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Stellar Evolution
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The birth, Life and Death of stars
Assigned Reading: up to Chapter 12
A main sequence star is the one which is
supported by hydrogen fusion
The main sequence
exists because stars
balance their weight
with energy outflows,
produced by nuclear
fusion in their core
41H --> 4He + energy ( E = mc2 )
mass(4He) = 0.993{mass(H)+mass(H)+mass(H)+mass(H)}
mass loss is 0.007{mass(4H)} = 5 x 10-29 kg
E=mc2=(5 x 10-29)(3 x 108)2 = 4 x 10-12 joules
How many fusions per second?
Solor Luminosity = 4 x 1026 joules/sec
4 x 1026 joules/sec
----------------------- = 1038 fusions/sec = 200 million tons/sec!!!
4 x 10-12 joules
Actually, about 500 million tons/sec are needed!
A main-sequence star can hold its structure for a very
long time. Why?
Time = c2 M / L = c2 M / M3.5 = 1 / M2.5
Thermal
Pressure
Gravitational
Contraction
How does a
star hold itself?
This balance between weight and
pressure is called hydrostatic
equilibrium. Four key equations
govern how stars work (shell by shell)
1.
Hydrostatic Equilibrium
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2.
Energy transport
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3.
Energy moves from hotter to cooler
places by radiation,
convection,conduction
Conservations of mass
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4.
Weight on each shell balanced by
pressure
Sum of all shells equals total mass of
star
Conservation of Energy
•
Total luminosity equals sum of
energy produced in each shell
The Solar Thermostat
Outward thermal pressure of core
is larger than inward gravitational
pressure
Core expands
Nuclear fusion rate
rises dramatically
Contracting core heats up
Core contracts
Expanding core cools
Nuclear fusion rate
drops dramatically
Outward thermal pressure
of core drops (and becomes
smaller than inward grav. pressure)
Pressure and Temperature of a Gas
Main Sequence Stars
•Main
Sequence stars are all fusing H to He in
their cores.
•Life time of a star is determined by its mass.
•Nature makes more low-mass stars than highmass stars. Low-mass stars also live longer. That
is why there are a lot more low-mass stars.
What happens after the main sequence (when
hydrogen in the core runs out)?
Low-End of Main Sequence
Very common stars, but very hard to see
This one is CHRX 73 A+B, a 0.3 Mo red dwarf plus a 15 MJ brown dwarf
High-End of Main Sequence
Very luminous byt very rare
Stars.
Very hard to measure the mass
Also, very hard to find stars with
M>100 Mo.
Large mass ejection
This one is Eta Carinae: two
Stars, one of 60 Mo and the
The other of 70 Mo.
When core hydrogen fusion ceases, a
main-sequence star becomes a giant
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The thermal pressure in the core can no longer support the
weight of the outer layers.
The enormous weight from the outer layers compresses
hydrogen in the layers just outside the core enough to initiate
shell hydrogen fusion.
This fusion takes place at very high temperatures and the new
thermal pressure causes the outer layers to expand into a giant
star.
Both the cooling/collapsing inert He core and the H-burning
shell contributes to energy output. Star overproduces energy: it
expands, surface cools, and becomes a luminous giant
Anatomy of a Star that is
leaving the Main Sequence
Hydrogen
fuel
Helium
“ash”
Hydrogen
burning core
shell
ABSOLUTELY NOT
IN SCALE:
In a 5 Mo star,
if core has size of a
quarter, envelope has
size of a baseball
diamond.
Yet, core contains
12% of mass
Up the red giant branch
Eventually, hydrogen will burn only in the outer parts of the
mostly-helium core. The star will swell to enormous size and
luminosity, and its temperature will drop, becoming a red
giant.
Sun in ~5 Gyr
Sun today
How does the Helium core push back?
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As matter compresses, it
becomes denser (and heats up!)
Eventually, the electrons are
forced to be too close together.
A quantum mechanical law called
the Pauli Exclusion Principle
restricts electrons from being in
the same state (i.e., keeps them
from being too close together).
The resulting outward pressure
which keeps the electrons apart
is called electron degeneracy
pressure – this is what supports
the core
Stars with M > 3 Mo never
develop degenerate He core
Indistinguishable particles
are not allowed to stay in
the same quantum state.
Helium fusion begins at the
center of a giant
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While the exterior layers expand, the helium core continues to
contract and eventually becomes hot enough (100 million Kelvin)
for helium to begin to fuse into carbon (if M > 0.5 Mo)
 Carbon ash is deposited in core and eventually a heliumburning shell develops. This shell is itself surrounded by a
shell of hydrogen undergoing nuclear fusion.
He fuses through a number of reactions, generally
referred to as the “3-a” reactions
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He + He + He = C + energy
… and produces an element “crucial” to our existence:
 CARBON
For a star with M<Msun, the carbon core
never gets hot enough to ignite nuclear
fusion (star needs 600,000,000 K to do so).
After helium fusion gets going…
The Sun will expand and cool again, becoming a red (super)
giant. Earth, cooked to a cinder during the red giant phase, will
be engulfed and vaporized within the Sun. At the end of this
stage, the Sun’s core will consist mostly of carbon, with a little
oxygen.
For low mass stars
Planetary Nebula
At the center of the nebula
there is the dying star.
Destiny of stars with roughly M
< 8Mo
M <0.4 Mo He WD
M < 4 Mo, C WD
M < 8 Mo, C + O + Si WD
Nuclear burning in massive stars (>4 Mo)
The lead-up to
disaster in
massive stars
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Iron cores do not
immediately collapse due
to electron degeneracy
pressure.
If the density continues to
rise, eventually the
electrons are forced to
combine with the protons
– resulting in neutrons.
What comes next … is
core collapse.
Massive Star Explosions: Supernovae
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The gravitational collapse of the core releases an
enormous amount of energy.
All the shells ignite, and the stars literally explodes
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100 times the total amount of energy produced by the Sun
in its lifetime is released in a matter of seconds.
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For a few days, the star is ~as luminous as a whole galaxy!!!
Then luminosity decays in following months:
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It can fully disintegrates, nothing is left of it (Type Ia)
Or a neutron star or black hole (core cadaver) is left (Type II)
E.g. A Type Ia SN dims by a factor of 100 in about 170 days
Chart of light intensity versus time is called “Light Curve”
(see fig13-13, page 300).
Supernova 1987a before/after
Supernova Remnant Cassiopeia A
End Products of Stars
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M > 8 Msun  Supernova + neutron star or a
black hole
0.08 Msun < M < 8 Msun  White dwarf
M < 0.08 Msun  Brown dwarf (fusion never
starts)
Stellar Evolution in a Nutshell
M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mass controls the
evolution of a star!
Mcore > 3MSun
O
All of the Heavy Elements are Made During Supernovae
The Key Point in the Production of
Elements in the Universe
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Hydrogen and Helium are initially created in the
Big Bang
Stars process Hydrogen and Helium into heavier
elements (elements lighter than iron) during
their lives.
Elements heavier than iron are generated only in
the deaths of high mass stars (supernovae).
We were all once fuel for a stellar furnace.
Parts of us were formed in a supernova.
Where does the energy come from in a star like the Sun? Why?
Nuclear fusion.
What elements can such a star produce?
Carbon and Oxygen.
Why cannot the star produce heavier element?
not enough mass to reach the temperature.
Why more massive stars have higher central temperatures?
high pressure to balance the gravity.
What is the heaviest element that can be fused into in a star? Why?
Iron, which is the most bound nucleus.