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Spendthrift Stars. The sun's period of tunneling must have been smalltime stuff, for the sun has remained an average, ordinary, well-adjusted star. This is probably just as well for the sun's tender planets. When a star gathers too much interstellar material, a spectacular fate awaits it. Its great mass forces it to burn up its hydrogen at an abnormal rate. It shines with a steel-blue light, perhaps 1,000 times more brilliant than the sun. Such spendthrift stars are called "supergiants." Like overly ambitious men who burn themselves out, they come to an early end. The hydrogen in a supergiant is consumed, says Hoyle, in some 500 million years, while a prudent star, e.g., the sun, makes its smaller portion last 50 billion years.
What happens when the hydrogen of the spendthrift star has all turned into helium? With no more energy being generated in its interior, says Hoyle, the great star begins to contract. Its matter, falling toward the center, makes the interior hotter & hotter. Simultaneously the whole mass, which has been revolving slowly as most stars do, begins to spin faster as it shrinks, just as a skater spins faster when he reduces his "effective diameter" by letting his outstretched arms fall to his sides. Eventually the star is spinning so fast that portions of it may fly off into space, exposing briefly the hot interior and causing one of those stellar flare-ups that astronomers call a "nova."
Sometimes the spinning, contracting star does not sputter its matter away. Sometimes it goes on contracting, spinning faster & faster, getting hotter & hotter. Calculations show, says Hoyle, that in its last days such a star must be a fearsome object indeed. It is smaller than the earth, but a cubic inch of material from near its center weighs about a billion tons. Its surface, emitting a blast of X rays, revolves at 100 million m.p.h.
When the temperature of the doomed star's interior approaches some 300 times that of the interior of the sun, vast numbers of free neutrons come suddenly into being. Nuclear reactions take place which form heavy elements (iron, uranium) out of the predominant helium. Such reactions absorb energy and so reduce suddenly the temperature of the star's interior. This is the end. The star collapses, releasing so much gravitational energy in a matter of minutes that much of its substance is blown away in a stupendous explosion. The outer layers fly off as incandescent gas, at millions of miles per hour. For a few days the detonating star shines brighter than all the ten billion stars in the galaxy put together. Soon the great flash dies down. All that remains of the spendthrift star is a faint "white dwarf"—its XA'/I Se' burned-out nucleus.
Where Planets Come From. Such a monster detonation is called a supernova.
Astronomers believe that in the "local " or Milky Way galaxy of ten billion-odd stars a supernova blows up each two or three nundred years.