Characteristics

Definition: A Black hole is an area in space in which the pull of gravity is so great that not even light can escape it once it enters. Since the escape velocity from a black hole is greater than light, escaping it is impossible.

Event Horizon is the area surrounding a black hole where the escape velocity is equal to the speed of light. Therefore we generally represent this as:

Event Horizon = c = (3.0E^10)

The event horizon can be thought of as a massive sphere. In fact, you can think of this sphere as the "surface" of the black hole, although the black hole's mass all lies well within this "surface". In fact, the distance from Singularity to the event horizon is the Schwarzschild Radius. (http://www.aspsky.org/education/tnl/24/24.html, 4/15/02)

Schwarzschild Radius is the distance from the center of a black hole to its event horizon. Before Russian physicist Karl Schwarzschild calculated this distance, the boundaries of black holes were a mystery. His calculations were spurred by Einstein's theory of general relativity, which induced the ability to calculate gravity at high speeds and large masses. The equation for calculating the Schwarzschild radius is:

Where M is the mass of the black hole, G is the universal gravitational constant, and c is the speed of light. The "radius" is essentially the distance from the center of the black hole, singularity, to the edge of the black hole, the event horizon.

Singularity is a located at the direct center of a black hole. It is also a place of infinite space time curvature. This point exists due to the star's entire density being crushed to a single point having, then infinite density.

Escape Velocity: The escape velocity is the speed at which one must travel to escape the gravitational pull of a given field. With black holes, the escape velocity is always greater than the speed of light. Because nothing can travel faster than the speed of light, and no signals can leave the black hole, the only indication of its presence is gravity 3. Though nothing can escape a black hole, we can calculate the rate that it would need to have.

Where m equals the mass of the black hole, G equals the universal constant, and r is the Schwarzschild radius. The following table lists the Schwartzschild radii and escape velocities of several objects 3.

Three properties of Black Holes:
1. Mass
2. Spin
3. Electrical Charge

A black hole must have at least 3 stellar masses, otherwise it could not form on its own.

How a Black Hole can be detected: Since no light escapes from them, black holes obviously cannot be seen. However, they do emit X-rays because all matter that is being pulled into them by their gravitational forces will be charged by heating and compression, which causes the emission of X-rays.

Where to find them: many are thought to be at the center of galaxies, such as our own

Mini Black holes: These are black holes that did not have the mass to form on their own power, but Steven Hawking theorized that these types of black holes may exist by virtue of the fact that when the universe was formed, gravitational forces and pressures may have forced a mass to collapse on itself, thus forming a black hole that is smaller than normal.

Super Massive Black Holes: These are black holes that have masses a few million times that of the sun or more and are speculated to be the power behind the nuclear activities that form galaxies.

White Holes: This is another theory that hasn't been proven, but can be correctly solved by the Schwarzschild equation: A black hole might actually emit light or particles from its event horizon, perhaps into another universe.

Wormholes: This is the linkage of a white hole and a black hole, such that what falls into a black hole would be emitted by a white hole, thus allowing "travel" between the two.

( http://antwrp.gsfc.nasa.gov/apod/ap020110.html)

The Chandra Observatory took this picture of the Milky Way. In it you can see white dwarfs, neutron stars and black holes. The white area in the center supposedly contains the supermassive black hole that is thought to be at the center of our Milky Way galaxy.

The Evaporation of Black Holes:
Stephen Hawking has postulated a theory that states that Black Holes actually evaporate over time. This is because energy is used from the radiation and mass is slowly decreased. As mass decreases however, the radiation emitted increases, and the hole eventually vanishes. No one is really sure what happens after that. Some scientists believe that a small bit of the hole remains as a stable remnant.
(http://cosmology.berkeley.edu/Education/BHfaq.html#q3, 4/20/02)

What if I fell into a black hole?
If you fell into a black hole, on approach, you'd feel weightless. However, once you cross the event horizon, it wouldn't take long for the tidal forces of the black hole to start pulling at your feet then the rest of you, until you're ripped apart, within the matter of seconds.

However, if a friend in a space shuttle were sitting behind you and watching you fall into that hole, all the while at a safe distance, that person would forever see you falling towards the hole, but never cross the event horizon.

Why? Because the closer you get to it, the longer and longer it takes light to get back from where you are, as the gravitation affects light itself. So that person will never see you cross the event horizon because as you approach it, the light is slower in getting back to her, and once you cross it, light can't escape anyways and you can't be seen again.
(http://cosmology.berkeley.edu/Education/BHfaq.html#q3, 4/20/02)

Cited:

Freedman, Roger A. and William J. Kauffman. Universe. W.H Freedman and Company. New York. 2002.
Rees, Martin. "The Edges of Space and Time, (Gravitational forces of black holes)". Astronomy. July 1998, pg 48.
Guidry, Michael. "Escape Velocities of Light". University of Kentucky.
http://www.aspsky.org/education/tnl/24/24.html, 4/15/02
http://cosmology.berkeley.edu/Education/BHfaq.html#q3, 4/20/02
http://chandra.as.utexas.edu/~kormendy/bhsearch.html, 4/20/02
http://www.geocities.com/autotheist/Physics/bh.htm, 4/20/02