Download 29 October: Dead Stars 3

White Dwarf Stars
The burnt-out cores of low mass,
post-main sequence stars
As cores contract, the density goes to
“astronomical” levels, matter acts in funny
ways
• Gas in this room, the “perfect gas law”
PV=nRT. Pressure depends on both
density and temperature
• Extremely dense, “degenerate” gas
PV=Kn. Pressure depends only on
density
• Demo
The structure of a star: a balance between
gravity and gas pressure
Self gravity
Gas pressure
Technical term: hydrostatic equilibrium
radius
Equations give radius of white dwarf as a
function of its mass
mass
What one might expect for how R
depends on M
What the solution really is for a white dwarf star
Main features to note about white dwarf
solution
• Note the size: objects with
masses like the sun, but
radii like the Earth
• The size becomes smaller
with increasing mass
• There is an upper limit (the
Chandrasekhar mass) to
the mass of a white dwarf
Point to emphasize: white dwarfs are real!
Many white dwarfs have been cataloged
http://www.astronomy.villanova.edu/WDCatalog/index.html
Note in particular 40 Eridani B, which is in
The sky now, and can be seen with small
telescopes
More information on 40 Eridani B
All Star Line Up.webarchive
What happens to
massive stars?
Cores are chemically differentiated
And too massive to be held up
By electron degeneracy force…
The cores collapse
Core collapse of a massive star has two
consequences
• Massive explosion
(1044 Joules)
• Production of a
neutron star
Formation of a neutron star from stellar
core
• As core collapses, matter becomes
compressed
• Electrons and protons forced together e+p >
n + nu (neutronization)
• Core of the becomes a neutron fluid
• Neutronization produces a burst of neutrinos
• Neutron fluid in core becomes degenerate
and rigid
The physics of a self-gravitating neutron
blob (neutron star)
• Radius versus
mass relation for
neutron star
• Notice size of
neutron star
• Masses extend
above
Chandrasekhar
limit
Theoretical prediction of the existence of
a neutron star
• The remnant after the explosion of a massive
star
• An object having the mass of the Sun (or
more) but in an object with the diameter of
Iowa City!
• An equivalent to the Chandrasekhar mass
(largest possible mass of a neutron star)
• Do they exist?