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Lecture 10

Lecture 10-Chapter 18-The Bizarre Stellar Graveyard


Department
Astronomy & Astrophysics
Course Code
AST201H1
Professor
Stefan Mochnacki
Lecture
10

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Chapter 18 ± The Bizarre Stellar Graveyard
White Dwarfs
x A white dwarf is the exposed core of a star that has died and shed its outer layers in a
planetary nebula.
x 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
x 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.
x The cores of low-mass stars never become hot enough to fuse helium, therefore
ends up as helium white dwarfs.
x Intermediate-mass stars progress to carbon burning, which leave behind white
dwarfs containing large amounts of oxygen or even heavier elements.
x 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.
x Degeneracy pressure supports the inert helium core of a low-mass red giant, so
this core is a white dwarf buried inside a star.
x 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.
x 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
x 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
x A white dwarf in a close binary system can gradually gain mass if its companion is
a main-sequence or giant star.
x 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.
x The small size and high density of a white dwarf make its surface gravity far
stronger than that of a protostar, which means its accretiRQGLVVRUELWDOVSHHGLV
faster.
x 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.
x $FFUHWLRQFDQSURYLGHD³GHDG´ZKLWHGZDUIZLWKDQHZHQHUJ\VRXUFH as long as
its companion keeps feeding matter into the accretion disk.
Novae
x The gas spilling onto an accreting white dwarf comes from the upper layers of its
companion star that is composed mostly of hydrogen.
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