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Ch 10 & 11 (The Deaths of Stars)

Ch 10 & 11 (The Deaths of Stars) - Chapter 10 The...

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The Deaths of Stars Chapter 10
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We have seen how a star resists it’s own gravity by generating energy through nuclear fusion. The energy keeps it’s interior hot, and the resulting high pressure balances gravity and prevents the star from collapsing. A star, however, has limited fuel. When a star exhausts it’s fuel, gravity wins and the star dies. There are three final states of a star. Most stars (including the sun) will become a white dwarf. The Most massive stars will become either a neutron star or a black hole.
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The End of a Star’s Life When all the nuclear fuel in a star is used up, gravity will win over pressure and the star will die. High-mass stars will die first, in a gigantic explosion, called a supernova. Less massive stars will die in a less dramatic event, called a nova
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10-1 Giant Stars A star remains on the main-sequence for 90% of its total lifetime. When its hydrogen fuel is exhausted, it begins to fuse helium (if it is massive enough) and swells into a giant star. At this point it leaves the main sequence. However, it remains in the giant stage for only about 10% of its total lifetime. The giant star stage is the first step in the death of a star.
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Expansion into a Giant As the hydrogen in the core is used up, the core begins to contract and heat up. When this happens, hydrogen in a spherical shell just outside the core can get hot enough to begin fusing there. This spherical shell grows outward as the star ages. The heat from the burning spherical shell flows outward, forcing the star to expand and become a giant star. When a star becomes a giant, it’s position on an H-R diagram moves up and to the right.
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The helium core is being squeezed tighter and tighter. It contains helium nuclei and electrons at very high densities . Electrons confined to the core at extremely high densities can transform into an exotic state of matter called degenerate matter . Degenerate Matter
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This degenerate matter has unusual properties 1. Even though it is a gas, it has the consistency of hardened steel. 2. Changing the temperature of this gas has almost no effect on its pressure. White dwarfs are made out of degenerate matter.
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Helium Fusion A higher temperature is needed to fuse helium than to fuse hydrogen. When the core of a star finally reaches 100 million K, it begins to fuse helium. Stars less massive than about 0.4 solar masses never get hot enough to ignite helium. When helium fusion starts 1. The star is making energy in its core (helium fusion) and in a shell around its core (hydrogen fusion). 2. The point representing the star in an H-R diagram changes direction again (moves down & to the left).
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As Helium Fusion Proceeds Helium fusion produces carbon and oxygen atoms. These atoms require even higher temperatures to fuse.
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