Download Lecture 15, PPT version

Midterm Exam #2
Tuesday, March 23
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Closed book
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Will cover Lecture 8 (Special Relativity) through Lecture 14 (Star
Formation) only
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If a topic is in the book, but was not covered in class, it will not be on the
exam!
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Some combination of multiple choice, short answer, short calculation
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Equations, constants will all be given
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Standard calculators allowed
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Cell phones, PDAs, computers not allowed
Outline - March 18, 2010
• Protostar to “Main Sequence” star (pgs. 556-57)
• HR Diagram revisit
• Define “low”, “intermediate” and “high” mass stars (pg. 565)
• Evolution and death of low mass stars (pgs. 566-572)
• Evolution and death of high mass stars (pgs. 572-581)
Protostar to Main Sequence Star
Protostar becomes a main
sequence star with the onset
of hydrogen fusion
H-R Diagram Review
About 90% of stars in the
sky are “Main Sequence”
stars
All main sequence stars
are stable (gravity exactly
balances pressure) and
energy source is fusion of
HYDROGEN to form
HELIUM
What are all of the “nonmain squence” objects
on the HR diagram?
It’s all about stellar evolution…
What determines where a star is on the HR diagram?
Its evolutionary state!
What determines how a star evolves?
Its “main sequence” mass!
Luminosity on the MS and lifetime on the MS depend on the star’s
“main sequence” mass:
L = (M / Msun)3 Lsun
Main sequence lifetime = 10 / (M / Msun)2 billion years
Main Sequence Lifetime
(about 90% of a star’s total lifetime)
The less massive is a star, the less fuel it has, but the longer it will
last on the main sequence.
The more massive is a star, the more fuel it has, but the shorter it
will last on the main sequence.
“Low-mass” stars: born with M < 2 Msun
“Intermediate-mass” stars: born with 2 Msun < M < 8 Msun
“High-mass” stars: born with M > 8 Msun
Main Sequence lifetime of 0.2 Msun star = about 500 billion years
Main Sequence lifetime of a 10 Msun star = about 100 million years
Evolution of “Low-Mass” vs. “High-Mass” Stars
Recall: stability of star is all about pressure-gravity balance
Main Sequence star: pressure comes from conversion of H to He
Problem: He nucleus has 2 protons (H nucleus has 1 proton), so it
takes a higher density and higher temperature to fuse He than it does
to fuse H
Cores of MS stars: not hot enough or dense enough to fuse He
In a main sequence star, He is a nuclear “ash” - it doesn’t
contribute to “holding up” the star against gravitational collapse.
Build-up of Inert Helium Core
Eventually, the star builds up a substantial He
core, with H “burning” in a shell around the
core.
The H burns into layers of the star that are
thinner, and thinner, making it harder to
hold the star up against gravitational
collapse.
The He core can provide a little bit of help
by contracting (conversion of gravitational
energy, just like a protostar).
As the core contracts, the outer envelope
expands and the star leaves the main
sequence.
Evolution of a Low-Mass Star
As He core contracts, the star moves up the
HR diagram.
As outer envelope expands, the star becomes
physically larger (increases luminosity) and the
surface temperature cools (becomes redder).
Star becomes a Red Giant.
Onset of He burning in the core happens quite
suddenly (helium “flash”) once the
temperature and density of the core are
high enough to fuse He.
Helium flash doesn’t disrupt the star
(localized region of 1/1000 of the star), but
does cause the core to expand a little bit
(and envelope shrinks in response).
Red Giant Phase for Low Mass Stars
Core is now 100 million Kelvin (about 10x
hotter than when the star was a main
sequence star)
Two sources of energy:
1. H to He in a shell
2. He to C (“triple alpha” process) in the core
“Triple Alpha” Process
(nuclear fusion of Helium to produce Carbon)
Red Giants are Truly Enormous
(sun as a red giant results in end of life on Earth)
5 billion
years in
the future
Today
When the sun becomes a Red Giant it will engulf Mercury, and perhaps Venus.
The surface temperature of the sun will be about 1/2 its current temperature, but
the sun will be so large that it will take up half the noon-time sky!
End of life on earth - we’ll be toasted.
Final Stage of Evolution of Low-Mass Star
It’s only a matter of time before the star gets in trouble again…
This time it’s CARBON ash that has sunk to the center (non-burning
carbon core, surrounded by a shell of He burning, surrounded by a
shell of H burning).
Most low mass stars can repeat the core contraction process, and
ignite Carbon fusion (which produces Oxygen).
But, once a significant amount of oxygen has built up in the
core, it’s game over for the star!!
Death of a Low Mass Star
Carbon-Oxygen core contracts in an attempt to help hold the star up against
gravitational collapse; but there isn’t enough mass in the star to make the
temperature and density high enough to fuse the oxgygen
Core shrinks down to about the size of the earth, and can’t go any farther
because of a quantum mechanical effect
Can only compress electrons so far - this is what stops the core contraction
Pressure in the core is provided by “degenerate electron gas” and the core
becomes stable (no longer contracting)
“Burning fronts” (H, He, C) plow out into the very light, fluffy layers of the
(enormous!) star, and the outer layers of the star “lift off” due to radiation
pressure
Formation of White Dwarf and Planetary Nebula
(end of a low-mass star)
Outer layers of star lift off, revealing small, hot core = White Dwarf
Initially, the white dwarf is very hot, but it cools off because it has no
internal source of energy (will eventually be black!)
Sirius A
Sirius B (white dwarf
companion to Sirius A)
Planetary Nebulae
(have nothing to do with planets!)
Gas from original envelope of star is heated by the white dwarf
Evolutionary Track on the HR Diagram
(Low-Mass Star)
Evolution of High-Mass Stars
Unlike low mass stars, high mass
stars make a steady transition
from H fusion in the core to He
fusion in the core (no “helium
flash”), to O fusion in the core,
and they keep on going to heavier
chemical elements.
High-mass stars evolve off the
main sequence to become
“supergiant” stars.
“Onion Layers” of Fusion in a High-Mass Star
Star undergoes cycles of
core contraction and
envelope expansion,
fusing heavier and
heavier chemical
elements, until an iron
core forms.
Once silicon starts to
fuse, the star has about
a week to live.
Timescales of Fusion
(Mstar = 20 Msun)
H fusion in core: 10 million years
He fusion in core: 1 million years
C fusion in core: 1000 years
O Fusion in core: 1 year
Si fusion in core: 1 week
What’s so special about Iron (Fe)?
Fusion of nucleii that are lighter than iron result in a net gain of
energy (takes less energy to bring the nucleii close together than
you get from mass loss)
Fusion of nucleii that are as heavy or heavier than iron result in
a net loss of energy (takes more energy to bring the nucleii
close together than you get from mass loss)
Bottom line: star can’t use iron as a nuclear fuel to support
itself from gravitational collapse, because fusing iron is a losing
proposition in the energy balance!
Death of a High-Mass Star
Supernova: Implosion followed by Explosion
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Once substantial amount of iron has built up, star implodes on itself
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Core reaches temperature of 10 billion Kelvin (= tremendously high energy
photons), the nuclei are split apart into protons and neutrons
(“photodisintegration”)
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In less than 1 second, the star undoes most of the effects of nuclear fusion that
happened in the previous 11 million years!!!!!
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High-energy photons are absorbed, giving rise to loss of thermal energy in the core,
the core becomes even more unstable, and the collapse accelerates
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Protons and electrons in the core combine together (“neutronization”), resulting in
nothing but neutrons in the core
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Collapse continues until it’s not possible to squeeze the neutrons together any
tighter (size of core = size of Manhattan)
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Collapse starts to slow, but overshoots and outer layers of star are driven out
into space (perhaps by “bounce” off the neutron core) in a massive explosion
Supernovae Generate Tremendous Amounts of Energy
At their maximum
brightness, supernovae
are as bright as an
entire galaxy.
Peak luminosity is
about 1051 ergs = the
sun’s total output of
energy over 10 billion
years!
How long does a supernova last?
“Type II” supernovae
are exploding highmass stars
“Type Ia” supernovae
are something else
entirely (and involve
binary star systems)
Why should you care about supernovae?
• Extraordinarily bright, so can use them to measure distances
to galaxies that are very far away: b = L / (4 d2)
• Supernovae are the source of all heavy chemical elements!
• The heavy chemical elements are produced during the
explosion itself, when there is more than enough energy to
fuse nuclei heavier than iron (doesn’t matter that there is a
“net loss” of energy - the star is already VERY far out of
equilibrium)
Supernova Remnants
(high-mass star guts)
Cycle of Star Formation and Supernovae
• Stars form out of gas in the ISM, evolve, and blow much of
themselves back into the ISM
• Massive stars create heavy chemical elements during the
explosions, which “enriches” the ISM with heavy chemical
elements
• New stars form, and make yet more heavy chemical elements
• It takes about 500 cycles of massive star formation to account
for all the heavy chemical elements in the universe
• More than enough time for this to happen (universe is 14
billion years old, massive stars take a few million years to evolve
and explode)