supernovae and nucleosynthesis of elements > fe death of low-mass star: white dwarf white dwarfs...
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Supernovae and nucleosynthesis of elements > Fe
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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
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A white dwarf can accrete mass from its companion
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Tycho’s supernova of 1572
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Expanding at 6 million mph
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Kepler’s supernova of 1609
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QuickTime™ and a decompressor
are needed to see this picture.
Supernovae outshine the
whole galaxy!
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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
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light curve shows how luminosity changes with time
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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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QuickTime™ and a decompressor
are needed to see this picture.
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A neutron star:
A few km in diameter, supported against gravity by degeneracy pressure of neutrons
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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
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Pulsar at center of Crab Nebula pulses 30 times per second
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Pulsars
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Thought Question
Could there be neutron stars that appear as pulsars to other civilizations but not to us?
A. YesB. No
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Thought Question
Could there be neutron stars that appear as pulsars to other civilizations but not to us?
A. YesB. No
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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
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• Neutron degeneracy pressure can no longer support a neutron star against gravity if its mass is > about 3 Msun
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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.
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Escape Velocity
Initial KineticEnergy
Final Gravitational Potential Energy
=
€
1
2mv 2 =
GmM
r
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
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Light would not be able to escape Earth’s surface if you could shrink it to < 1 cm
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“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.
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The event horizon of a 3 MSun black hole is a few km
Neutron star
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A black hole’s mass strongly warps space and time in vicinity of event horizon
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What would it be like to visit a black hole?
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Light waves take extra time to climb out of a deep hole in spacetime, leading to a gravitational redshift
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Time passes more slowly near the event horizon
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Thought Question
Is it easy or hard to fall into a black hole?
A. EasyB. Hard
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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
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Do black holes really exist?
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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)
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Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes
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One famous X-ray binary with a likely black hole is in the constellation Cygnus
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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
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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
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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
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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
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If the Sun shrank into a black hole, its gravity would be different only near the event horizon
Black holes don’t suck!
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Event horizon is larger for black holes of larger mass
Black holes have only mass, spin and charge