outer layer and collapse around its carbon core, turning into a “white dwarf” about the size of the Earth. It will move across the main sequence of the H-R diagram toward the bottom left of the diagram. It will become smaller and less bright but with a high surface temperature. Over billions of years, it will cool until eventually any surviving astronomers will start calling it a “black dwarf.” Smaller stars than the Sun will burn much longer. When they run out of fuel, they may simply collapse into a white If you’re wearing a gold dwarf, or they may brieÀy burn helium and ring, it was forged in a then collapse into a white dwarf. supernova and is on very The death of really large stars is much temporary loan to you! more spectacular. When they run out of hydrogen they, too, become unstable. They start burning helium, and when that’s gone, temperatures will still be high enough for them to burn their way through carbon and a number of other elements including oxygen and silicon. Eventually, they start producing iron (which has 26 protons in its nucleus), and the temperature at their core reaches 4 billion degrees Celsius. Cesare Emiliani describes this dramatic phase in the death of a large star: A star 25 times more massive than the Sun will exhaust the hydrogen in its core in a few million years, will burn helium for half a million years, and—as the core continues to contract and the temperature continues to rise—will burn carbon for 600 years, oxygen for 6 months, and silicon for 1 day. (Emiliani, The Scienti¿c Companion, p. 61) When its iron core shuts down, the star will collapse catastrophically in just a second, before exploding in a “supernova.” Most of its mass will be hurled into space. Its core will collapse into a “neutron star,” a ball of matter so dense that all its atoms are crushed into their nucleus. If the supernova is large enough the collapse will go even further, to form a “black hole.” A black hole is an object so dense that not even light can escape its gravitational pull. In the extreme temperatures of a supernova, the remaining elements of the periodic table can be manufactured in just a few seconds, all the way
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