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If the captured matter fails to ignite, however, the dwarf's mass increases until it approaches the point -- known as Chandrasekhar's limit, for University of Chicago Astronomer Subrahmanyan Chandrasekhar, who first characterized it -- at which its own gravity will overcome even the powerful repulsive force between electrons. When the dwarf's mass reaches about 1.4 times that of the sun (the exact figure depends on the star's makeup), the star suddenly begins to collapse again, heating up so violently that its core ignites in a sudden thermonuclear fire. The result: a supernova. "It takes half a second for the flame to cross the whole white dwarf," says Santa Cruz's Woosley. "So much energy is released that the entire star is disrupted. It blows itself to smithereens." Such an exploding star is known as a Type I supernova; historical accounts of the rate at which Brahe's and Kepler's supernovas dimmed suggest to modern astronomers that both were probably Type I.
Even if a star begins life with as much as eight times the mass of the sun, it has more than likely ejected so much matter from its outer layers in the course of evolving it ends up with a mass below Chandrasekhar's limit. Hence it will become a white dwarf and a candidate for either stable, long-term cooling or, if it has a close companion, nova- or supernova-hood. In fact, since a white dwarf has inevitably lost its outer, hydrogen-rich layers (no matter what its original size), the lack of detectable hydrogen in a supernova explosion typically identifies it as a Type I.
If the stellar mass exceeds eight times that of the sun, however, the star has a short, spectacular life, turning into a red supergiant and ending its life by exploding as a Type II supernova. Says Woosley: "Big stars burn the candle at both ends, and they go out in style." After only 7 million years of existence, according to Woosley, the fast-burning star has probably fused all its hydrogen into helium and begins to contract. The compression drives the temperature up to 180 million degrees Celsius, more than high enough to begin fusing helium atoms and releasing more energy. The star then expands again, remaining stable for about 600,000 years, until all the helium atoms have been fused into carbon and oxygen. Then, in successively shorter intervals and with ever higher temperatures, the star expands and contracts, its fires dying down, then blazing hotter, gradually fusing lighter elements into heavier ones, until in just one day, its silicon is fused into iron.
And that is the end of the line. The structure of iron atoms prevents them from being fused into a heavier element under those conditions. At this point the star resembles an iron-cored onion, with an outermost shell of hydrogen and nested inner shells of some 20 other elements, including silicon, sulfur, calcium, argon, chlorine, potassium, neon, magnesium, aluminum and phosphorus.