Lecture 35 Stellar Remnants

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Lecture 35Stellar Remnants
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Neutron Stars

• Supernova remnant
• Tightly packed neutron core.
• Size 20 km (small asteroid or city)
• Mass 1.4-3 M?
• Density very high
• 1 tsp. gt 100,000,000 tons on Earth.
• Rotates many times per second
• Conservation of Angular Momentum

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• Strong magnetic field
• When star collapses, magnetic field is
  concentrated.

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Neutron Star -- HST
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Pulsars

• 1967 Jocelyn Bell
• Observed object emitting pulses of radio waves.
• Pulses repeated every 1.34 seconds

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• 100s more have been found.
• Some pulse in optical, X-rays, or gamma rays.
• Periods 0.03-0.3 sec.
• Some associated with supernova remnants.

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Crab Nebula
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Crab Pulsar
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• Hewitt proposed it is a rapidly rotating neutron
  star beaming radiation.
• Magnetic pole and rotational axis not quite lined
  up.
• Strong magnetic field.
• Charged particles at poles of magnetic fields and
  emit large amounts of energy.

Lighthouse Model
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• Not all neutron stars are seen to pulse
• Beam may not be pointed at the Earth
• Unknown if all neutron stars are pulsars

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Formation of Black Holes

• If core of star has M gt3 M? the neutron pressure
  cannot hold up the core
• Nothing known left to stop collapse.
• Becomes a singularity -- object with infinite
  density and infinitely small size.

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Why are Black Holes Black?

• Escape velocity velocity needed to escape the
  gravitational pull of an object.
• On Earth 11 km/s
• As size decreases, gravity on the surface of the
  star increases so a larger velocity is needed to
  escape surface.
• When the escape velocity at the surface becomes
  greater than the speed of light no light can
  escape.

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Schwarzschild Radius
Radius an object needs at which the escape
velocity on the surface would be greater than the
speed of light.
Black Hole Light cannot escape surface (c ?
escape velocity)
Radius ? Schwarzschild Radius
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Event Horizon

• If BH is smaller than Schwarzchild Radius, the
  distance from the BH at which light cannot escape
  event horizon.
• No events or communication inside the event
  horizon can be observed.

Event Horizon
Light cannot escape
Black Hole
Light can escape
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Properties of Black Holes

• All matter warps space.
• Like weight on rubber sheet.

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Evidence for Black Holes

• Isolated black holes are hard to observe.
• No light emitted.
• Can observe Gravitational Lensing

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Evidence for Black Holes

• Can observe how BHs gravity affects nearby
  objects.
• Unseen companion
• Accretion disk
• X-ray emission

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Cygnus X-1
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Black Holes in Galaxies

• BHs may have formed in center when galaxy formed.
• Mass of billions of stars in size of SS (Keplers
  3rd Law).
• Black hole likely in center of the Milky Way.

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Black Hole -- HST
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Traveling into a Black Hole -- Tidal Forces

• Very strong.
• Difference in forces of gravity on near and far
  side would pull object apart.

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Time Dilation

• From outside, observer sees clock on board tick
  more and more slowly than outside of craft.
• Closer to black hole, the slower time appears to
  run.
• At event horizon, clock appears to stop!
• Never see craft fall into BH

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• BUT person on board craft would not notice any
  change
• Why? If light used as clock, it would have lower
  frequency
• Result of General Relativity

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Inside of a Black Hole

• Scientists to not know for sure what is inside of
  a black hole.
• Theories of physics break down for such high
  densities.
• Hard to make new model since BH cannot be
  observed.