Download Lecture 19 Review

Lecture 19 Review
We have talked about our Sun. From a large gas cloud it collapses to a protostar,
heating up, compressing, finally igniting. In the process planets are formed from some
remaining high angular momentum solar gas and dust particles. A great deal of gas
and dust is blown off in the process. This all happens in 100,000 years. The sun
arrives on the main sequence of the H&R diagram where it will burns hydrogen stably
for 10 billion years. Lacking hydrogen fuel in the core, the core collapses while a shell
of hydrogen continues to burn around the core. The outer atmosphere cools, expands,
and becomes more tenuous. A smaller core begins to burn helium. This red giant
phase may last 2 billion years. Finally gravity wins and the Sun collapses to a white
dwarf with a mass of M ~ .6MSun , the rest blown away. Degenerate electron pressure
maintains the Sun at a size about R ~ 2RSun . With time the Sun will cool and evolve to
a black dwarf.
All stars, lighter or heavier, evolve through a protostar stage to the main sequence
where they burn hydrogen. The more massive the star, the faster the hydrogen core
fuel is consumed. In the end, all stars become unstable when their hydrogen fusion
source is exhausted. Instability produces strong stellar winds in which considerable
gas is expelled. Doppler shifts show expelled gas with speeds of 1500 to 1900 km/s.
This gas produces planetary nebulae.
In the end gravity wins.
.08 to .25 MSun - These stars can only generate enough pressure to burn hydrogen.
Their lifetime is greater than that of the Big Bang. They collapse to a helium white
dwarf with R ~ RSun , density 105 to 108 g/cc (1 g/cc = water).
.25 to 8 MSun - Similar to the Sun. For stars approaching 8 MSun greater temperatures
and pressures are exerted on the core. Enough pressure can be exerted on an 8 MSun
star to burn carbon, oxygen, neon, and magnesium. Up to 80% of the mass is expelled
in the process. A degenerate white dwarf is left with mass from 1.2 MSun to 1.4 MSun .
1.4 MSun is called the Chandrasahkar Limit - Above this limit electron degeneracy
pressure can no longer sustain the surface and further collapse occurs.
M > 8MSun Further burning produces iron. The slow neutron process produces some
heavy elements. There are no more fusion processes. Beyond this point it takes energy
to make a heavier element. At this point gravitational collapse occurs followed by a
catastrophic rebound. A fast neutron process produces heavy elements all the way up
to Plutonium. There is a strong neutron burst and a supernova explosion. For a few
days the supernova has a luminosity L = 10 billion Lsun and then begins to dim. A
rapidly expanding nebula marks the spot of the event.
The Type 1 supernova results from a
binary star system where one of the
stars is a very large white dwarf at the
Chandrasahkar limit. If the
companion star happens to be
shedding large amounts of gas, then
the white dwarf becomes a supernova.
The Type 2 supernova is just a very
large star.
The question is, what happens after the
supernova occurs? The answer
depends on how much mass is left.
If what remains is between 1.4 and 2.5 MSun , then under great pressure the
electrons combine with protons to form neutrons. The density is ~ 1014 g/cc and the
diameter is on the order of 20 km, the size of a fair sized city. This is a neutron star.
Conservation of angular momentum produces a very high rotation rate. Escaping
electrons flow out rotating magnetic field lines and produce synchrotron radiation.
This radiation is directional and acts like a light house beacon. The result is radiation
that oscillates. When observed these are called pulsars.
If what remains is M > 2.5 MSun then gravity really wins big time, you end up with an
object whose gravitational force is so large that even light cannot escape the attraction.
This is a black hole, and you can’t see it, no matter how close or far you are from it.
However, if the black hole is part of a binary system, infalling gasses from the other
star heat up, ionize, and emit x-rays. While you can’t see the black hole, the x-rays are
an indiction that the black hole is there.