Black holes are possibly the most famous of all the stellar phenomena. Probably because they are so mysterious and their truth eludes us. We will never be able to actually see a black hole with the naked eye, but through experiments we can be relatively sure that they do exist. The actual Definition of a black hole is kind of sketchy, but most people say that they are just a non-dimensional (or very small diameter) singularity (A singularity is "a point in space-time in which the space-time curvature becomes infinite").
(A Brief History of Time, 186) surrounded by a kind of membrane called its event horizon. The event horizon is simply the boundary of a black hole. More specifically, it is the sphere around the black hole inside which nothing can pass without being inevitably eaten by the hole. By any definition, a black hole is a point in space in which time and space are so distorted that they form a never-ending infinite loop.
This is caused by the immense gravitational effects resulting after a massive star has died. Not just any star can die and become a black hole. The mass of the star, before it starts its death cycle, has to be above a certain limit. This is called the Chandrasekhar limit. (It is named for Subrahmanyan Chandrasekhar; an Indian that worked closely with Sir Arthur Eddington, an expert on the subject of general relativity. ) "Chandrasekhar calculated that a cold star of more than about one and a half times the mass of the sun would not be able to support itself against its own gravity." (A Brief History of Time, 82) Therefore, anything larger would, more than likely, collapse into a black hole.
"Though we cannot 'see' a black hole itself (since not even light can escape the hole's gravitational field), we may see the hole's effects on nearby matter. For example, if gas from a nearby star were sucked towards the black hole, the intense gravitation al (sic) energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole." (How Can We See Black Holes? ) Also, there could be bulges in nearby stars that would indicate a very powerful nearby force. There is a more direct way to detect a black hole because "black holes ain't so black"; they may be in a sense... painted. (A Brief History of Time, 99) It's difficult to imagine anything emanating from a black hole because you already know of the immense gravitational forces that are in action around the hole.
But the matter doesn't really emanate from inside the black hole; it comes from the "empty" space around the event horizon. You would think of it as empty, but that would be impossible; it would violate the uncertainty principle (the more accurately you know one thing, the less accurately you can know another). The space could not be exactly empty "because then it would have both a precise value (zero) and a precise rate of change (also zero). There must be a certain minimum amount of uncertainty, or quantum fluctuations, in the value of the field. One can think of these fluctuations as pairs of particles of light or gravity that appear together at some time, move apart, and then come back together again and annihilate each other. These particles are virtual particles like the particles that carry the gravitational force of the sun: unlike real particles, they cannot be observed directly with a particle detector.
However, their indirect effects, such as small changes in the energy of electron orbits in atoms, can be measured and agree with the theoretical predictions to a remarkable degree of accuracy. The uncertainty principle also predicts that there will be similar virtual pairs of matter particles, such as electrons or quarks. In this case, however, one member of the pair will be a particle and the other an antiparticle (the antiparticles of light and gravity are the same as the particles)." (A Brief History of Time, 105-106) To sum the previous sentences up: Every particle has an antiparticle; the particle and antiparticle try to find each other and annihilate themselves. As I said before, the particle and antiparticle pairs find each other, come together, move apart, and then annihilate. "Move apart" needs to be emphasized because this is the part of the process that explains our painted holes. Sometimes when they move apart, the antiparticle gets caught inside the event horizon and the real particle escapes (The particle that escapes is always positive and the antiparticle is always negative because real particles are always positive).
These escaping particles look like they " re emitting from inside the black hole. And if particles look like they " re escaping from the hole, the hole has a "temperature"; although it is not much above absolute zero. Moving away from painted holes, I would now like to explain what would happen if you were unfortunate enough to fall into a black hole (if we can find a way to get there). The events that would comprise the fall would be different depending upon the size of the black hole. Just because you pass the event horizon doesn't mean that you automatically get stretched into infinity and die. In a very, very large black hole it would be hours before you would experience any extreme discomfort from the gravity.
But, all that time before you die is boring, so let's just skip to the singularity. When you start to experience the effects of the gravity drastically, you will stretch like a spaghetti noodle. That is because you expand vertically and contract horizontally. To imagine this, just think of a fleet of Kamikaze astronauts coming toward a point on the earth's surface (they are in a perfectly square formation). When they start to get close to the earth they have to get closer together for them to hit their target on the earth's surface. And the astronauts that are closer to the earth have a stronger gravitational force on them (caused by the earth) that makes them go faster than their counterparts in the back of the formation that are still in space.
Therefore, they expand vertically (outward from the earth in this case) and contract horizontally. Now if you think of the Kamikaze astronauts's pace ships as your atoms in your body, you can see why you would turn into a big heaping pile of Chef Boyar dee if you do have the tough luck to be sucked into a black hole.