Lecture 26:
The Bizarre Stellar Graveyard:
White Dwarfs
and Neutron Stars
Stellar Corpses

White dwarf : inert core left after a
low-mass star has ceased nuclear
burning and ejected its outer
envelopes
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supported by electron degeneracy
pressure
neutron star: core of a massive star
that has exploded in a supernova
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supported by neutron degeneracy
pressure
White Dwarfs
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Most white dwarfs are mainly carbon.
Very low mass stars cannot fuse
helium and so leave behind their
helium cores
Intermediate mass stars may progress
beyond carbon burning but not all the
way to iron – they leave can leave cores
of oxygen or heavier elements
More massive wd are bigger
Mass-radius relation
radius
of earth
Chandrasekhar
limit
The Chandrasekhar limit
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for masses larger than 1.4 Msun,
electron degeneracy pressure cannot
support the mass because electrons
would have to move faster than the
speed of light
therefore it was predicted that white
dwarfs with masses larger than this
limit cannot exist
none are observed
Sirius A and B
1.8 Msun
1.2 Msun
3 Msun
White dwarfs cool
at constant radius
White dwarfs in
close binary systems
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if a white dwarf is close to another
star it can steal some of its mass
the mass forms an accretion disk
and accelerates due to conservation
of angular momentum
a new shell of fresh hydrogen can
then accumulate around the dead
white dwarf
the Algol paradox
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the star system Algol
contains a 3.7 Msun main
sequence star and a 0.8 Msun
subgiant.
paradox: the more massive
star should be more evolved
the sub-giant used to be
more massive and lost mass
to its companion
in the future, the process
may be reversed!
White dwarf Novae
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if the shell of hydrogen builds up to
10 million K then shell fusion
burning can begin –
the star flares up in a nova, as
bright as 100,000 suns for a few
weeks
winds blow off most of the new mass
new mass starts to accrete, and the
whole process repeats…
Nova remnant
White dwarf supernovae
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if the accreted mass causes the star
to exceed the Chandrasekher limit
then the carbon core starts to
collapse and heat up
because the core is degenerate, there
is no ‘safety valve’ and the
temperature increases in a runaway
process
the core explodes and produces a
supernova
SN Light Curves
Neutron stars
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created by collapse of the iron core in a
massive star
about 10 km across and 1 Msun!
escape velocity from the surface is
about half the speed of light
like a giant atomic nucleus held
together by gravity
Neutron star in our Galaxy
a little history…
white dwarfs more
massive than 1.4 Msun
will collapse!
S. Chandrasekhar
neutron degeneracy
pressure could halt
the collapse for
more massive
objects…
No way!
Sir Arthur Eddington
Lev Landau
Pulsars
Sorry
Sir Eddington!
Jocelyn Bell
Giant Lighthouses
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Neutron stars
should have very
strong magnetic
fields
these fields
produce jets along
the axis of the
magnetic field
the jets sweep
around the sky as
the star rotates
Pulsar in the
Crab Nebula
X-ray image
At the heart of the Crab
A fast-moving Pulsar
Neutron stars have
superconducting, superfluid cores
Pulsars lose energy to their
surroundings and slow down
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electrons moving in a magnetic field
emit radiation (synchrotron).
this energy loss causes the rotation
of the neutron star to slow down
over time
for example, the period of the Crab
pulsar increases by 3 x 10-8 seconds
per day
in general, old pulsars rotate slower
than young ones.
Neutron stars
in close binary systems
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if mass is stripped from a close
companion, it causes the rotation to
speed up (conservation of angular
momentum)
millisecond pulsars (which must
rotate 100-1000 times per second)
are believed to be made in this way
The Black Widow Pulsar
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high energy radiation from the pulsar is
destroying its companion star
X-ray binaries
X-ray pulses from Centaurus X-3
The End