Neutron Stars and Black Holes

1
Neutron Stars and Black Holes
0
Chapter 11
2
Neutron Stars
0
A supernova explosion of an M gt 8 Msun star blows
away its outer layers.
Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object.
The central core will collapse into a compact
object of a few Msun.
Typical size R 10 km
Mass M 1.4 3 Msun
Density r 1014 g/cm3
? Piece of neutron star matter of the size of a
sugar cube has a mass of 100 million tons!!!
3
Discovery of Pulsars
0
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
4
The Crab Pulsar
0
Pulsar wind jets
Remnant of a supernova observed in A.D. 1054
5
The Crab Pulsar
0
Visual image
X-ray image
6
Light curves of the Crab Pulsar
0
7
The Lighthouse Model of Pulsars
0
A pulsars magnetic field has a dipole structure,
just like Earth.
Radiation is emitted mostly along the magnetic
poles.
8
Images of Pulsars and other Neutron Stars
0
The Vela pulsar moving through interstellar space
The Crab Nebula and pulsar
9
The Effects of Pulsar Winds
0
Pulsars blow off a constant stream (wind) of
high-energy particles pulsar winds
10
Proper Motion of Neutron Stars
0
Some neutron stars are moving rapidly through
interstellar space.
This might be a result of anisotropies during the
supernova explosion forming the neutron star.
11
Binary Pulsars
0
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.
12
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
13
Compact Objects with Disks and Jets
0
Black holes and neutron stars can be part of a
binary system.
Matter gets pulled off from the companion star,
forming an accretion disk.
gt Strong X-ray source!
Heats up to a few million K.
14
The X-Ray Burster 4U 1820-30
0
Several bursting X-ray sources have been observed
Rapid outburst followed by gradual decay
Optical
Ultraviolet
15
Pulsar Planets
0
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.
16
Black Holes
0
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!
17
Escape Velocity
0
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
If you could compress Earth to a smaller radius
gt higher escape velocity from the surface.
vesc
18
The Schwarzschild Radius
0
gt There is a limiting radius where the escape
velocity reaches the speed of light, c
Vesc c
2GM
____
Rs
c2
G gravitational constant
M mass
Rs is called the Schwarzschild radius.
19
Schwarzschild Radius and Event Horizon
0
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

20
0
21
Black Holes Have No Hair
0
Matter forming a black hole is losing almost all
of its properties.
black holes are completely determined by 3
quantities
mass
angular momentum
(electric charge)
22
The Gravitational Field of a Black Hole
0
Gravitational Potential
Distance from central mass
The gravitational potential (and gravitational
attraction force) at the Schwarzschild radius of
a black hole becomes infinite.
23
General Relativity Effects Near Black Holes
0
An astronaut descending down towards the event
horizon of the black hole will be stretched
vertically (tidal effects) and squeezed laterally.
24
General Relativity Effects Near Black Holes (II)
0
Time dilation
Clocks starting at 1200 at each point. After 3
hours (for an observer far away from the black
hole)
Clocks closer to the black hole run more slowly.
Time dilation becomes infinite at the event
horizon.
Event horizon
25
General Relativity Effects Near Black Holes (III)
0
gravitational redshift
All wavelengths of emissions from near the event
horizon are stretched (redshifted). ? Frequencies
are lowered.
Event horizon
26
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!
27
0
Compact object with gt 3 Msun must be a black hole!
28
Jets of Energy from Compact Objects
0
Some X-ray binaries show jets perpendicular to
the accretion disk
29
Model of the X-Ray Binary SS 433
0
Optical spectrum shows spectral lines from
material in the jet.
Two sets of lines one blue-shifted, one
red-shifted
Line systems shift back and forth across each
other due to jet precession
30
Gamma-Ray Bursts (GRBs)
0
Short ( a few s), bright bursts of gamma-rays
GRB of May 10, 1999 1 day after the GRB
2 days after the GRB
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.
Probably related to the deaths of very massive (gt
25 Msun) stars.