Announcements March 31

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AnnouncementsMarch 31

• Exam III Wednesday, April 5
• Covers Chapters 16-18,S2,S3 (Stellar evolution,
  neutron stars, black holes)
• Monday Start Chapter 19 (Milky Way Galaxy)
• Observing Test
• If your last name starts with A-L, you must pass
  this test by Thursday April 20.
• If you name begins with M-Z, by Thursday May 4
  (last Thursday before final exams).
• Clear sky patrol hours now 9 pm 11 pm if clear
  weather!
• Can check status at 5pm Clear Sky Status Page
  on website
• Observing list is on the course website (Obs.
  Project).

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Black Holes
Black holes form when matter collapses to a point
- a singularity. Nothing not even light can
escape from within the event horizon above a
black hole. The event horizon is one
Schwarzschild radius (RS) from the singularity.
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As the gravitational field of an object
increases, the curvature of space-time near its
surface increases to the point (for black holes)
where not even light can escape.
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Light Bending Near a Black Hole
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Falling Into a Black Hole

• A probe falling into a black hole
• would be distorted by the immense
  gravitational forces
• photons leaving the probe would lose more
  and more energy
• they would be redshifted to longer
  wavelengths.
• time on the probe would appear to move
  slower and slower
• to the observer who sent it in
• Time becomes infinitely long as probe
  approaches event horizon!

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Black Holes
are not cosmic vacuum cleaners!
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Observing Black Holes
We can see accreting black holes in binary star
systems via their X-ray emission.
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X-ray Exotica
Compact objects give rise to a wide variety of
phenomena, all of which have associated X-ray
emission.
The Black Widow Pulsar
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X-ray Exotica
Jets from a black-hole binary
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Summary of Stellar Evolution Death

• Initial mass lt 0.4 Msun ? He white dwarf
• Initial mass 0.4-4 Msun
• ? planetary nebul
• ? C-O white dwarf
• Iniital mass 4-8 Msun ? planetary nebula/
• white dwarf likely
• Initial Mass 8-25 Msun
• ? supernova
• ? neutron star
• Initial mass gt 25 Msun
• ? supernova
• ? black hole

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If we apply General Relativity to a collapsing
stellar core, we find that it can be sufficiently
dense to trap light in its gravity.
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Wormholes Fact or Fiction This diagram shows an
Einstein-Rosen bridge where a black hole becomes
a tunnel to a parallel universe.
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A non-rotating black hole has only a center and
a surface

• The black hole is surrounded by an event horizon
  which is the sphere for which light cannot escape
• The distance between the black hole and its event
  horizon is the Schwarzschild radius (RSch
  2GM/c2)
• The center of the black hole is a point of
  infinite density and zero volume, called a
  singularity

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Falling into a black hole is an infinite voyage
as 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.
Falling into a black hole revisited What would
it be like?
Note A black hole of Mass M has EXACTLY the same
gravitational force outside the event horizon as
an ordinary object of mass M
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Myths and Truths about journeys to Black holes

• 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)
• Signals sent from the freely falling observer a
  regular intervals (clock ticks) would be
• Dilated (take a longer time interval as measured
  by distant receiver
• Redshifted
• Once inside the event horizon, no communication
  with external world ever possible
• But incoming signals from external world can
  enter
• The tidal force near the event horizon
• A black hole of mas M has exactly the same
  gravitational field as an ordinary mass M at
  large distances

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Certain binary star systems probably contain
black holes

• Black holes cannot been seen because they do not
  emit nor reflect light
• Black holes that are in binary systems might be
  able to be detected
• As material races toward a black hole, it heats
  and emits X-rays

This star is unrelated (foreground star)
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Q. A distant observer watches an object falling
directly into a black hole. The object emits
pulses of blue light. What is seen as the object
approaches the event horizon?

• Object speeds up, emits blue light
• Object speeds up, emits red light
• Object slows down, emits blue light
• Object slows down, emits red light
• Object travels a constant speed, emits no light

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Black holes often found in binary systems
X-rays emitted from hot accretion disk around
black hole
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Black holes in galactic cores has masses between
106 and 109 solar masses!
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Supermassive black holes at the centers of many
galaxies
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Black holes evaporate

• Virtual particles that appear in pairs near an
  event horizon may not be able to mutually
  annihilate each other if only one manages to
  survive a trip along the event horizon.

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Rotating Black Holes!
Structure of a Kerr (Rotating) Black Hole
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Q. What is the most common way to detect a
black hole?

• Redshifted optical emission from jets
• Radio pulses from rotating searchlight
• Periodic Gamma ray bursts
• Observations of matter crashing into event
  horizon
• X ray emission form accretion disk surrounding
  black hole