Lecture 10-Chapter 18-The Bizarre Stellar Graveyard

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University of Toronto St. George
Astronomy & Astrophysics
Stefan Mochnacki

Chapter 18 The Bizarre Stellar Graveyard White Dwarfs N A white dwarf is the exposed core of a star that has died and shed its outer layers in a planetary nebula. N White dwarfs exist in a state of balance because the outward push of electron degeneracy pressure matches the inward crush of its gravity. White Dwarf Composition, Density, and Size N A star like the size of our Sun becomes carbon white dwarfs, because the Sun fuses helium into carbon in their final stage of life. N The cores of low-mass stars never become hot enough to fuse helium, therefore ends up as helium white dwarfs. N Intermediate-mass stars progress to carbon burning, which leave behind white dwarfs containing large amounts of oxygen or even heavier elements. N More massive white dwarfs are actually smaller in size than less massive ones, because its greater gravity can compress its matter to a much greater density. N Degeneracy pressure supports the inert helium core of a low-mass red giant, so this core is a white dwarf buried inside a star. N As the hydrogen-burning shell above it deposits more helium ash onto the degenerate core, the mass of the core increases, as it also contracts, increasing temperature and fusion rate. N As long as the core remains inert and fusion occurs in a shell overlying it, the luminosity of the red giant must steadily increase. The White Dwarf Limit N Because electrons cannot travel faster than the speed of light, no white dwarf can have a mass greater than 1.4 times the mass of the sun, since electron speeds are higher in more massive white dwarfs. A White Dwarf in a Close Binary System Accretion Disks N A white dwarf in a close binary system can gradually gain mass if its companion is a main-sequence or giant star. N Accretion disks form around white dwarfs when mass from the companion first spills to it for much the same reason that infalling gas creates protostellar disks around protostars. N The small size and high density of a white dwarf make its surface gravity far stronger than that of a protostar, which means its accreti43L8N847-L9,O8500L8 faster. N With higher orbital speed, more orbital energy can be turned into heat, making the disk around a white dwarf much hotter than the one around a protostar. N ..709L43.,3574;L0,0,ZKL90Z,71ZL9K,30Z0307J84:7.0 as long as its companion keeps feeding matter into the accretion disk. Novae N The gas spilling onto an accreting white dwarf comes from the upper layers of its companion star that is composed mostly of hydrogen. www.notesolution.com
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