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The death of stars and stellar
remnants
•He White dwarfs
•He White dwarf and planetary nebulae
•Carbon White Dwarfs
•Neon-Silicon-Oxigen White Dwarfs
•Two types of Supernovae
•Type Ia, the exploding stars disintegrates
•Type II (core collapse), the star leaves remnants:
•Neutron stars (basically, a neutron white dwarf,
I.e. degenerate gas of neutrons)
•Black holes
Assigned Reading
Ch. 13 and Ch. 14,
excluding 14.3 (eliminated from syllabus)
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Stellar evolution can be deeply altered
in a binary system: mass transfer
An originally
massive star can
loose mass
and become less
massive (longer
life)
A nearly dead star can
be rejuvenated by
accretion of fresh fuel
Planetary Nebula
At the center of the nebula there is
the dying star.
This is a white dwarf, where small
and hot: it photo-ionizes the nebula
The nebula formed out of the mass
loss during the red super-giant
phase.
Destiny of stars with roughly M <
8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD
Novae are nuclear explosions on the
surface of white dwarf and neutron stars
Brightness changes by a factor of 4000!
Two basic types of supernovae
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Type Ia – from the
thermonuclear detonation of a
white dwarf with M ~ 1.4 Msun
after accreting matter from its
companion.
(1.4 Msun is called Chandrasekhar limit)
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Type II – from core collapse of a
massive star  neutron star or
black hole.
Type Ia: White Dwarf Supernova
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If a White Dwarf accretes enough matter
from a companion star, it will eventually
nova.
If, after the nova, it does not shed all the
mass it gained, it will continue to accrete
mass until it novas again.
If this process continues (accretion, nova,
accretion, nova, etc.) such that the WD
continues to gain mass, once it has a
mass of 1.4Msun, the core will collapse,
carbon fusion will occur simultaneously
throughout the core, and the WD will
supernova.
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How might it be possible for a White
Dwarf to flare back to life?
Remnant from a Type Ia supernova:
A lot of irons!
The Sun will never supernova
because
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It will become a white dwarf before it
has the chance.
Its surface temperature is not high
enough.
It is not large enough.
It is not massive enough.
Remnant from a Type II supernova
Crab Nebula
The supernova
explosion that
created the Crab was
seen on about July 4,
1054 AD.
Neutron stars
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A neutron star --- a
giant nucleus --- is
formed from the
collapse of a massive
star.
Supported by neutron
degeneracy pressure.
Only about 10 km in
radius.
A teaspoon full would
contain 108 tons!
Very hot and with
very strong magnetic
field
Jocelyn Bell
Neutron stars
discovered as
pulsar
SNR N157B in the LMC
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pulsar
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16ms period
The fastest young
pulsar known
Pulsar, as a light house
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A fast rotating,
magnetized
neutron star.
Emits both strong
radiation (radio)
and jets pf highenergy particles.
Jets not very well
understood; their
existence is due to
the rotation and to
the presence of
magnetic fields
Pulsar Evolution
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Pulsar slow down their rotation
Pulsar emits radiation (0.1%) and high energy
plasma (99.9%): it looses energy
This energy is replenished at the expense of
rotational energy
Eventually, pulsar slows down, radio beams
become weaker.
Many pulsars not observable, because beams
do not sweep Earth; or they have been kicked
off their nebula (asymmetric collapse) or
slowed down too quickly (because they had
ultra strong magnetic field)
The Limit of Neutron
Degeneracy
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The upper limit on the mass of stars
supported by neutron degeneracy
pressure is about 3.0 MSun (predicted
by Lev Landau)
If the remaining core contains more
mass, neutron degeneracy pressure
is insufficient to stop the collapse.
In fact, nothing can stop the collapse,
and the star becomes a black hole.
Review Questions
1.
2.
3.
4.
What are type-Ia supernovae?
What do a type-II supernova leave
behind?
Why does a neutron star spin fast?
What is a pulsar?
Black holes
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When the ball of neutrons collapses,
it forms a singularity – a small region
in space with small volume and the
mass of the parent material.
a singularity in space, from which nothing can escape, even light!
The most interesting aspects of a black hole are not what it’s
made of, but what effect is has on the space and time around it.
If we apply General Relativity to a collapsed star, we find that it
can be sufficiently dense to trap light in its gravity.
The Size of a Black Hole
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The extent of a black
hole is called its event
horizon. Nothing
escapes the event
horizon!
The radius of the event
horizon is the
Schwarzschild radius
given by:
Rs = 2GM/c2
Some Examples of Black Hole Sizes
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A 3MSun black hole would have a Schwarzschild
radius of ~10km. It would fit in Amherst.
A 3 billion MSun black hole would have a radius
of 60 AU – just twice the radius of our solar
system.
Some primordial black holes may have been
created with a mass equal to that of Mount
Everest. They would have a radius of just
1.5x10-15 m – smaller than a hydrogen atom!
What would a Black
Hole look like?
Gravitational lensing
Some Odd Properties of Space
Around a Black Hole
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Light emitted near the surface of a
black hole is redshifted as it leaves
the intense gravitational field.
For someone far away, time seems to
runs more slowly near the surface of
a black hole. An astronaut falling
into a black hole would seem to take
forever to fall in.
Gravitational Redshifts
A photon will
give up energy
while climbing
away from a mass.
It is trading its
own energy for
gravitational
potential energy.
Survey Question
If your buddy were falling into a black hole, what
kind of telescope would you need in order to see
him/her wave goodbye as they crossed the event
horizon?
1) A large radio telescope.
2) A large infrared telescope.
3) A large visible light telescope?
4) A large X-ray telescope?
Black Holes Don’t Suck!
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Many people are under the
impression that the gravity of black
holes is so strong that they suck in
everything around them.
Imagine what would happen if the
Sun were to instantly turn into a
black hole. What would happen to
the Earth?
Black Holes Don’t Suck!
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Since the mass of the Sun and Earth
don’t change, and the Earth is no
further from the Sun than it was
before, the force on the Earth would
remain exactly the same. The Earth
would continue to orbit the black
hole at a distance of 1 AU!
Black Holes Don’t Suck!
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So why are black holes so infamous?
• The reason is that the mass is so compact
that you can get within a few kilometers of a
full solar mass of material. Today, if you
stood on the surface of the Sun, much of the
material is hundreds of thousands of
kilometers away. With a black hole, the
mass is so concentrated that you can get
very close to the full mass.
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Gravity strength is extreme near a B.H.
The tidal forces
near a moderate
sized black hole
are lethal!
How Do We See A Black Hole?
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Short answer … we don’t. But we
can see radiation from the material
falling into one.
When matter falls into a B.H. it gets
very, very hot. It emits X-ray.
Candidate B.H.’s are powerful X-ray
emitters, especially if they show very
rapid variability (=small size)
Evidence for Black Holes
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If black holes are black, how do we
know that they exist?
The star HD 226868 is an excellent
example. It is a B supergiant.
The spectral lines in the star clearly
show that it is in a binary system
with a period of 5.6 days, however,
we see no companion star.
It is one of the brightest X-ray sources
in the sky and is called Cygnus X-1
HD 226868
Cygnus X-1
The blue supergiant is so large, that its outer atmosphere can be
drawn into the black hole. As the material spirals into the black
hole, it heats up to millions of degrees and emits X-ray radiation.
What did you think?
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Are black holes just holes in space?
No, black holes contain highly compressed matter
(with infinitely small volume) at their center. They
are not empty.
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What is at the surface of a black hole?
The surface of a black hole, called the event horizon,
is empty space-there is no stationary matter there.
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What power or force enables black holes to
draw things in?
The only force that pulls things in is the gravitational
attraction of the matter in the black hole.
Time runs more slowly in the
presence of a gravitational field.
1s
Strobe light
No gravitational field.
Time runs more slowly in the
presence of a gravitational field.
Strobe light
(according to the clock)
1s
Big gravitational field.
Discussion Question
Your doomed friend remembers that he has a rocket that
he can use to temporarily stop his descent into the black
hole. With visions of heroism in your head, you tie a rope
to your waist and jump out of your spaceship to go and
rescue him. How does time appear (to you) to progress
for you and your friend as you approach him?
1) Your own time seems to run normally and your friend’s
time seems to run faster and faster as you approach him.
2) Your own time seems to run slower and slower as you
fall and your friend’s time seems to continue to run at
the same slow rate.