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
Made from core of massive star that had
supernova
explosion
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
Graduate student, Jocelyn Bell discovered pulses in her radio data (1967)
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
Schwarzschild radius:
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
