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After the Main Sequence...
Eventually a star will run out of H in its core, so it can't carry out H > He fusion.
Rule of thumb: when the energy output changes in the core, the core compresses and its temperature rises. This causes the surface to expand and cool.
6 Red Giant When the core runs out of H, it collapses and heats up. This increases the rate of H > He fusion in a shell around the core. Eventually the core temperature increases to 100 million K and He begins to fuse into carbon (C) [and C + He fuses into oxygen (O)]. This increase in temp/energy output in the core expands the star's surface, which cools off. The star's surface will be larger, cooler (and redder), and more luminous. surface temp: decreases to ~ 4000 K, then increases to ~ 5000 K size: increases to ~ 100 x Sun, then decreases to ~ 10 x Sun lasts ~ 150 million years
7 Supergiant
The star runs out of He in the core, which again collapses
and heats up. This increases the fusion rate in the shells
above the corean inner shell that still fusing He > C and an
outer shell fusing H > He. This swells the surface of the star
into a supergiant.
surface temp: ~4000K
size: ~ 500 x Sun
lasts ~ 10,000 years
8 Planetary Nebula
The supergiant is unstableits outer layers get pushed off
into space as a planetary nebula. This has a side effect of
enriching the ISM with heavier elements, especially the C.
lasts: ~100,000 years
9 White Dwarf
The star's mass is too small to fuse C (or O) in the core into
anything else. Gravity compresses the core until it's stopped
by the electron degeneracy pressure. A small, hot core is left
to gradually cool off over eons.
temp: ~ 100,000 K and dropping
size: about the size of Earth
Deaths of HighMass Stars
Stars over ~ 8x Sun's mass can fuse carbon in their cores
when they get to the supergiant phase. They can even fuse
elements up through iron in their cores (but not heavier than
iron).
With this different "fusion path," the star has a very different
end to its life. With the production of iron in the core, fusion
shuts down suddenly and the core collapses almost
instantaneously upon itself. (You've got the coreabout the
size of a white dwarf/Earthcollapsing to just a few km
across.) The overlying shells of the star pile up on the core
and rebound (bounce back). The star will be blown apart as
a supernova.
The supernova can outshine an entire galaxy (~
100,000,000,000 stars) for a few weeks. The energy of the
explosion can fuse elements heavier than iron, and it sends
them off into space to be a part of the next generation of star
formation.
Supernova Remnant Cores
If less than about 3 x the Sun's mass is left in the core, a ball
of neutrons will be lefta neutron starwhich is about as
heavy as a star but only a few km across.
As the inner gets smaller, it "condenses" the magnetic field
of the star, and its rotation rate greatly increases. The
neutron star will emit beams of radiation along its magnetic
field, and sweep those beams across space as it rotates. If
we happen to be in the path of a beam, we'll get pulses of
energy like from a lighthousea pulsar.
If over about 3 x the Sun's mass is left, then gravity collapses
it completely into a black hole.
Einstein's 2 postulates (starting points to reconcile EM &
laws of motion):
1) The laws of nature are the same for all observers,
regardless of their motion.
2) The speed of light is the same for all observers,
regardless of their motion.
He first worked this out forspecial relativitydifferent
observers are not accelerating relative to each other (but can
be moving). Let's look at some of the consequences of
these assumptions/postulates in special relativity:
The speed of light is an upper limit on all speeds. Space and time are connected "Spacetime" "Length contraction" the length of an object decreases as
it moves faster.
"Time dilation" time passes more slowly the faster you go. ("Twin paradox") mass increases the faster you move
E = mc² > energy = mass x (speed of light)² There is an equivalence between mass and energy. Either can be converted into the other. (Fusion in a star: H > He. The He produced has slightly less mass than the H you start with. The mass that's "lost" is actually converted to energy, and it's the source of the energy of fusion.)
Einstein next worked the consequences of his 2 postulates for general relativity observers can be accelerating relative to each other. It turned out to be a model of gravity.
Consequences: (all the consequences of special relativity apply) Gravity can "pull" light (It doesn't slow the light, but it stretches the wavelengthwe call it a gravitational redshift.) Gravity (or mass) bends spacetime spacetime is dynamicit's part of the process of nature time slows around mass
Measured effect: bending of starlight by the Sun.
Is relativity a theory of "relativism?"
Material or objects far outside the blackhole would not notice
any change in gravitational pull, orbits, etc. for an object that
collapses to become a black hole. (The mass of the object
and the distance to it wouldn't change.)
Material near the black hole would
experience severe tidal forces, which become more and
more extreme closer to the event horizon.
be pulverized by tidal forces
orbital motions would be very fast:
friction and/or rapid motions produce:
heat material to extreme temperature
cause some of material to fall in toward b.h.
light emitted would be redshifted by the b.h.'s gravity.
material falling in would take longer & longer to fall in due to
time dilation
Title: May 21 12:08 PM (10 of 11)
We can't see a black hole directly, but we could look for the
material around it: hot gas orbiting a small object very
rapidly.
Candidates for stellarmass black holes:
object mass (x Sun's) size how far away
Cygnus X1 10 < 300 km 7000 ly
LMC X3 10? 180,000 ly
A062000 3.8 few km 2700 ly
Title: May 21 12:20 PM (11 of 11)
