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

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The Deaths of StarsChapter 10
We have seen how a star resists it’s own gravity bygenerating energy through nuclear fusion.The energykeeps it’s interior hot, and the resulting high pressurebalances gravity and prevents the star from collapsing.A star, however, has limited fuel.When a star exhaustsit’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 Mostmassive stars will become either a neutron star or a blackhole.
The End of a Star’s LifeWhen 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 giganticexplosion, called a supernova.Less massive stars willdie in a less dramaticevent, called a nova
10-1Giant StarsA star remains on the main-sequence for 90% of its totallifetime.When its hydrogen fuel is exhausted, it begins tofuse helium (if it is massive enough) and swells into a giantstar.At this point it leaves the main sequence.However, itremains in the giant stage for only about 10% of its totallifetime.The giant star stage is the first step in the deathof a star.
Expansion into a GiantAs the hydrogen in the core is used up, the corebegins to contract and heat up.When this happens,hydrogen in a spherical shell just outside the core canget hot enough to begin fusing there.This sphericalshell grows outward as the star ages.The heat from theburning spherical shell flows outward, forcing the starto expand and become a giant star.When a star becomes a giant, it’s position on an H-Rdiagram moves up and to the right.
The helium core is being squeezed tighter and tighter.Itcontains helium nuclei andelectrons at very highdensities.Electrons confined to the core at extremelyhigh densities can transform into an exotic state of mattercalleddegenerate matter.Degenerate Matter
This degenerate matter has unusual properties1. Even though it is a gas, it has the consistency ofhardened steel.2. Changing the temperature of this gas has almost noeffect on its pressure.White dwarfs are made out of degenerate matter.
Helium FusionA higher temperature is needed to fuse helium thanto fuse hydrogen.When the core of a star finallyreaches 100 million K, it begins to fuse helium.Starsless massive than about 0.4 solar masses never get hotenough to ignite helium.When helium fusion starts1. 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 diagramchanges direction again (moves down & to the left).
As Helium Fusion ProceedsHelium fusion produces carbon and oxygen atoms.These atoms require even higher temperatures to fuse.

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Term
Fall
Professor
susanbontems
Tags
Mass Transfer, Supernova, White dwarf

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