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). - PowerPoint PPT Presentation

About This Presentation
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).

Description:

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 – PowerPoint PPT presentation

Number of Views:3059
Avg rating:3.0/5.0
Slides: 67
Provided by: MarkusBo5
Category:

less

Transcript and Presenter's Notes

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).
2
Neutron Stars and Black Holes
  • Chapter 14

3
Guidepost
The preceding chapters have traced the story of
stars from their birth as clouds of gas in the
interstellar medium to their final collapse. This
chapter finishes the story by discussing the
kinds of objects that remain after a massive star
dies. How strange and wonderful that we humans
can talk about places in the universe where
gravity is so strong it bends space, slows time,
and curves light back on itself! To carry on
these discussions, astronomers have learned to
use the language of relativity. Throughout this
chapter, remember that our generalized
discussions are made possible by astronomers
studying general relativity in all its
mathematical sophistication. That is, our
understanding rests on a rich foundation of
theory. This chapter ends the story of
individual stars. The next three chapters,
however, extend that story to include the giant
communities in which stars live the galaxies.
4
Outline
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. Black Holes Have No
Hair D. A Leap into a Black Hole E. The Search
for Black Holes
5
Outline (continued)
III. Compact Objects with Disks and Jets A.
X-Ray Bursters B. Accretion Disk
Observations C. Jets of Energy from Compact
Objects D. Gamma-Ray Bursts
6
Neutron Stars
A supernova explosion of a M gt 8 Msun star blows
away its outer layers.
The central core will collapse into a compact
object of a few Msun.
7
Formation of Neutron Stars
Compact objects more massive than the
Chandrasekhar Limit (1.4 Msun) collapse further.
? 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
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!!!
9
Discovery of Pulsars
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
Neutron star surface has a temperature of 1
million K.
Cas A in X-rays
Wiens displacement law, lmax 3,000,000 nm
/ TK gives a maximum wavelength of lmax 3
nm, which corresponds to X-rays.
11
Pulsar Periods
Over time, pulsars lose energy and angular
momentum
gt Pulsar rotation is gradually slowing down.
12
Lighthouse Model of Pulsars
A Pulsars magnetic field has a dipole structure,
just like Earth.
Radiation is emitted mostly along the magnetic
poles.
13
Neutron Star
(SLIDESHOW MODE ONLY)
14
Images of Pulsars and Other Neutron Stars
The vela Pulsar moving through interstellar space
The Crab nebula and pulsar
15
The Crab Pulsar
Pulsar wind jets
Remnant of a supernova observed in A.D. 1054
16
The Crab Pulsar (2)
Visual image
X-ray image
17
Light Curves of the Crab Pulsar
18
Proper Motion of Neutron Stars
Some neutron stars are moving rapidly through
interstellar space.
This might be a result of anisotropies during the
supernova explosion forming the neutron star
19
Binary Pulsars
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.
20
Neutron Stars in Binary Systems X-ray Binaries
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
21
Pulsar Planets
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
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
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
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
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 Radius of Black Hole
(SLIDESHOW MODE ONLY)
27
Black Holes in Supernova Remnants
Some supernova remnants with no pulsar / neutron
star in the center may contain black holes.
28
Schwarzschild Radii
29
Black Holes Have No Hair
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)
30
General Relativity Effects Near Black Holes
An astronaut descending down towards the event
horizon of the BH will be stretched vertically
(tidal effects) and squeezed laterally.
31
General Relativity Effects Near Black Holes (2)
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
32
General Relativity Effects Near Black Holes (3)
Gravitational Red Shift
All wavelengths of emissions from near the event
horizon are stretched (red shifted). ?
Frequencies are lowered.
Event Horizon
33
Observing Black Holes
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!
34
End States of Stars
(SLIDESHOW MODE ONLY)
35
Candidates for Black Hole
Compact object with gt 3 Msun must be a black hole!
36
Compact Objects with Disks and Jets
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.
37
X-Ray Bursters
Several bursting X-ray sources have been observed
Rapid outburst followed by gradual decay
Repeated outbursts The longer the interval, the
stronger the burst
38
The X-Ray Burster 4U 1820-30
In the cluster NGC 6624
Ultraviolet
Optical
39
Black-Hole vs. Neutron-Star Binaries
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.
40
Black Hole X-Ray Binaries
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
41
Radio Jet Signatures
The radio jets of the Galactic black-hole
candidate GRS 1915105
42
Model of the X-Ray Binary SS 433
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
43
Gamma-Ray Bursts (GRBs)
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.
44
New Terms
neutron star pulsar lighthouse model pulsar
wind glitch magnetar gravitational
radiation millisecond pulsar singularity black
hole event horizon Schwarzschild radius (RS) Kerr
black hole ergosphere time dilation gravitational
red shift X-ray burster
quasi-periodic oscillations (QPOs) gamma-ray
burster soft gamma-ray repeater
(SGR) hypernova collapsar  
45
Discussion Questions
1. Has the existence of neutron stars been
sufficiently tested to be called a theory, or
should it be called a hypothesis? What about the
existence of black holes? 2. Why would you
expect an accretion disk around a star the size
of the sun to be cooler than an accretion disk
around a compact object? 3. In this chapter, we
imagined what would happen if we jumped into a
Schwarzschild black hole. From what you have
read, what do you think would happen to you if
you jumped into a Kerr black hole?
46
Quiz Questions
1. What is the lower limit for the mass of
neutron stars? a. About 0.08 solar masses. b.
About 0.4 solar masses c. Exactly 1 solar
mass. d. About 1.4 solar masses. e. Between 2 and
3 solar masses.
47
Quiz Questions
2. White dwarfs and neutron stars are both end
products of stellar evolution. White dwarfs are
composed of mostly carbon, oxygen, and electrons,
whereas neutron stars are composed of mostly
neutrons. What happens to the protons in the
atomic nuclei and the degenerate electrons that
were inside the star that creates a neutron
star? a. The protons and electrons, being
charged particles, are all ejected during the
supernova event by the strong magnetic field. b.
The electrons and protons combine to form
neutrons and neutrinos. c. Protons decay into
neutrons and positrons and neutrinos. The
positrons combine with the electrons and form
pairs of gamma rays. d. The protons remain in the
neutron star and the electrons are ejected by the
magnetic field. e. Our understanding of atomic
nuclei is not yet good enough to determine what
happens to them.
48
Quiz Questions
3. Why don't we use visible-wavelength telescopes
to locate neutron stars? a. Neutron stars have
wavelengths of maximum intensity in the X-ray
band of the electromagnetic spectrum. b. Neutron
stars are too cold to emit visible light. c.
Neutron stars have strong magnetic fields. d.
Neutron stars pulse too rapidly. e. Neutron stars
are very small.
49
Quiz Questions
4. What prevents neutron stars from contracting
to a smaller size? a. Gas pressure fueled by
hydrogen fusion. b. Gas pressure fueled by helium
fusion. c. Gas pressure fueled by carbon
fusion. d. Degenerate neutrons. e. Degenerate
electrons.
50
Quiz Questions
5. Why do we expect that neutron stars spin
rapidly? a. The law of conservation of
energy. b. The law of conservation of angular
momentum. c. Equatorial jets spin them up. d.
Both a and b above. e. All of the above.
51
Quiz Questions
6. Why are pulsars so hot? a. Helium flash. b.
The CNO cycle proceeds rapidly. c. Active fusion
of elements heavier than carbon. d. Gravitational
energy was converted into thermal energy during
formation. e. Active fusion of elements heavier
than iron produces vast amounts of energy.
52
Quiz Questions
7. Why does the short length of pulsar pulses
eliminate normal stars as possible pulsars? a.
Normal stars are unchanging and eternal. b.
Normal stars are too cool to emit radio
pulses. c. Normal stars do not have strong enough
magnetic fields. d. The small size of normal
stars prohibits the emission of pulses this
short. e. An object cannot emit pulses that are
shorter than the time it takes light to cross its
diameter.
53
Quiz Questions
8. How can a neutron star not be a pulsar? a.
Its magnetic field may be too weak to generate
beams of radiation. b. A pulsar may be too old
and rotate too slowly to pulse. c. A pulsar's
magnetic field may not sweep past Earth. d. Both
a and b above. e. All of the above.
54
Quiz Questions
9. What type of pulsars have been observed to
emit visible pulses? a. Young pulsars. b. Slow
pulsars. c. Old pulsars. d. Nearby pulsars. e.
Both b and c above.
55
Quiz Questions
10. Why are all pulsars not located in supernova
remnants? a. Pulsars persist longer than
supernova remnants. b. Some pulsars given high
velocities upon formation can flee the scene of
destruction. c. Some pulsars are the result of
mergers rather than supernova events. d. Both a
and b above. e. All of the above.
56
Quiz Questions
11. Why do the millisecond pulsars spin so
fast? a. They have all been formed recently. b.
The impact of a large planet has spun them up. c.
Accretion of matter from a nearby binary
companion spins them up. d. Their strong magnetic
field couples with the interstellar magnetic
field. e. They really spin at half their pulse
rate, as both magnetic poles sweep past Earth.
57
Quiz Questions
12. Which of the following is an accurate
description of the Schwarzschild radius? a. It
is the radius of a black hole singularity. b. It
is the radius to which an object must shrink to
become a black hole. c. It is the radius of the
event horizon surrounding a black hole
singularity. d. Both a and b above. e. Both b and
c above.
58
Quiz Questions
13. What is the difference between a
Schwarzschild black hole and a Kerr black
hole? a. Mass. b. Electric charge. c. Angular
momentum. d. Both b and c above. e. All of the
above.
59
Quiz Questions
14. What changes would occur if the Sun were
replaced with a one-solar-mass black hole? a.
Earth's orbit would not change. b. Earth would be
sucked into the Sun. c. The planets would
disappear from view. d. Extreme tidal forces
would severely fracture Earth's lithosphere. e.
Both a and c above.
60
Quiz Questions
15. What property does matter inside the event
horizon of a black hole retain? a. Mass. b. Mass
and angular momentum. c. Mass, angular momentum,
and electric charge. d. Mass, angular momentum,
electric charge, and temperature. e. Mass,
angular momentum, electric charge, temperature,
and luminosity.
61
Quiz Questions
16. Which of the following describes the
gravitational red shift? a. The reddening of
starlight by interstellar dust grains. b. A
reduction in the energy of photons as they move
away from objects. c. The angular change in a
star's position when observed during a solar
eclipse. d. The alternating Doppler effect due to
two bodies whose orbital plane contains our line
of sight. e. We are unable to tell whether an
object is moving away from us or we are moving
away from the object.
62
Quiz Questions
17. What observational evidence do we have that
stellar death black holes really exist? a.
Hollowed-out green spheres are sucking up matter
in star forming regions and emitting gamma
rays. b. Some X-ray binaries have an unseen
object with masses greater than 3 solar
masses. c. Some X-ray binaries emit pulses of
radiation at radio wavelengths. d. We see areas
that block the light from more distant
objects. e. The observed glitches in the periods
of pulsars.
63
Quiz Questions
18. What is the source of the continuous X-rays
emitted by a close binary system that contains a
compact object? a. The systems magnetic field
accelerates ionized matter, emitting synchrotron
radiation. b. An accretion disk around the
compact object is heated by friction. c. Matter
impacting the surface of the compact object. d.
The companion star is superheated by tidal
forces. e. The hot surface of the compact object.
64
Quiz Questions
19. Some X-ray novae emit bursts of energy and
others do not. In addition, those with energy
bursts are about 100 times as luminous as those
without bursts. What type of compact objects are
responsible for these two types of X-ray
novae? a. Those with bursts contain black holes
and those without bursts contain neutron
stars. b. Those with bursts contain neutron stars
and those without bursts contain black holes. c.
Those with bursts contain white dwarfs and those
without bursts contain neutron stars. d. Those
with bursts contain neutron stars and those
without bursts contain white dwarfs. e. Those
with bursts contain black holes and those without
bursts contain white dwarfs.
65
Quiz Questions
20. What may be responsible for the observed
gamma ray bursters? a. Neutron star merger. b.
Hypernovae. c. Magnetars. d. Both a and b
above. e. All of the above.
66
Answers
1. d 2. b 3. e 4. d 5. b 6. d 7. e 8. e 9. a 10. d
11. c 12. e 13. c 14. e 15. c 16. b 17. b 18. b 19
. b 20. d
Write a Comment
User Comments (0)
About PowerShow.com