Lecture 15
April 1, 2008
Stellar Graveyards

Key Concepts:

What is a white dwarf?
What are degeneracy pressure and the Chandrasekhar limit?
What is a white dwarf supernova?
How do massive stars evolve after their "main sequence" phase?
What process leads to the implosion of massive stars?
What are neutron stars and pulsars? and how do they work?

White Dwarf, Black Dwarf

White dwarf is a naked compact (Earth size), hot core of a Sun-like star.
It is revealed after the red giant and planetary nebular phase.
A white dwarf has no internal energy source, so it cools slowly by radiating away energy. When a WD is completely cooled, it becomes a hunk of stellar soot or a "black dwarf".
These dead stellar cores could account for some of the "dark" mass of the Milky Way galaxy.
Quiz 15A: Interstellar compactor

Degeneracy and the Chandrasekhar Limit

exclusion principle: two particles cannot be at the same place at the same time
electron degeneracy pressure: when atoms are pushed close together (as in a white dwarf) so that electron shells are "touching", quantum mechanical pressure resists this action.
Chandrasekhar limit: if enough mass (about 1.4 solar mass) is piled on a WD, gravity overwhelms electron degeneracy pressure, and the WD collapses (an implosion).
Quiz 15B: Piling it on...

White Dwarf Nova and Supernova

Many or most stars are in binary or multiple systems.
A WD can grow in mass by tidally stripping and stealing gas from its companion -- watch the movie (by J. Blondin).
Nova is a WD that brightens suddenly by 100-1000 times normal brightness. The accreted material forms a disk around the WD, and fusion ignites hydrogen when enough mass accumulates.
Supernova: When a WD grows to a mass larger than the Chandrasekhar limit (about 1.4 solar mass), gravity overwhelms electron degeneracy pressure, and the WD collapses through an implosion.
Quiz 15C: WD SN is a standard candle.

Evolution of a Massive (>8 solar mass) Star

"Nuclear synthesis" -- formation of heavier atoms like carbon or oxygen by fusion (Carl Sagan: "we are all star stuff")
Fusion of heavier elements

electric force stronger for heavier atoms, and higher temperature is needed to overcome Coulomb barrier (100 million K to fuse helium)
3 helium atoms form a carbon atom ("triple alpha process")

fusing heavier atoms require higher T but less energy comes out
core collapse: iron cannot burn and release energy, no more source of heat and pressure to counter gravity --> supernova
either a neutron star or a black hole is formed.

This figure show the mass per nuclear particle as a function of atomic mass. The average mass per nuclear particle declines from hydrogen to iron and then increases again.

Remember Einstein: mass is energy.

Fusion Process in a 25 solar mass star

Fusion Process	Minimum Temperature	Energy Released	Duration
4 H --> He	5 million K	0.71%	7 million years
3 He --> C	100 million K	0.07%	500 thousand years
C + C --> Ne + He	600 million K	0.02%	300 years
Ne + Ne --> Mg + O	1500 million K	0.01%	8 months
O + O --> Si + He	2000 million K	0.03%	3 months
Si + Si --> Fe	2500 million K	0.03%	1 day

Supernova: an Explosive End (also an implosion)

Massive star ("Type II") supernova: like a massive train that is climbing a steep high mountain, running out of fuel spells an even bigger (and predictable) doom for a massive star!

Neutron stars and Pulsars

The iron core of a massive star shrinks and heats. Electron degeneracy pressures cannot halt the collapse, and electrons and protons combine to form neutrons.

H + e + energy --> n + ν

Now neutron degeneracy pressure kicks in, and this pressure may be enough to hold up the star against further collapse.
A NS is only about 10 km in size (1/100 of Earth or WD)

density of the Sun: about 1 gram per CC
density of WD: about 1 million gram (1/2 ton) per CC (about 100 times smaller in size)
density of NS: more than 1 trillion gram (million ton) per CC (another 100 times smaller in size)
Quiz 15D: galactic compactor II

strong gravity due to small size: V_esc ~ 1/2 c. Strong gravity alters the geometry of space and time around a NS (through a general relativistic effect; see Lecture 24) and bend light around it.
Pulsar:

first discovered by Jocelyn Bell in 1967 ("Little Green Men")
rapid rotation from conservation of angular momentum: more than 10,000 times smaller than the Sun, so spinning more than 10,000 times faster, with a rotation period of about 1 second.
magnetic field one trillion times stronger than on Earth
radiation is beamed along the magnetic poles like a pair of flashlights.
A pulsar is an extremely precise clock (and slowly slows down at a rate precisely measured and predictable)
Chandra and Hubble movies of the Crab pulsar (inside the Crab Nebula)
millisecond pulsars: re-awakened pulsars in binary systems spinning at a rate of 1000 times per second.