BLACK HOLES

1. BLACK HOLES Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. Singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. Here it's no longer meaningful to speak of space and time, much less spacetime. Jumbled up at the singularity, space and time cease to exist as we know them.

2. Eventually, all possible nuclear fuel is used up by a star and the core collapses. How far it collapses, into what kind of object, and at what rate, is determined by the star's final mass and the remaining outward pressure that the burnt-up nuclear residue (largely iron) can muster. If the star is sufficiently massive or compressible, it may collapse to a black hole. If it is less massive or made of stiffer material, its fate is different: it may become a white dwarf or a neutron star.

3. White Dwarf When small stars (up to 8 times the size of the Sun) exhaust their nuclear fuel, they typically shed large amounts of matter, leaving a core that eventually cools and contracts gravitationally to about the size of the Earth. The result is a white dwarf: the more massive it is, the greater its inward gravitational pull, and the smaller it becomes

4. A teaspoonful of white dwarf material would weigh five-and-a-half tons or more in the Earth's gravity! Yet a white dwarf can contract no further; its electrons resist further compression by exerting an outward pressure that counteracts gravity. There are many white dwarfs in our galaxy, but most are too dim to be seen. One of the first to be discovered was Sirius B, the dense companion star to Sirius.

5. Sirius B has another claim to fame. This white dwarf star fueled a debate in the 1920s between leading astrophysicists Subrahmanyan Chandrasekhar and Sir Authur Eddington. At issue was the following question: How far can a star possibly collapse? And for a given mass, what will it collapse into?

6. Chandrasekhar derived a relation ship between the star's mass and its radius which sets an upper limit to the mass a white dwarf can have, beyond which it will collapse to a neutron star or, if sufficiently massive, to a black hole. Calculations put the "Chandrasekhar Limit" at 1.4 solar masses. Decades later Chandrasekhar's fundamental contributions were recognized when he won the 1983 Nobel Prize in Physics.

7. Neutron Stars More massive stars tend to burn hotter and faster. Once all the nuclear fuel has been exhausted, such stars quickly collapse, shedding much of their mass in dramatic explosions called supernovae The most recent event of this kind was observed in 1987 when a star weighing the equivalent of 20 suns blew up in a neighboring galaxy 160,000 light years away.

8. If after such an explosion, the remaining material is greater than 1.4 solar masses, it will contract into an unimaginably dense core made solely of neutrons. Neutron stars are so dense a teaspoonful would weigh 100 million tons! Eventually astronomers may discover the telltale signs of a neutron star exactly where the old star met its doom, though as yet none has been detected.

9. If the star's final mass exceeds much beyond 2 solar masses, there is no outward force that can resist gravity. The core continues to collapse to a critical size or circumference beyond which there is only one fate: to form a black hole. Black Hole at Cygnus X-1

10. Anatomy of a Black Hole By definition a black hole is a region where matter collapses to infinite density, and where, as a result, the curvature of spacetime is extreme. Moreover, the intense gravitational field of the black hole prevents any light or other electromagnetic radiation from escaping.

11. The Event Horizon Applying the Einstein Field Equations to collapsing stars, German astrophysicist Karl Schwarzschild deduced the critical radius for a given mass at which matter would collapse into an infinitely dense state known as a singularity. For a black hole whose mass equals 10 suns, this radius is about 30 kilometers or 19 miles, which translates into a critical circumference of 189 kilometers or 118 miles.

12. If you envision the simplest three-dimensional geometry for a black hole, that is a sphere (known as a Schwarzschild black hole), the black hole's surface is known as the event horizon. Behind this horizon, the inward pull of gravity is overwhelming and no information about the black hole's interior can escape to the outer universe.

13. Apparent versus Event Horizon As a doomed star reaches its critical circumference, an "apparent" event horizon forms suddenly. Why "apparent?" Because it separates light rays that are trapped inside a black hole from those that can move away from it. However, some light rays that are moving away at a given instant of time may find themselves trapped later if more matter or energy falls into the black hole, increasing its gravitational pull. The event horizon is traced out by "critical" light rays that will never escape or fall in.

14. Even before the star meets its final doom, the event horizon forms at the center, balloons out and breaks through the star's surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of "surface" or "skin" beyond which we can venture but cannot see. Imagine what happens as you approach the horizon, then cross the threshold.

15. The Limits of Physical Law Newton and Einstein may have looked at the universe very differently, but they would have agreed on one thing: all physical laws are inherently bound up with a coherent fabric of space and time. At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizarre realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity.

16. It's no surprise that throughout his life Einstein rejected the possibility of singularities. So disturbing were the implications that, by the late 1960s, physicists conjectured that the universe forbade "naked singularities." After all, if a singularity were "naked," it could alter the whole universe unpredictably. All singularities within the universe must therefore be "clothed."

17. http://archive.ncsa.uiuc.edu/Cyberia/Expo/information-pavilion.htmlhttp://archive.ncsa.uiuc.edu/Cyberia/Expo/information-pavilion.html