Jessica Horn: Chapter 13 Notes

Chapter 13 The Bizarre Stellar Graveyard
13.1 White Dwarfs

• White dwarfs are remaining coves of dead stars
• Electron degeneracy pressure supports them against gravity
• White dwarfs cool off and grow dimmer with time

Size of a White Dwarf

• Quantum mechanics say that electrons must move faster as they are squeezed into a very small space
• As a white dwarf's mass approaches 1.4MSun, its electrons must move at nearly the speed of light
• Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4M Sun, the white dwarf limit (aka Chandrasechar limit)
• A star that started with less mass gains mass from its companion
• Eventually, the mass-losing star will become a white dwarf

Accretion Disks

• Mass falling toward a white dwarf from its close binary companion has some angular momentum
• The matter therefore orbits white dwarf in an accretion disk
• Friction btwn orbiting rings of matter is disk transforms angular momentum...

Nova

• The temp of accreted matter eventually becomes hot enough for hydrogen fusion
• Fusion begins suddenly and explosively, causing a nova 
• The nova star system temporarily appears much brighter 
• The explosion drives accreted matter out into space

Two Types of Supernova

• Massive star supernova:Iron core of massive star reaches white dwarf limit and collapses into a neutron star, causing an explosion
• White Dwarf Supernova: Carbon fusion suddenly begins as white dwarf in close binary system reaches white dwarf limit, causing a total explosion
• One way to tell supernova types apart is with a light curve showing how luminosity changes with time

Nova or Supernova?

• Supernovae are much much more luminous than novae
• Nova: H to He fusion of a layer of accreted matter; white dwarf left intact

Supernova Types: Massive Star or White Dwarf

• Light curves differ
• Spectra differ (exploding white dwarfs don't have hydrogen absorption lines)

13.2 Neutron Stars

• Neutron Star: is the ball of neutrons left behind by a massive-star supernova
• The degeneracy pressure of neutrons supports a neutron star against gravity 
• Electron degeneracy pressure goes away bc electrons combine with protons, making neutrons and neutrinos 
• Neutrons collapse to the center, forming a neutron star
• A neutron star is about the same size as a small city

Pulsars

• A pulsar is a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis
• The radiation beams sweep through spacelike lighthouse beams as the neutron star rotates
• Pulsars spin fast bc the core's spin speeds up as it...
• Matter falling toward a neutron star forms an accretion disk...
• Accreting matter adds angular momentum to a neutron star, increasing its spin

X-Ray Burst

• Matter accreting onto a neutron star can eventually become hot enough for helium to fuse
• The sudden onset of fusion produces X-ray

13.3 Black Holes: Gravities Ultimate Victory

• Black Hole- an object whose gravity is so powerful that not even light can escape it
• Some massive star supernovae can make a black hole even if mass...
• Light would not be able to escape Earth's surface if you could shrink it to less than 1 cm

Surface of Black Hole

• "Surface" of black hole is the radius at which the escape velocity is the speed of light
• This spherical surface is known as the event horizon
• The radius of the event horizon is known as Schwarzchild radius
• A black hole's mass strongly warps space and time in the vicinity of the event horizon

No Escape

• Nothing can escape from within the even horizon bc nothing can go faster than light
• No escape means there is no more contact with something that falls in it. It increased the hole's mass, changes its spin or charge, but otherwise loses its identity

Singularity

• Beyond the neutron star limit, no known force can resist the crush of gravity
• As far as we know, gravity crushes all the matter into a single point known as a singularity
• If Sun shrank into a black hole, its gravity would be different only near the event horizon
• Black holes don't suck!
• Light waves take extra time to climb out of a deep hold in spacetime, leading to a gravitational redshift 
• Time passes more slowly near event horizon

Black Hole Verification

• Need to measure mass
• Uses orbital properties of companion
• Measure velocity and distance of orbiting gas
• It's a black hole if it's not a star and its mass exceed the neutron star limit

13.4 The Origin of Gamma-Ray Bursts

• Brief bursts of gamma rays coming from space were first detected in 1960s
• Observations from the 90s showed that many gamma-ray bursts were coming from distant galaxies
• They must be among the most powerful explosions in the universe-could be the formation of black holes
• Observations show that at least some gamma-ray bursts are produced by supernova explosions