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	BLACK HOLES AND BEYOND

	Black holes began to come out of the theorists’ closet and into reality when astronomers started to get a handle on how stars die. The vast majority of stars in the universe have masses similar to or smaller than the Sun’s. After billions of years, a small star exhausts its nuclear fuel and “puffs off” its outer layers, forming a beautiful, gaseous envelope called a planetary nebula and leaving behind the star’s core as a white dwarf—a hot cinder with the mass of the Sun crushed to the size of Earth.


	Not all stars start out as small as the Sun, however, and they can end their lives in a far more spectacular fashion. A star that begins life with more than roughly 40 times the mass of the Sun eventually builds up a core of pure iron that can no longer generate energy to balance the inward pull of gravity. The core then collapses, sending a shock wave through the rest of the star that destroys it. The resulting supernova shines brilliantly, giving off as much light as 10 billion Suns. In 1939, physicists J. Robert Oppenheimer and Hartland Snyder showed that the final state of the star, which would weigh more than three Suns, would be a region of space-time cut off from the rest of the universe—a black hole, as physicist John Wheeler dubbed these dark objects 30 years later.


	Theory predicted that black holes exist, but few scientists actually believed it. And even if black holes did exist, how could anyone hope to see an object that gives off no light? In one of the great ironies of the universe, the region surrounding a black hole can glow brilliantly. The intense gravity in the vicinity of a black hole causes any matter that comes close—say gas drawn from a companion star—to get sucked in. As the matter swirls around the black hole, it moves faster and faster, and friction among the atoms heats the gas to millions of degrees. Anything that hot emits lots of X-rays, which can be seen from observatories in Earth’s orbit.

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