot escape. Anything that crosses the event horizon of a black hole is lost from this universe forever.

There are many curious properties about black holes. One involves time. According to general relativity, intense gravity is equivalent to very rapid acceleration. This implies great speed, which means that time can slow down. Atoms near the event horizon of a black hole are affected by this time dilation. Emitted photons have lower frequencies, and therefore longer wavelengths. The light is gravitationally redshifted.

Similarly, consider a person at the event horizon of a black hole. Light travels away from that person at the speed of light. But light isn't going anywhere -- because the gravity is so intense, light isn't escaping from the hole at all. How can light be traveling at 300,000 km/s and not be going anywhere? The answer is simple: time must stop at the event horizon.

An alternative description of light bending is that that space-time itself is distorted by the presence of mass. (In other words, you can say that light always travels in a straight line through space, but that space itself is curved.) The amount of curvature depends on the strength of the gravitational field. Normally, space is only curved a small amount, but strong gravitational fields can create large warps in space. The limiting case is a black hole, which seals its section of space off from the rest of space. Can one travel through a black hole and come out someplace else? Perhaps. But one factor that usually ignored is tides. As one gets very close to a black hole (or a neutron star for that matter), the difference in the graviational pull from one side a person to another can be enormous. The tides can literally rip a person apart. (This theme is used again and again in various works of the science-fiction writer Larry Niven.) So, while you might possibly get to another universe (after an infinite amount of time has elapsed in this universe), you would be in many tiny pieces.