Pulsars

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Pulsars

• A pulsar is a neutron star that beams radiation
  along a magnetic axis that is not aligned with
  the rotation axis

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Pulsars

• The radiation beams sweep through space like
  lighthouse beams as the neutron star rotates

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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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What can happen 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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A black hole is an object whose gravity is so
powerful that not even light can escape it.
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Supernovae/Supernova Remnants

• Massive stars fuse heavier elements, up to Iron
  (Fe)
• Core is billions of Kelvin and greater than
  Chandrasekhar Limit (1.4 Msun)
• Rapid collapse to neutron star
• Rebound of core results in expulsion of outer
  layers ? Supernova Remnant

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Before/After!
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Tycho SNR (1572)
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(No Transcript)
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Supernova 1987A (light took 170,000 years to get
here!)
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Black HolesThe Science Behind The Science
Fiction

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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
• 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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Eluding Gravitys Grasp

Escape Velocity
Escape Velocity Speed Needed To Escape An
Objects Gravitational Pull
Mass M Radius R
Earth Vesc 27,000 miles/hour (11 km/s) Sun
Vesc 1.4 million miles/hour (600 km/s)
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Dark Stars Rev. John Michell (1783)
Pierre-Simon Laplace (1796)
Speed of light ? 1 billion miles/hour (3x105 km/s)

• What if a star were so small, escape speed gt
  speed of light?
• A star we couldnt see!

Earth mass R ? 1 inch Solar mass R ? 2
miles
Vesc speed of light ?
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Surface of a Black Hole

• 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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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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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.

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Mass versus radius for a neutron star
Objects too heavy to be neutron stars collapse to
black holes
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Neutron Star Limit

• Neutron 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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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.

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Singularity

• The shrunken star too small to be measured but
  with indefinite density

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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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Einsteins theory of gravity is built on the
principle that

1. The speed of light is constant.
2. As an object speeds up its clock runs faster.
3. The effects of gravity cannot be distinguished
   from the effects of acceleration.
4. Motion is a relative state.

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How about if there is wind?
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Speed of light is constant
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Our conceptions of space and time has to be
changed.

• Facts
• Regardless of speed or direction, observers
  always measure the speed of light to be the same
  value.
• Speed of light is maximum possible speed.
• Consequences
• The length of an object decreases as its speed
  increases
• Clocks passing by you run more slowly than do
  clocks at rest (example solar wind particles)

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Time dilation
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Special Relativity Length Contraction
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Equivalence principle
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Gravitational redshift
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Gravity deforms space-time
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Precession of Mercurys orbit
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Gravity bends the path of light
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Geodesics in curved spacetime
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Gravity bends the path of light
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Light waves are stretched out leading to a
gravitational redshift
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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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Falling into a black hole
Falling into a black hole gravitational tidal
forces pull spacetime in such a way that time
becomes infinitely long (as viewed by distant
observer). The falling observer sees ordinary
free fall in a finite time.
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Falling into a black holes

• With a sufficiently large black hole, a freely
  falling observer would pass right through the
  event horizon in a finite time, would be not feel
  the event horizon.
• A distant observer watching the freely falling
  observer would never see her fall through the
  event horizon (takes an infinite time).
• Falling into smaller black hole, the freely
  falling observer would be ripped apart by tidal
  effects.

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Falling into a black hole

• Signals sent from the freely falling observer
  would be time dilated and redshifted.
• Once inside the event horizon, no communication
  with the universe outside the event horizon is
  possible.
• But incoming signals from external world can
  enter.
• Time travel and other fairy tales

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Seeing black holes
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Seeing black holes
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How do we know its a BH?

• Nature is tricky couldnt it be another small
  star like a neutron star or a
  white dwarf?
• Measure mass of X-ray star by motion of its
  companion (a star like the sun)
• Mass gt 3 solar
• masses ? BH!
• Roughly a dozen BHs found this way (tip of the
  iceberg)

Chandrasekhar
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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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One famous X-ray binary with a likely black hole
is in the constellation Cygnus
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Gamma-Ray Bursts

• Brief bursts of gamma-rays coming from space were
  first detected in the 1960s

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• Observations in the 1990s showed that many
  gamma-ray bursts were coming from very distant
  galaxies
• They must be among the most powerful explosions
  in the universecould be the formation of a black
  hole

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Supernovae and Gamma-Ray Bursts

• Observations show that at least some gamma-ray
  bursts may be produced by supernova explosions
• Some others may come from collisions between
  neutron stars

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Quasars

• Small, powerful source of energy thought to be
  cores of distant spiral galaxies

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Quasars and Active Galaxies

• Active galaxies are galaxies with exceptionally
  bright and compact nuclear regions, called
  Active Galactic Nuclei (AGN).
• The energy source of AGN is ultimately gravity,
  in the form of accretion of gas onto a
  super- massive black hole, one the most
  efficient engines in the Universe.

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THE AGN ZOO jet-powered radio lobes
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What is the energy source of AGN?
The one characteristic that all AGN share is fast
variability, from which astronomers infer the
size of the central engine.
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A Unified Model for AGN
Are the different classes of AGN truly different
beasts? In the Unified Model for AGN, the
apparent differences are mainly due to
inclination effects. The ingredients are the
hole, the disk, the jet, some orbiting clouds of
gas, plus a dusty torus that surrounds the inner
disk.
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A Unified Model for AGN
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A Unified Model for AGN observational
confirmations