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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 supported by electron degeneracy
pressure neutron star: core of a massive
star that has exploded in a supernova supported by neutron degeneracy
pressure
White Dwarfs
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
Chandrasekharlimit
The Chandrasekhar limit
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
3 Msun1.8 Msun
1.2 Msun
White dwarfs cool at constant radius
White dwarfs in close binary systems
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
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 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
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
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!
neutron degeneracy pressure could halt
the collapse for more massive
objects…
No way!
S. Chandrasekhar
Sir Arthur EddingtonLev Landau
Pulsars
Jocelyn Bell
Sorry Sir Eddington!
Giant Lighthouses
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
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
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
high energy radiation from the pulsar is destroying its companion star
X-ray binaries
X-ray pulses from Centaurus X-3
The End