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 Chandrasekhar limit

The Chandrasekhar limit for masses larger than 1.4 M sun, 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 M sun 1.8 M sun 1.2 M sun

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 M sun main sequence star and a 0.8 M sun 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 M sun ! 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 M sun will collapse! neutron degeneracy pressure could halt the collapse for more massive objects… No way! S. Chandrasekhar Sir Arthur Eddington Lev 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 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 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