Review: How does a star

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Supernovae and nucleosynthesis of elements gt Fe
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Death of low-mass star White Dwarf

• White dwarfs are the remaining cores once fusion
  stops
• Electron degeneracy pressure supports them
  against gravity
• Cool and grow dimmer over time

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A white dwarf can accrete mass from its companion
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Tychos supernova of 1572
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Expanding at 6 million mph
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Keplers supernova of 1609
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Supernovae outshine the whole galaxy!
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Two kinds of supernovae
Type I White dwarf supernova White dwarf near
1.4 Msun accretes matter from red giant
companion, causing supernova explosion Type II
Massive star supernova Massive star builds up
1.4 Msun core and collapses into a neutron star,
gravitational PE released in explosion
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light curve shows how luminosity changes with
time
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r process and s process elements

• Nuclear fusion in all stars only produces up to
  Fe-56
• Slow neutron capture (s process) forms up to
  Bi-209 in low-mass stars
• High temps in SN creates elements up to Ca-254
• Rapid neutron capture (r process) create
  neutron-rich isotopes which decay into more
  stable neutron-rich elements
• Neutron flux during SN is 1022 neutrons per
  square centimeter per second
• neutron captures occur much faster than decays

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A neutron star A few km in diameter, supported
against gravity by degeneracy pressure of neutrons
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Discovery of Neutron Stars

• Using a radio telescope in 1967, Jocelyn Bell
  discovered very rapid pulses of radio emission
  coming from a single point on the sky
• The pulses were coming from a spinning neutron
  stara pulsar

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Pulsar at center of Crab Nebula pulses 30 times
per second
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Pulsars
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Thought Question

• Could there be neutron stars that appear as
  pulsars to other civilizations but not to us?
• A. Yes
• B. No

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Thought Question

• Could there be neutron stars that appear as
  pulsars to other civilizations but not to us?
• A. Yes
• B. No

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What happens if the neutron star has more mass
than can be supported by neutron degeneracy
pressure?

1. It will collapse further and become a black hole
2. It will spin even faster, and fling material out
   into space
3. Neutron degeneracy pressure can never be overcome
   by gravity

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• Neutron degeneracy pressure can no longer support
  a neutron star against gravity if its mass is gt
  about 3 Msun

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18.3 Black Holes Gravitys Ultimate Victory
A black hole is an object whose gravity is so
powerful that not even light can escape it.
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Escape Velocity
Final Gravitational Potential Energy
Initial Kinetic Energy

Where m is your mass, M is the mass of the
object that you are trying to escape from, and r
is your distance from that object
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Light would not be able to escape Earths surface
if you could shrink it to lt 1 cm
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Surface of a Black Hole

• The surface of a black hole is the distance at
  which the escape velocity equals the speed of
  light.
• This spherical surface event horizon.
• The radius of the event horizon is known as the
  Schwarzschild radius.

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Neutron star
The event horizon of a 3 MSun black hole is a
few km
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A black holes mass strongly warps space and time
in vicinity of event horizon
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What would it be like to visit a black hole?
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Light waves take extra time to climb out of a
deep hole in spacetime, leading to a
gravitational redshift
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Time passes more slowly near the event horizon
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Thought Question

• Is it easy or hard to fall into a black hole?
• A. Easy
• B. Hard

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Tidal forces near the event horizon of a 3 MSun
black hole would be lethal to humans Tidal
forces would be gentler near a supermassive black
hole because its radius is much bigger
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Do black holes really exist?
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Black Hole Verification

• Need to measure mass
• Use orbital properties of companion
• Measure velocity and distance of orbiting gas
• Its a black hole if its not a star and its mass
  exceeds the neutron star limit (3 MSun)

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Some X-ray binaries contain compact objects of
mass exceeding 3 MSun which are likely to be
black holes
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One famous X-ray binary with a likely black hole
is in the constellation Cygnus
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Thought Question

• How does the radius of the event horizon change
  when you add mass to a black hole?
• A. Increases
• B. Decreases
• C. Stays the same

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Thought Question

• How does the radius of the event horizon change
  when you add mass to a black hole?
• A. Increases
• B. Decreases
• C. Stays the same

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Thought Question

• Is it easy or hard to fall into a black hole?
• A. Easy
• B. Hard

Hint A black hole with the same mass as the Sun
wouldnt be much bigger than a college campus
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Thought Question

• Is it easy or hard to fall into a black hole?
• A. Easy
• B. Hard

Hint A black hole with the same mass as the Sun
wouldnt be much bigger than a college campus
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If the Sun shrank into a black hole, its gravity
would be different only near the event horizon
Black holes dont suck!
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Event horizon is larger for black holes of larger
mass Black holes have only mass, spin and charge