Astronomy 1021 Chapter Notes - Chapter 14: Inverse-Square Law, Eta Carinae, Type Ia Supernova
29 May 2018
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Astronomy 1021
Chapter 14
White Dwarfs
•White dwarf- remnants of low-mass star; exposed core of a star that died and shed its outer layers
in a planetary nebula; hot when it first forms (b/c old core of a star) and glow brightly in X-ray,
cools over time
•Large mass but small radius (same size as Earth but mass of the Sun) = dim and very high
density; more massive white dwarfs are smaller (stacking pillow analogy)
•White dwarf remains stable b/c electron degeneracy pressure- electrons are as closely packed tog.
as possible, exert outward push to counterbalance inward crush of gravity
•Composition- reflects products of star’s final fusion stage- He (lowest mass stars), C (Sun), C/O
(intermediate); white dwarfs are too cool to fuse past Carbon
-Teaspoon of white dwarf matter would weigh several tons
•White Dwarf Limit- max mass is 1.4MSun, also called Chandrasekhar limit (it cannot resist
gravity and collapses)
White Dwarf in a Close Binary System
•Left to itself, white dwarf cools until it becomes a black dwarf, but diff. for white dwarf in a close
binary system
•Accretion Disks- White dwarf in close binary system will gain mass from companion giant/main-
sequence star
-Mass spills over from giant star to white dwarf, clump orbits faster and faster as it falls inward
towards white dwarf’s surface and forms a disk (Called accretion disk)
-Accretion disk compresses white dwarf, interacts with magnetic fields and rips material off from
magnetic fields (beams)
-Inner gas orbits faster than outer, causes disk to lose energy and settle on white dwarf; this gives
white dwarf new source of energy (gas in the disk is very hot b/c GPE converts to thermal energy);
hydrogen gas can also spill
•Novae- gravity compresses H gas that lands on white dwarf into a thin surface layer, increasing
the pressure and temp, when temp at the bottom of layer reaches 10 million K, hydrogen fusion
ignites
-Fusion starts in the H layer, causing white dwarf to shine as a nova for a few weeks, produces 100
000LSun
-Nova occurs when shell becomes hot enough for a burst of H fusion that drives gas outward into
space
-Ex. 2007 Nova in Scorpius, 1992 Nova in Cygnus- light from nova illuminated nebula which wasn’t
seen before, detonation blasted material in expanding shell
-Material that accreted on surface layer expands, creating a nova remnant; accretion resumes after
nova explosion; accretion continues as white dwarf keeps accreting more mass from companion;
accretion causes ‘recurring novae’ (10s to 1000s of years b/w bursts)
