Chapter 13 The Bizarre Stellar Graveyard

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Chapter 13The Bizarre Stellar Graveyard
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13.1 White Dwarfs

• Our Goals for Learning
• What is a white dwarf?
• What can happen to a white dwarf in a close
  binary system?

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What is a white dwarf?
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White dwarfs are the remaining cores of dead
stars Electron degeneracy pressure supports them
against gravity
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White dwarfs cool off and grow dimmer with time
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A white dwarf is about the same size as Earth
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White dwarfs shrink when you add mass to them
because their gravity gets stronger
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Shrinkage of White Dwarfs

• Quantum mechanics says that electrons in the same
  place cannot be in the same state
• Adding mass to a white dwarf increases its
  gravity, forcing electrons into a smaller space
• In order to avoid being in the same state some of
  the electrons need to move faster
• Is there a limit to how much you can shrink a
  white dwarf?

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The White Dwarf Limit Einsteins theory of
relativity says that nothing can move faster than
light When electron speeds in white dwarf
approach speed of light, electron degeneracy
pressure can no longer support it Chandrasekhar
found (at age 20!) that this happens when a white
dwarfs mass reaches 1.4 MSun
S. Chandrasekhar
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What can happen to a white dwarf in a close
binary system?
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Star that started with less mass gains mass from
its companion Eventually the mass-losing star
will become a white dwarf What happens next?
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White dwarfs gravity pulls matter off of giant
companion, but angular momentum prevents the
matter from falling straight in Infalling matter
forms an accretion disk around the white dwarf
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Friction in disk makes it hot, causing it to
glow Friction also removes angular momentum from
inner regions of disk, allowing them to sink onto
white dwarf
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Thought Question

• What would gas in disk do if there were no
  friction?
• A. It would orbit indefinitely.
• B. It would eventually fall in.
• C. It would blow away.

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Thought Question

• What would gas in disk do if there were no
  friction?
• A. It would orbit indefinitely.
• B. It would eventually fall in.
• C. It would blow away.

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Hydrogen that accretes onto a neutron star builds
up in a shell on the surface When base of
shell gets hot enough, hydrogen fusion suddenly
begins leading to a nova
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Nova explosion generates a burst of light lasting
a few weeks and expels much of the accreted gas
into space
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Thought Question

• What happens to a white dwarf when it accretes
  enough matter to reach the 1.4 MSun limit?
• A. It explodes
• B. It collapses into a neutron star
• C. It gradually begins fusing carbon in its
  core

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Thought Question

• What happens to a white dwarf when it accretes
  enough matter to reach the 1.4 MSun limit?
• A. It explodes
• B. It collapses into a neutron star
• C. It gradually begins fusing carbon in its
  core

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

• Supernovae are MUCH MUCH more luminous!!! (about
  10 million times)
• Nova H to He fusion of a layer, white dwarf left
  intact
• Supernova complete explosion of white dwarf,
  nothing left behind

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Supernova Type Massive Star or White Dwarf?

• Light curves differ
• Spectra differ (exploding white dwarfs dont have
  hydrogen absorption lines)

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What have we learned?

• What is a white dwarf?
• A white dwarf is the core left over from a
  low-mass star, supported against the crush of
  gravity by electron degeneracy pressure.
• What can happen to a white dwarf in a close
  binary system?
• A white dwarf in a close binary system can
  acquire hydrogen from its companion through an
  accretion disk. As hydrogen builds up on the
  white dwarfs surface, it may ignite with nuclear
  fusion to make a nova.

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13.2 Neutron Stars

• Our Goals for Learning
• What is a neutron star?
• How were neutron stars discovered?
• What can happen to a neutron star in a close
  binary system?

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What is a neutron star?
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A neutron star is the ball of neutrons left
behind by a massive-star supernova Degeneracy
pressure of neutrons supports a neutron star
against gravity
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Electron degeneracy pressure goes away because
electrons combine with protons, making neutrons
and neutrinos Neutrons collapse to the center,
forming a neutron star
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A neutron star is about the same size as a small
city
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How were neutron stars discovered?
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• The first neutron star was discovered by Bell
  Burnell in 1967
• Using a radio telescope she noticed very regular
  pulses of radio emission coming from a single
  part of the sky

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Pulsars are neutron stars that give off very
regular pulses of radiation
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Pulsar at center of Crab Nebula pulses 30 times
per second
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Pulsars are rotating neutron stars that act like
lighthouses Beams of radiation coming from
poles look like pulses as they sweep by Earth
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A pulsars rotation is not aligned with magnetic
poles
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X-rays
Visible light
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Why Pulsars must be Neutron Stars
Circumference of NS 2p (radius) 60
km Spin Rate of Fast Pulsars 1000 cycles per
second Surface Rotation Velocity 60,000
km/s 20 speed of light
escape velocity from NS
Anything else would be torn to pieces!
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Pulsars spin fast because cores spin speeds up
as it collapses into neutron star Conservation
of angular momentum
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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 to a neutron star in a close binary
system?
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Matter falling toward a neutron star forms an
accretion disk, just as in a white-dwarf binary
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Accreting matter adds angular momentum to a
neutron star, increasing its spin Episodes of
fusion on the surface lead to X-ray bursts
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Thought Question

• According to conservation of angular momentum,
  what would happen if a star orbiting in a
  direction opposite the neutrons star rotation
  fell onto a neutron star?
• The neutron stars rotation would speed up.
• The neutron stars rotation would slow down.
• Nothing, the directions would cancel each other
  out.

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Thought Question

• According to conservation of angular momentum,
  what would happen if a star orbiting in a
  direction opposite the neutrons star rotation
  fell onto a neutron star?
• The neutron stars rotation would speed up.
• The neutron stars rotation would slow down.
• Nothing, the directions would cancel each other
  out.

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Thought Question

• If you dropped a mountain onto the surface of a
  neutron star, what would happen?
• It would heat up (conservation of energy)
• It would flatten out (gravitational compression)
• It would bounce off.
• It would turn into pure neutrons and emit
  neutrinos.

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Thought Question

• If you dropped a mountain onto the surface of a
  neutron star, what would happen?
• It would heat up (conservation of energy)
• It would flatten out (gravitational compression)
• It would bounce off.
• It would turn into pure neutrons and emit
  neutrinos.

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What have we learned?

• What is a neutron star?
• A neutron star is the ball of neutrons created by
  the collapse of the iron core in a massive star
  supernova.
• How were neutron stars discovered?
• Neutron stars spin rapidly when they are born,
  and their strong magnetic fields can direct beams
  of radiation that sweep through space as the
  neutron star spins. We see such neutron stars as
  pulsars, and these pulsars provided the first
  direct evidence for the existence of neutron
  stars.

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What have we learned?

• What can happen to a neutron star in a close
  binary system?
• Neutron stars in close binary systems can accrete
  hydrogen from their companions, forming dense,
  hot accretion disks. The hot gas emits strongly
  in X rays, so we see these systems as X-ray
  binaries. In some of these systems, frequent
  bursts of helium fusion ignite on the neutron
  stars surface, emitting X-ray bursts.

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13.3 Black Holes Gravitys Ultimate Victory

• Our Goals for Learning
• What is a black hole?
• What would it be like to visit a black hole?
• Do black holes really exist?

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What is a black hole?
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A black hole is an object whose gravity is so
powerful that not even light can escape it.
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Thought Question

• What happens to the escape velocity from an
  object if you shrink it?
• A. It increases
• B. It decreases
• C. It stays the same

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Thought Question

• What happens to the escape velocity from an
  object if you shrink it?
• A. It increases
• B. It decreases
• C. It stays the same
• Hint

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Thought Question

• What happens to the escape velocity from an
  object if you shrink it?
• A. It increases
• B. It decreases
• C. It stays the same
• Hint

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Escape Velocity
Initial Kinetic Energy
Final Gravitational Potential Energy

(escape velocity)2 G x (mass)

2 (radius)
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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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The surface of a black hole is the radius at
which the escape velocity equals the speed of
light. This spherical surface is known as the
event horizon. The radius of the event horizon
is known as the Schwarzschild radius.
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A black holes mass strongly warps space and time
in vicinity of event horizon
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No Escape
Nothing can escape from within the event horizon
because nothing can go faster than light. No
escape means there is no more contact with
something that falls in. It increases the hole
mass, changes the spin or charge, but otherwise
loses its identity.
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Neutron Star Limit

• Quantum mechanics says that neutrons in the same
  place cannot be in the same state
• Neutron degeneracy pressure can no longer support
  a neutron star against gravity if its mass
  exceeds about 3 Msun
• Some massive star supernovae can make black hole
  if enough mass falls onto core

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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.
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Neutron star
3 MSun Black Hole
The event horizon of a 3 MSun black hole is also
about as big as a small city
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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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What would it be like to visit a black hole?
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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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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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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?
• 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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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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What have we learned?

• What is a black hole?
• A black hole is a place where gravity has crushed
  matter into oblivion, creating a true hole in the
  universe from which nothing can ever escape, not
  even light.

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What have we learned?

• What would it be like to visit a black hole?
• You could orbit a black hole just like any other
  object of the same mass. However, youd see
  strange effects for an object falling toward the
  black hole
• Time would seem to run slowly for the object
• Its light would be increasingly redshifted as it
  approached the black hole.
• The object would never quite reach the event
  horizon, but it would soon disappear from view as
  its light became so redshifted that no instrument
  could detect it.

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What have we learned?

• Do black holes really exist?
• No known force can stop the collapse of a stellar
  corpse with a mass above the neutron star limit
  of 2 to 3 solar masses, and theoretical studies
  of supernovae suggest that such objects should
  sometimes form. Observational evidence supports
  this idea.

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13.4 The Mystery of Gamma-Ray Bursts

• Our Goals for Learning
• What causes gamma ray bursts?

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What causes gamma ray bursts?
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Gamma ray bursts may signal the births of new
black holes
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At least some gamma ray bursts come from
supernovae in very distant galaxies
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What have we learned?

• What causes gamma ray bursts?
• Gamma-ray bursts occur in distant galaxies and
  are the most powerful bursts of energy we observe
  anywhere in the universe. No one knows their
  precise cause, although at least some appear to
  come from unusually powerful supernovae.