# Chapter 18  Stellar Death

After it runs out of fuel, what prevents a star from collapsing?

1.   electron degeneracy in white dwarfs whose mass is   < 1.4 MSun

2.   nuclear degeneracy in neutron stars whose mass is   < 3 MSun

3.   objects whose mass is > 3 MSun do collapse

The corpses of stars = white dwarfs, neutron stars & black holes

White Dwarfs:

Made of core of old star (i.e. could be helium, carbon, oxygen, etc. + electrons)
Example: SiriusB in Sirius A B visual binary
Dense!!          Ex: a W.D. with mass of the Sun has radius of the Earth
The greater the mass of the W.D.
the smaller the size
the greater the electron speeds (quantum mechanics)
=> the greater the electron degeneracy pressure countering gravity
Chandrasekhar Limit: 1.4 MSun:
even if electron speeds approach c, still not enough pressure to support mass
so star-core continues to collapse => neutron star or black hole

Why living with another star can be complicated:
White dwarf stars in close binary systems (Cataclysmic variables)
Nova: sketch, observation
too much mass => sudden burning of core (white dwarf supernova)

Neutron stars

Like a super nucleus made of neutrons, but held together by gravity not strong force
Incredibly dense
A neutron star with Sun's mass plus some would have 10 km diameter
Paperclip => mass of Mount Everest
Huge gravity
30% gravitational redshift

Pulsars

tidied plot
actual data
Another pulsar: Crab Nebula Pulsar in optical
Sometimes see double pulse pattern in various parts of spectrum
The model:
rotating, magnetic neutron star, emits light from its magnetic poles
acts like a lighthouse
Have seen pulsars inside SNRs
(ex: Cassiopeia A, IC 443, Crab)
conclude that pulsars are rotating neutron stars made by SN explosions
Spin Rates:
Crab pulsar spins 30 rotations/sec, so period = 1/30 = 0.033sec
Generally spin rates slow as pulsar ages
Pulsar can be spun-up by accretion: schematic, result = millisecond pulsars
Accreted H -> He -> flash burning: X-ray Bursters

Black Holes

A black hole will result if there is more than 3MSun left over after SN

Remember Einstein: spacetime is actually curved, especially near
very massive objects. So, light (and objects) take "bent paths" when
near massive objects       Example

A black hole's   curved spacetime

RS = 2GM/c2    about 3km per MSun

Near the event horizon:

Relativistic time dilation, gravitational redshift, large tidal forces

Evidence

Stellar mass BH  Cyg X-1

Supermassive galactic cores  M87

Gamma Ray Bursters

Possible explanations hypernova neutron star collisions