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	Frequently Asked Questions Send to a friend  Printable version  En Español Keywords Lifecycle of Stars  Death of Stars  Nuclear Fusion  Neutron Stars  Black Holes  Resources Brightest stars Extrasolar planets Latest news about exoplanets FAQs about Stars and Nebulae Astronomy Gift Shop 2009 Sky Almanac McDonald Observatory logo cap What happens to a star when it runs out of fuel?  In the last moments of its life, a star destined to explode as a supernova has a structure like an onion, with layers of elements outside the iron core. As a star expends its supply of hydrogen fuel, the helium produced in hydrogen fusion builds up steadily in the star's core. Eventually, this region becomes so dense and hot itself that helium begins to fuse into carbon in a reaction called the "triple alpha process." This type of fusion is much more energetic than hydrogen burning and consequently lasts for a much shorter time. For low-mass stars like the Sun, this is the final stage in its energy production. When the helium fuel is expended, the star puffs away its outer atmosphere in a few, final bursts of activity before quietly cooling off as a white dwarf. More massive stars share a more dramatic fate. Deep in their cores, the greater pressure of the star's enormous mass creates temperatures sufficient to fuse heavier elements -- carbon, neon, oxygen, and silicon. Each of these successive stages of nuclear fusion lasts an even shorter period of time than the previous one, and the race to the finish is on. With the burning of silicon, iron collects at the star's core, and this stage cues the grand finale. Iron represents a losing game for fusion -- it takes more energy to fuse iron than can be extracted from the process. As the central source of energy dies out, the outer layers of the star come crashing onto the iron core, crushing it into an exotic object called a neutron star, so dense that a spoonful weighs more than 100 million tons. The gas rebounds off the core's incredibly stiff surface, and the resulting shock wave rips the star apart in a massive explosion -- a supernova. For stars more than 10 times the mass of the Sun, a slightly different fate awaits. The neutron star that forms at the star's core is incapable of supporting the weight of the collapsing material, and it compresses further still. With no physical means of stopping the collapse, the star's central region becomes a black hole -- an area of infinite density --with a gravitational presence so strong that not even light can escape its boundary. Other FAQs Select FAQ category Earth, Moon, and the Sun The Solar System Space Probes and Human Exploration Stars and Nebulae Astronomers and Observatories Galaxies and Cosmology Stargazing and Star Lore » FAQs about StarDate Online Stars and Nebulae Links StarDate's Black Hole Encyclopedia Stars and Constellations Virtual Trips to Black Holes and Neutron Stars
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