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