<black.holes>

A black is an object whose gravitational pull is so great that nothing--not even light--can escape from it. At one time, black holes were entirely theoretical. But now their existence both in our own galaxy and elsewhere in the universe, has been confirmed. A black hole forms when a massive star dies and collapses under its own mass. Some black holes, billions of times more massive than the Sun, lie at the centers of distant galaxies. Others may have been forged in the first fiery moments of the universe.

Throw a ball into the air and gravity will bring it back down to Earth. But throw the ball fast enough--around 25,000 mph--and it will excape from the Earth's gravity into space. More massive objects than the Earth have stronger gravitational fields that demand higher escape velocites (EVs); the Sun's EV, for example, is about 100 times the Earth's.

Black holes are bodies so massive and dense that their theoretical EV is greater than the speed of light itself. This means that not even light can escape from such bodies, rendering them effectively invisible--hence the name.

A black hole is formed when a very large star--at least 30 times the mass of the Sun--dies and collapses under its own weight. It shrinks down to an infinitely dense point know as a singularity, at which point conventional mathematics can no longer cope. Around the singularity is an imaginary circle called the event horizon, beyond which all light from the object is turned back by the force of its own gravity. And since Einstein proved that gravity can not only capture light, but can also distort time and space, inside the event horizon the entire concept of space and time completely breaks down.

You could never see a person actually fall into a black hole: as they approached the event horizon, time would slow to thel point where it would take infinitely long to reach it! In the meantime, the black hole's gravitational pull on light would make them appear to fade away. As for the person, who knows? All we know for sure is that in a black hole, reality as we know ceases it exist.

A black hole is produced by a dying star. A star with a mass of about 10 - 20 times the mass of our Sun may produce a black hole at the end of its life. In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy to push outward. For most of a star's life, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight. For small stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully. This type of star is called the "white dwarf." When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely
collapses under its own weight.

To create a massive core a progenitor (ancestral) star would need to be 10 - 20 times more massive than our Sun. If the core is very massive (approximately 2.5 times more massive than the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole. Then the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is referred to as a singularity. When this happens, escape would require a velocity greater than the speed of light. No object can reach the speed of light. The distance from the black hole at which the escape velocity is just equal to the speed of light is called the event horizon. Anything, including light, that passes across the event horizon toward the black hole is forever trapped.

Where Mh is the mass of the black hole, G is Newtonís gravitational constant; 1,327 x 10^11 kilometers^3 per second^2 per solar mass, and c is the speed of light; 2.998 x 10^5 kilometers per second.

Where Mh is the mass of the black hole, G is Newtonís gravitational constant; 1,327 x 10^11 kilometers^3 per second^2 per solar mass, L is the distance from your head to the end of your abdomen, and C is the circumference of your orbit. FYI: In 1960 Alan Shepard experienced a 12 g force during re-entry of the Mercury spacecraft Freedom 7! Also Eli Beeding in 1958 was put in the Guinness Book of World Records for withstanding a force of 82.6 g for 0.04 seconds on a water-braked rocket sled!

Here's a table to show how much mass must be crammed into a certain volume as given be the Schwarzschild Radius: