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Chapter 14
Neutron Stars and
Black Holes
Guidepost
When stars like the sun die, they leave behind white dwarfs,
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 five
essential questions:
• How did scientists predict the existence of neutron stars?
• What is the evidence that neutron stars really exist?
• How did scientists predict the existence of black holes?
• What is the evidence that black holes really exist?
• What happens when matter falls into a neutron star or
black hole?
Guidepost
Answering these questions has challenged scientists to
create new theories and to test them critically. That raises
an important question about science:
• What checks are there against fraud in science?
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.
Outline
I. Neutron Stars
A. Theoretical Prediction of Neutron Stars
B. The Discovery of Pulsars
C. A Model Pulsar
D. Recognizing Neutron Stars
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
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
Formation of Neutron Stars
A supernova explosion of a M > 8 Msun
star blows away its outer layers.
The central core
will collapse into a
compact object of
~ a few Msun.
Compact objects more
massive than the
Chandrasekhar Limit
(1.4 Msun) collapse
beyond the formation of
a white dwarf.
 Pressure
becomes so high
that electrons and protons
combine to form stable
neutrons throughout the object:
p + e-  n + ne
 Neutron
Star
Formation of Neutron Stars (2)
Properties of Neutron Stars
Typical size: R ~ 10 km
Mass: M ~ 1.4 – 3 Msun
Density: r ~ 1014 g/cm3
 a piece of neutron star
matter of the size of a
sugar cube has a mass of
~ 100 million tons!!!
A neutron star
(more than the
mass of the
sun) would
comfortably fit
within the
Capital
Beltway!
Discovery of Pulsars
Angular momentum conservation
=> 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)
=> Rapidly pulsed (optical and radio) emission from
some objects interpreted as spin period of neutron stars
Pulsars / Neutron Stars
Neutron star surface has a temperature of
~ 1 million K.
Cas A in X-rays
Wien’s displacement law,
lmax = 3,000,000 nm / T[K]
gives a maximum wavelength of lmax = 3 nm,
which corresponds to X-rays.
Pulsar Periods
Over time, pulsars
lose energy and
angular
momentum.
=> Pulsar rotation
is gradually
slowing down.
Pulsar Winds
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
pulsar’s rotation.
Lighthouse Model of Pulsars
A Pulsar’s
magnetic field
has a dipole
structure, just
like Earth.
Radiation
is emitted
mostly
along the
magnetic
poles.
Images of Pulsars and Other Neutron Stars
The Vela Pulsar moving
through interstellar space
The Crab
nebula and
pulsar
The Crab Pulsar
Pulsar wind + jets
Remnant of a supernova observed in A.D. 1054
The Crab Pulsar (2)
Visual image
X-ray image
Light Curves of the Crab Pulsar
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.
Magnetars
Some neutron stars have magnetic fields ~ 1000
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.
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.
Neutron Stars in Binary Systems:
X-ray Binaries
Example: Her X-1
2 Msun (F-type) star
Neutron star
Orbital period =
1.7 days
Accretion disk material heats to
several million K => X-ray emission
Star eclipses neutron
star and accretion
disk periodically
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.
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 > 3 Msun
We know of no mechanism to halt the collapse
of a compact object with > 3 Msun.
It will collapse into a single point – a singularity:
=> A Black Hole!
Escape Velocity
Velocity needed
to escape Earth’s
gravity from the
surface: vesc ≈
11.6 km/s.
Now, gravitational
force decreases
with distance (~
1/d2) => Starting
out high above the
surface => lower
escape velocity.
vesc
vesc
vesc
If you could compress Earth to a smaller radius
=> higher escape velocity from the surface
The Schwarzschild Radius
=> There is a limiting radius
where the escape velocity
reaches the speed of light, c:
2GM
Rs = ____
c2
G = Universal const. of gravity
M = Mass
Rs is called the
Schwarzschild Radius.
Vesc = c
Schwarzschild Radius and Event Horizon
No object can
travel faster than
the speed of light
=> nothing (not
even light) can
escape from inside
the Schwarzschild
radius
• We have no way
of finding out what’s
happening inside
the Schwarzschild
radius.
 “Event horizon”
Black Holes in Supernova Remnants
Some
supernova
remnants with
no pulsar /
neutron star in
the center may
contain black
holes.
Schwarzschild Radii
“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)
General Relativity Effects
Near Black Holes (1)
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.
General Relativity Effects
Near Black Holes (2)
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.”
General Relativity Effects
Near Black Holes (3)
Time dilation
Clocks starting at
12:00 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
General Relativity Effects
Near Black Holes (4)
Gravitational Red Shift
All wavelengths of emissions
from near the event horizon
are stretched (red shifted).
 Frequencies are lowered
Event Horizon
Observing Black Holes
No light can escape a black hole
=> 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 > 3 Msun
=> Black hole!
Candidates for Black Hole
Compact object with
> 3 Msun must be a
black hole!
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.
=> Strong X-ray source!
Heats up to a few million K
X-Ray Bursters
Several bursting X-ray
sources have been
observed:
Rapid outburst
followed by
gradual decay
Repeated
outbursts:
The longer
the interval
before the
burst, the
stronger the
burst.
The X-Ray Burster 4U 1820-30
In the cluster NGC 6624
Optical
Ultraviolet
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.
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
Gamma-Ray Bursts (GRBs)
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.
Short (~ a few s),
bright bursts of
gamma-rays
A model for Gamma-Ray Bursts
At least some GRBs are
probably related to the deaths of
very massive (> 25 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”