Black holes are the densest, most massive singular objects in the universe. Formed in one of three main processes, they exert so much gravitational force that nothing - not even light - can escape their pull. Since nothing can ever come out, it is called a hole. Since not even light nor other electromagnetic radiation can escape, it is called a black hole.
Black holes come in several different sizes and types, which are discussed in the following sections. Black Hole Formation
Current theory holds that black holes form in three main ways. The first is that if a star has more than nine solar masses when it goes supernova, then it will collapse into a black hole. The reason that a neutron star stops collapsing is the strong nuclear force, the fundamental force that keeps the center of an atom from collapsing. However, once a star is this big, the gravitational force is so strong that it overwhelms the strong nuclear and collapses the atom completely. Now there is nothing to hold back collapse of the star, and it collapses into a point (or, in theory, a ring) of infinite density.
A second way for black holes to form is that, in some rare instances, two neutron stars will be locked in a binary relationship. Because of energy lost through gravitational radiation, they will slowly spiral in towards each other, and merge. When they merge, they will almost always form a black hole.
Finally, a third way was proposed by quantum cosmologist Stephen Hawking. He theorized that trillions of black holes were produced in the Big Bang, with some still existing today. This theory is not as widely accepted as the other two. Types and Sizes of Black Holes
A black hole is classified by the only three properties that it possesses: Mass, Spin, and Magnetic Field.
Currently, there are only two recognized mass classes of black hole: Stellar and Supermassive. The stellar black holes are star-sized and range in the 10-100 solar mass range. The supermassive black holes are at the cores of - what appear to be - every large galaxy, including our Milky Way. These range in the millions to even billions of solar masses.
Intermediate black holes are hotly debated. There has been no universally-accepted proof as to their existence, and many doubt there to be a reasonable mechanism by which they would form.
The simplest black hole has no spin and no magnetic field. This is called a Schwarzschild black hole. A black hole that has a field but no spin is called a Reissner-Nordstrøm black hole. One that has both a magnetic field and spin is called a Kerr black hole.
The differences are discussed in the next section - Black Hole Anatomy. Black Hole Anatomy
Schwarzschild Black Holes
To begin with the simplest type, a Schwarzschild black hole has two main components - a singularity and an event horizon. The singularity is what is left of the collapsed star, and is theoretically a point of 0 dimension with infinite density but finite mass. The event horizon is a region of space that is the "boundary" of the black hole. Within it, the escape velocity is faster than light, so it is past this point that nothing can escape. Reissner-Nordstrøm Black Holes
A step up is the Reissner-Nordstrøm black hole. It has the singularity and two event horizons. The outer event horizon is a boundary where time and space flip. This means that the singularity is no longer a point in space, but one in time. The inner event horizon flips space-time back to normal. Kerr Black Holes
A Kerr black hole adds another feature to the anatomy - an ergosphere. The ergosphere resides in an ellipsoidal region outside the outer event horizon. The ergosphere represents the last stable orbit, and the outer boundary is called the static limit. Outside of it, a hypothetical spaceship could maneuver freely. Inside, space-time is warped in such a way that a spaceship would be drawn along by its rotation.
An interesting point that comes up in the case of a spinning black hole is that of the naked singularity. The faster the black hole rotates, the larger the inner event horizon becomes, while the outer event horizon remains the same size. They become the same size when the rotational energy equals the mass energy of the black hole. If the rotational energy were to become more than the mass energy, the event horizons would vanish and what would be left is a "naked singularity" - a black hole whose only part is the singularity.
Yet another distinguishing feature of the Kerr black hole is that, since it rotates, the 0-D point that is the singularity in the Schwarzschild and Reissner-Nordstrøm black hole is spun into a ring of 0 thickness. Interesting theoretical physics can take place around this ring singularity. One consequence is that nothing can actually fall into it unless it approaches along a trajectory along the ring's side. Any other angle and the ring actually produces an antigravity field that repels matter.
NOTE: The only physical part of a black hole is the singularity. The other parts mentioned are mathematical boundaries. There is no physical barrier called an event horizon, but it marks the boundaries between types of space under the influences of the singularity.
Two other features can characterize a black hole - the accretion disk and jets.
An accretion disk is matter that is drawn to the black hole. In rotating black holes and/or ones with a magnetic field, the matter forms a disk due to the mechanical forces present. In a Schwarzschild black hole, the matter would be drawn in equally from all directions, and thus would form an omni-directional accretion cloud rather than disk.
The matter in accretion disks is gradually pulled into the black hole. As it gets closer, its speed increases, and it also gains energy. Accretion disks can be heated due to internal friction to temperatures as high as 3 billion K, and emit energetic radiation such as gamma rays. This radiation can be used to "weigh" the black hole. By using the doppler effect, astronomers can determine how fast the material is revolving around the black hole, and thus can infer its mass.
Jets form in Kerr black holes that have an accretion disk. The matter is funneled into a disk-shaped torus by the hole's spin and magnetic fields, but in the very narrow regions over the black hole's poles, matter can be energized to extremely high temperatures and speeds, escaping the black hole in the form of high-speed jets.
Finding Black Holes
No black hole has actually been imaged in a telescope. Actually, this is in itself impossible because, simply by definition, one cannot see "nothing." A black hole can only be spotted by observing how the material around it acts. Through this method, astronomers have observed many black holes; they usually are found in the center of galaxies, and some believe that every galaxy harbors a black hole in its center.
The rendering at the right depicts what a binary system with a black hole might look like, with it pulling matter off its companion star to form the accretion disk. NASA created this image.
Hypothetical Journey Through a Black Hole
What would happen if you were to fall into a black hole? As the you approach the black hole, your watch would begin to run slower than the watch of your colleagues on the spaceship. Also, your comrades notice that you begin to take on a reddish color. This is due to the warping of space in the vicinity of the hole. Then, just before you "enter" the hole (pass through the outer event horizon), your friends would see you apparently "frozen" there, just outside the event horizon and to them, your watch would have stopped (if they could observe it). They would never see you enter the hole, because at that distance from the singularity, an object must travel at the speed of light to maintain its distance. Thus your dim, red image would stay frozen in their eyes for as long as the hole exists.
However, from your vantage point, as you enter the black hole, nothing has changed. As you look "out" of the hole, the universe still looks relatively normal. However, you are drawn towards the singularity, and cannot escape its grasp. At this point, modern physics does not know what would happen. The most likely outcome is that you are compacted into a miniscule size upon the singularity.
However, you would not actually survive the fall into the hole. The immense warping of space around the hole would cause a spaghettification effect - you would be pulled apart because your feet (assuming they went feet first) would be far greater than the force on your head, and they you would be pulled as one pulls dough into a rope. This would be rather unpleasant, as well as fatal.
The idea of a white hole is the opposite of a black hole, and is entertained more in science fiction than in actual science journals. Some believe it is the "other side" of a black hole. It is theorized to spew matter and energy out. A flaw in this theory, as many scientists have noted, is that the matter ejected from the white hole would accumulate in the vicinity of the hole, and then collapse upon itself, forming a black hole.