AST201H1 Lecture 10: Lecture 10-Chapter 18-The Bizarre Stellar Graveyard
4 Aug 2010
School
Department
Course
Professor
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 accretiRQGLVN¶VRUELWDOVSHHGLV
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 $FFUHWLRQFDQSURYLGHD³GHDG´ZKLWHGZDUIZLWKDQHZHQHUJ\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.
www.notesolution.com
24
AST201H1 Full Course Notes
Verified Note
24 documents
Document Summary
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. 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. 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.
