Chapter 13 The Bizarre Stellar Graveyard
13.1 White Dwarfs
What is a White Dwarf?
White Dwarf: exposed core of a star that has died and shed its outer layers
quite hot when forms
slowly cools with time
have masses of stars but radii of earth (very dim)
support by electron degeneracy pressure (packed electrons push outward)
White Dwarf Composition, Density and Size
- A teaspoon of white dwarf matter would weigh several tons
- White dwarf formed by stars like our son would leave behind carbon
- Low mass stars form white dwarfs with helium
- More massive white dwarfs are smaller in size
The White Dwarf Limit (Chandrasekhar Limit)
- No white dwarf can have mass greater than 1.4M sun
What can happen to a white dwarf in a close binary system?
- Can slowly gain mass
- In a close binary system, gas from a companion star can spill toward a white dwarf
forming an accretion disk (new energy source) around it
- a nova is caused by hydrogen fusion on the surface of a white dwarf in a binary star
- 100,000 times as bright as sun, shines for a few weeks, not as bright as a supernova
White dwarf super novae
- when it reaches 1.4M of the sun carbon is ignited and star explodes
- massive star supernova occurs when massive star explodes
- light of white dwarf supernova fade steadily
- if a white dwarf gains enough matter to exceed the 1.4 solar mass white dwarf limit, it
will explode in a super nova 13.2 Neutron Stars
What is a neutron star?
- neutron degeneracy pressure: same as electron pressure pushing outwards in white
dwarfs but with neutrons instead
- a neutron star is a ball of neutrons just a few km in radius but with a mass like the sun
- a neutron star could fit in your hometown but its gravity would quickly destroy our planet
- 300,000 times as massive as earth
How were neutron stars discovered?
- Neutron stars can spin rapidly and emit beams of radiation along their magnetic poles
which we detect as pulses of radiation if the beams sweep by earth
- Pulses come from the star which spins as a result of conservation of angular momentum
- Pulsar: is a rotating neutron star that beams radiation along its magnetic axis
- All pulsars are neutron stars but not all neutron stars are pulsars
What can happen to a neutron star in a close binary system?
- X-ray binaries: close binaries that contain a