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Title: Note that the following lectures include animations and PowerPoint effects such as fly-ins and transitions that require you to be in PowerPoint's Slide Show mode (presentation mode).


1
Note that the following lectures include
animations and PowerPoint effects such as fly-ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
0
2
Neutron Stars and Black Holes
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  • Chapter 14

3
Guidepost
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  • In the previous two chapters you have traced the
    story of stars from birth to death. By now you
    are asking a simple question, Whats left? The
    answer depends on the mass of the star. You
    already know that stars like the sun produce
    white dwarf, but more massive stars leave behind
    the strangest beasts in the cosmic zoo.
  • Now you are ready to meet neutron stars and black
    holes, and your exploration will answer four
    important questions
  • How does theory predict the existence of neutron
    stars?
  • How do astronomers know neutron stars really
    exist?
  • How does theory predict the existence of black
    holes?
  • How can astronomers be sure that black holes
    really exist?

4
Guidepost
0
This chapter will show you more striking examples
of how astronomers combine observations and
theory to understand nature. This chapter ends
the story of individual stars, but it does not
end the story of stars. In the next chapter, you
will begin exploring the giant communities in
which stars live the galaxies.
5
Outline
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I. Neutron Stars A. Theoretical Prediction of
Neutron Stars B. The Discovery of Pulsars C. A
Model Pulsar D. The Evolution of Pulsars E.
Binary Pulsars F. The Fastest Pulsars G. Pulsar
Planets II. Black Holes A. Escape Velocity B.
Schwarzschild Black Holes C. Leaping into a
Black Hole D. The Search for Black Holes
6
Outline (continued)
0
III. Compact Objects with Disks and Jets A. Jets
of Energy from Compact Objects B. Gamma-Ray
Bursts
7
Formation of Neutron Stars
0
A supernova explosion of a M gt 8 Msun star blows
away its outer layers.
Compact objects more massive than the
Chandrasekhar Limit (1.4 Msun) collapse beyond
the formation of a white dwarf
The central core will collapse into a compact
object of a few Msun
? Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object
p e- ? n ne
? Neutron Star
8
Properties of Neutron Stars
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? a piece of neutron star matter of the size of a
sugar cube has a mass of 500 million tons!!!
Typical size R 10 km
Mass M 1.4 3 Msun
Density r 1015 g/cm3
A neutron star (more than the mass of the sun)
would comfortably fit within the Capital Beltway!
9
Discovery of Pulsars
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Angular momentum conservation
gt Collapsing stellar core spins up to periods of
a few milliseconds
Magnetic fields are amplified up to B 109
1015 G
(up to 1012 times the average magnetic field of
the sun)
gt Rapidly pulsed (optical and radio) emission
from some objects interpreted as spin period of
neutron stars
10
Pulsars / Neutron Stars
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Neutron star surface has a temperature of 1
million K
Wiens displacement law, lmax 3,000,000 nm
/ TK gives a maximum wavelength of lmax 3
nm, which corresponds to X-rays
11
Pulsar Winds
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Pulsars are emitting winds and jets of highly
energetic particles.
These winds carry away about 99.9 of the energy
released from the slowing-down of the pulsars
rotation.
12
Lighthouse Model of Pulsars
0
A Pulsars magnetic field has a dipole structure,
just like Earth.
Radiation is emitted mostly along the magnetic
poles.
13
Images of Pulsars and Other Neutron Stars
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The Vela Pulsar moving through interstellar space
The Crab nebula and pulsar
14
The Crab Pulsar
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Pulsar wind jets
Remnant of a supernova observed in A.D. 1054
15
The Crab Pulsar (2)
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Visual image
X-ray image
16
Light curves of the Crab Pulsar
0
Pulsar pulse shapes can be quite different in
different wavelength ranges (e.g., optical vs.
X-rays)
17
Proper Motion of Neutron Stars
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Some neutron stars are moving rapidly through
interstellar space.
This might be a result of anisotropies during the
supernova explosion, forming the neutron star.
18
Binary Pulsars
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Some pulsars form binaries with other neutron
stars (or black holes).
Radial velocities resulting from the orbital
motion lengthen the pulsar period when the pulsar
is moving away from Earth...
and shorten the pulsar period when it is
approaching Earth.
19
Neutron Stars in Binary Systems X-ray Binaries
0
Example Her X-1
Star eclipses neutron star and accretion disk
periodically
2 Msun (F-type) star
Neutron star
Orbital period 1.7 days
Accretion disk material heats to several million
K gt X-ray emission
20
Neutron Stars in Binary Systems X-ray Binaries
(2)
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Example Her X-1
Neutron-star X-ray binaries are often found in
star clusters where stars are crowded close
together.
21
Pulsar Planets
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Some pulsars have planets orbiting around them.
Just like in binary pulsars, this can be
discovered through variations of the pulsar
period.
As the planets orbit around the pulsar, they
cause it to wobble around, resulting in slight
changes of the observed pulsar period.
22
Black Holes
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Just like white dwarfs (Chandrasekhar limit 1.4
Msun), there is a mass limit for neutron stars
Neutron stars can not exist with masses gt 3 Msun
We know of no mechanism to halt the collapse of a
compact object with gt 3 Msun
It will collapse into a single point a
singularity
gt A Black Hole!
23
Escape Velocity
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Velocity needed to escape Earths gravity from
the surface vesc 11.6 km/s
vesc
Now, gravitational force decreases with distance
( 1/d2) gt Starting out high above the surface
gt lower escape velocity
vesc
vesc
If you could compress Earth to a smaller radius
gt higher escape velocity from the surface
24
The Schwarzschild Radius
0
gt There is a limiting radius where the escape
velocity reaches the speed of light, c
2GM
____
Rs
Vesc c
c2
G Universal const. of gravity
M Mass
Rs is called the Schwarzschild Radius
25
Schwarzschild Radius and Event Horizon
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No object can travel faster than the speed of
light.
gt Nothing (not even light) can escape from
inside the Schwarzschild radius.
  • We have no way of finding out whats happening
    inside the Schwarzschild radius.
  • Event horizon

26
Schwarzschild Radii
0
27
General Relativity Effects Near Black Holes (1)
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At a distance, the gravitational fields of a
black hole and a star of the same mass are
virtually identical.
At small distances, the much deeper gravitational
potential will become noticeable.
28
General Relativity Effects Near Black Holes (2)
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An astronaut descending down towards the event
horizon of the BH will be stretched vertically
(tidal effects) and squeezed laterally.
This effect is called spaghettification.
29
General Relativity Effects Near Black Holes (3)
0
Time dilation
Clocks starting at 1200 at each point After 3
hours (for an observer far away from the BH)
Clocks closer to the BH run more slowly.
Time dilation becomes infinite at the event
horizon.
Event Horizon
30
General Relativity Effects Near Black Holes (4)
0
Gravitational Red Shift
All wavelengths of emissions from near the event
horizon are stretched (red shifted). ?
Frequencies are lowered
Event Horizon
31
Observing Black Holes
0
No light can escape a black hole.
gt Black holes can not be observed directly.
If an invisible compact object is part of a
binary, we can estimate its mass from the orbital
period and radial velocity.
Mass gt 3 Msun gt Black hole!
32
Candidates for Black Hole
0
Compact object with gt 3 Msun must be a black hole!
33
Black-Hole vs. Neutron-Star Binaries
0
Black Holes Accreted matter disappears beyond
the event horizon without a trace.
Neutron Stars Accreted matter produces an X-ray
flash as it impacts on the neutron star surface.
34
Black Hole X-Ray Binaries
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Accretion disks around black holes
Strong X-ray sources
Rapidly, erratically variable (with flickering on
time scales of less than a second)
Sometimes Quasi-periodic oscillations (QPOs)
Sometimes Radio-emitting jets
35
Gamma-Ray Bursts (GRBs)
0
Short ( a few s), bright bursts of gamma-rays
GRB a few hours after the GRB
Same field, 13 years earlier
Later discovered with X-ray and optical
afterglows lasting several hours a few days
Many have now been associated with host galaxies
at large (cosmological) distances.
36
A model for Gamma-Ray Bursts
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At least some GRBs are probably related to the
deaths of very massive (gt 20 Msun) stars.
In a supernova-like explosion of stars this
massive, the core might collapse not to a neutron
star, but directly to a black hole.
Such stellar explosions are termed hypernovae.
37
Magnetars
0
Some neutron stars have magnetic fields 100
times stronger even than normal neutron stars.
These care called Magnetars.
Earthquake-like ruptures in the surface crust of
Magnetars cause bursts of soft gamma-rays.
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