Qualities of a Students

BLACK HOLE
Some regions of space exert such powerful gravity that they suck in any matter that comes too close. That matter—whether it is a comet, a planet, or a cloud of gas—is crushed to infinite density. The gravity is so intense that it tugs at time and space, slowing down time and stretching out space. Not even light can escape the fierce gravitational pull, so the regions remain black and invisible. The U.S. physicist John Wheeler named these dark, devouring voids black holes.
Black holes seem more like science fiction than reality. But since 1916, when Albert Einstein developed his general theory of relativity, scientists have known that black holes may exist, and in 1994 astronomers confirmed the existence of a black hole.
Einstein's theory suggested that if gravity could become strong enough, it would rob light of all its energy, trapping it in the same way in which it traps a planet's atmosphere. For gravity to be that strong, however, its source would have to be an extremely dense object, one with a very large mass compressed into a very small space. In 1916 the German astronomer Karl Schwarzschild calculated just how compressed a star would have to be for its gravity to trap light. According to Schwarzschild's calculations, a star the size of the Sun—864,950 miles (1,392,000 kilometers) in diameter—would have to shrink to less than 1.9 miles (3 kilometers) wide.
In 1939 the U.S. physicists J. Robert Oppenheimer and Hartland S. Snyder discovered that it is possible for stars much larger than the Sun to become this small. For most of their lives, stars remain a constant size because they contain a balance of forces: energy generated by nuclear reactions in the star's core expands the star outward, while the force of gravity pulls it in. Eventually, in billions of years, the star exhausts its nuclear fuel and collapses under its own weight. Oppenheimer and Snyder proved that if the star is more than 3.2 times as massive as the Sun, nothing can stop the...