chapter18bb

chapter18bb - Chapter 18 The Bizarre Stellar Graveyard What...

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Chapter 18 The Bizarre Stellar Graveyard
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What is a white dwarf?
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White Dwarfs White dwarfs are the remaining cores of dead stars Electron degeneracy pressure supports them against gravity
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White dwarfs cool off and grow dimmer with time
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Size of a White Dwarf White dwarfs with same mass as Sun are about same size as Earth Higher mass white dwarfs are smaller
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The White Dwarf Limit Quantum mechanics says that electrons must move faster as they are squeezed into a very small space • As a white dwarf’s mass approaches 1.4 M Sun , its electrons must move at nearly the speed of light Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4 M Sun , the white dwarf limit (or Chandrasekhar limit )
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What can happen to a white dwarf in a close binary system?
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Star that started with less mass gains mass from its companion Eventually the mass- losing star will become a white dwarf What happens next?
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Accretion Disks Mass falling toward a white dwarf from its close binary companion has some angular momentum The matter therefore orbits the white dwarf in an accretion disk
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Accretion Disks Friction between orbiting rings of matter in the disk transfers angular momentum outward and causes the disk to heat up and glow
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Nova The temperature of accreted matter eventually becomes hot enough for hydrogen fusion Fusion begins suddenly and explosively, causing a nova
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Nova The nova star system temporarily appears much brighter The explosion drives accreted matter out into space
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Two Types of Supernova Massive star supernova: Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing explosion White dwarf supernova: Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing total explosion
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One way to tell supernova types apart is with a light curve showing how luminosity changes with time
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Nova or Supernova? Supernovae are MUCH MUCH more luminous!!!
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chapter18bb - Chapter 18 The Bizarre Stellar Graveyard What...

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