01:750:109 Chapter Notes - Chapter 10.1: Starflight, Triple-Alpha Process, Planetary Nebula
White Dwarfs and Neutron Stars
●Low-mass stars like the Sun leave behind white dwarfs when they die
●High-mass stars die in titanic explosions known as supernovae, leaving behind neutron stars or
black holes
What are White dwarfs?
●A white dwarf
is the exposed core of a low-mass star that has died and shed its outer layers in a
planetary nebula
○Hot when it first forms bc it was recently inside of a star
○Cools with time
●Are stellar in mass but small in size (radius) which is why they are dim compared to stars like the
Sun
●BUT!! Hottest white dwarfs can shine brightly in high-energy ultraviolet and X-ray light
●White dwarf's star-like mass and small size makes gravity very strong near its surface
●Exists in a state of balance with the outward push of electron degeneracy pressure
to match the
inward push of gravity
○Electrons packed closely together as nature allows
White Dwarf Composition, Density, and Size
●Composition reflects the products of the star's final fusion stage
○White dwarf left behind by a 1MSun star consists mostly of carbon, since stars like the
Sun fuse helium into carbon in their final stage of life
●Typical white dwarf has a mass similar to that of the Sun compressed into a volume the size of
Earth
○Density of a white dwarf so high that a teaspoon of its material would weigh as much as
a small truck
●More massive white dwarfs are smaller in size than less massive ones
○Ex: a 1.3 Msun white dwarf is half the diameter of a 1.0 Msun white dwarf
○More massive white dwarf is smaller bc its stronger gravity compresses its matter to a
greater density
○The most massive white dwarfs are therefore the smallest
The White Dwarf Limit
●Electron speeds are higher in more massive white dwarfs
○Leads to a fundamental limit on the maximum mass of a white dwarf
○Theoretical calculations show that electron speeds would reach the speed of light in a
white dwarf with a mass about 1.4 times the mass of the Sun
●Bc neither electrons nor anything else can travel faster than the speed of light, no white dwarf
can have a mass greater than this 1.4 MSun white dwarf limit (also called the Chandrasekhar
limit)
White Dwarfs and Accretion
●Left to itself, a white dwarf will never shine as brightly as the star it once was
○No source of fuel for fusion
Document Summary
Low-mass stars like the sun leave behind white dwarfs when they die. High-mass stars die in titanic explosions known as supernovae, leaving behind neutron stars or black holes. A white dwarf is the exposed core of a low-mass star that has died and shed its outer layers in a planetary nebula. Hot when it first forms bc it was recently inside of a star. Are stellar in mass but small in size (radius) which is why they are dim compared to stars like the. Hottest white dwarfs can shine brightly in high-energy ultraviolet and x-ray light. White dwarf"s star-like mass and small size makes gravity very strong near its surface. Exists in a state of balance with the outward push of electron degeneracy pressure to match the inward push of gravity. Electrons packed closely together as nature allows. Composition reflects the products of the star"s final fusion stage.
