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

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Supernovae outshine the

whole galaxy!

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

Two kinds of supernovae

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

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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. YesB. No

Thought Question

Could there be neutron stars that appear as pulsars to other civilizations but not to us?

A. YesB. 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 KineticEnergy

Final Gravitational Potential Energy

=

€

12

mv 2 = GmMr

Where m is your mass, M is the mass of the object that you are trying to escape from, andr 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.

The event horizon of a 3 MSun black hole is a few km

Neutron star

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. EasyB. 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. IncreasesB. DecreasesC. Stays the same

Thought Question

How does the radius of the event horizon change when you add mass to a black hole?

A. IncreasesB. DecreasesC. Stays the same

Thought Question

Is it easy or hard to fall into a black hole?

A. EasyB. 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. EasyB. 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