Supernovae and nucleosynthesis
of elements > Fe
Death of low-mass star: White Dwarf
• White dwarfs are
the remaining cores
once fusion stops
• Electron
degeneracy pressure
supports them
against gravity
• Cool and grow
dimmer over time
A white dwarf can accrete mass
from its companion
Tycho’s supernova of 1572
Expanding at 6 million mph
Kepler’s supernova of 1609
Supernovae
outshine the
whole galaxy!
QuickTime™ and a
decompressor
are needed to see this picture.
Two kinds of supernovae
Type I: White dwarf supernova
White dwarf near 1.4 Msun accretes matter from red
giant companion, causing supernova explosion
Type II: Massive star supernova
Massive star builds up 1.4 Msun core and collapses into
a neutron star, gravitational PE released in explosion
light curve shows how luminosity changes with time
r process and s process elements
•
•
•
•
•
•
Nuclear fusion in all stars only produces up to Fe-56
Slow neutron capture (s process) forms up to Bi-209 in
low-mass stars
High temps in SN creates elements up to Ca-254
Rapid neutron capture (r process) create neutron-rich
isotopes which decay into more stable neutron-rich
elements
Neutron flux during SN is 1022 neutrons per square
centimeter per second
neutron captures occur much faster than decays
QuickTime™ and a
decompressor
are needed to see this picture.
A neutron star:
A few km in
diameter,
supported
against gravity
by degeneracy
pressure of
neutrons
Discovery of Neutron Stars
• Using a radio telescope in 1967, Jocelyn Bell
discovered very rapid pulses of radio emission
coming from a single point on the sky
• The pulses were coming from a spinning neutron
star—a pulsar
Pulsar at center
of Crab Nebula
pulses 30 times
per second
Pulsars
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
Thought Question
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
What happens if the neutron star has
more mass than can be supported by
neutron degeneracy pressure?
1. It will collapse further and become a black
hole
2. It will spin even faster, and fling material
out into space
3. Neutron degeneracy pressure can never be
overcome by gravity
• Neutron degeneracy pressure can no longer
support a neutron star against gravity if its mass is
> about 3 Msun
18.3 Black Holes: Gravity’s Ultimate
Victory
A black hole is
an object whose
gravity is so
powerful that not
even light can
escape it.
Escape Velocity
Initial Kinetic
Energy
=
Final Gravitational
Potential Energy
1 2 GmM
mv 
2
r
Where m is your mass,
M is the mass of the object that you are trying to escape from, and
r is your distance from that object

Light
would not
be able to
escape
Earth’s
surface if
you could
shrink it to
< 1 cm
“Surface” of a Black Hole
• The “surface” of a black hole is the distance at which
the escape velocity equals the speed of light.
• This spherical surface = event horizon.
• The radius of the event horizon is known as the
Schwarzschild radius.
Neutron star
The event horizon of a 3 MSun black hole is a few km
A black hole’s mass strongly warps space
and time in vicinity of event horizon
What would it be like to visit a
black hole?
Light waves take extra time to climb out of a deep hole in
spacetime, leading to a gravitational redshift
Time passes more slowly near the event horizon
Thought Question
Is it easy or hard to fall into a black hole?
A. Easy
B. Hard
Tidal forces near the
event horizon of a
3 MSun black hole
would be lethal to
humans
Tidal forces would be
gentler near a
supermassive black
hole because its radius
is much bigger
Do black holes really exist?
Black Hole Verification
•
•
Need to measure mass
— Use orbital properties of companion
— Measure velocity and distance of orbiting gas
It’s a black hole if it’s not a star and its mass
exceeds the neutron star limit (~3 MSun)
Some X-ray binaries contain compact objects of mass
exceeding 3 MSun which are likely to be black holes
One famous X-ray binary with a likely black hole is in
the constellation Cygnus
Thought Question
How does the radius of the event horizon change
when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same
Thought Question
How does the radius of the event horizon change
when you add mass to a black hole?
A. Increases
B. Decreases
C. Stays the same
Thought Question
Is it easy or hard to fall into a black hole?
A. Easy
B. Hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus
Thought Question
Is it easy or hard to fall into a black hole?
A. Easy
B. Hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus
If the Sun shrank
into a black hole, its
gravity would be
different only near
the event horizon
Black holes don’t suck!
Event
horizon is
larger for
black holes
of larger
mass
Black
holes have
only mass,
spin and
charge