Download Stellar Evolution

How Stars Evolve
• Pressure and temperature
– Normal gases
– Degenerate gases
• The fate of the Sun
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Red giant phase
Horizontal branch
Asymptotic branch
Planetary nebula
White dwarf
Normal gas
• Pressure is the force exerted by atoms in a gas
• Temperature is how fast atoms in a gas move
• Hotter

atoms move faster  higher
pressure
• Cooler

atoms move slower 
lower pressure
Pressure balances gravity, keeps stars from collapsing
Degenerate gas
• Very high density
• Motion of atoms is not due to kinetic
energy, but instead due to quantum
mechanical motions
• Pressure no longer depends on temperature
• This type of gas is sometimes found in the
cores of stars
Fermi exclusion principle
• No two electrons can occupy the same
quantum state
• Quantum state = energy level + spin
• Electron spin = up or down
Electron orbits
Only two electrons (one up, one down) can go into each
energy level
Electron energy levels
• Only two electrons (one up,
one down) can go into each
energy level.
• In a degenerate gas, all low
energy levels are filled.
• Electrons have energy, and
therefore are in motion and exert
pressure even if temperature is
zero.
Which of the following is a key difference
between the pressure in a normal gas and in a
degenerate gas?
1. Degenerate pressure exists whether matter
is present or not.
2. In a degenerate gas pressure varies rapidly
with time.
3. In a degenerate gas, pressure does not
depend on temperature.
4. In a degenerate gas, pressure does not
depend on density.
The Fate of the Sun
• How will the Sun evolve over time?
• What will be its eventual fate?
Sun’s Structure
• Core
– Where nuclear fusion
occurs
• Envelope
– Supplies gravity to keep
core hot and dense
Main Sequence Evolution
• Core starts with same
fraction of hydrogen as
whole star
• Fusion changes H  He
• Core gradually shrinks and
Sun gets hotter and more
luminous
Gradual change in size of Sun
Now 40% brighter, 6% larger, 5% hotter
Main Sequence Evolution
• Fusion changes H  He
• Core depletes of H
• Eventually there is not
enough H to maintain
energy generation in the
core
• Core starts to collapse
Red Giant Phase
• He core
– No nuclear fusion
– Gravitational contraction
produces energy
• H layer
– Nuclear fusion
• Envelope
– Expands because of
increased energy production
– Cools because of increased
surface area
Sun’s Red Giant Phase
HR diagram
Giant phase is when core has been fully converted to Helium
A star moves into the giant phase
when:
1. It eats three magic beans
2. The core becomes helium and fusion in
the core stops.
3. Fusion begins in the core
4. The core becomes helium and all fusion in
the star stops.
Broken Thermostat
• As the core contracts,
H begins fusing to He
in a shell around the
core
• Luminosity increases
because the core
thermostat is broken—
the increasing fusion
rate in the shell does
not stop the core from
contracting
Helium fusion
Helium fusion does not begin right away because it
requires higher temperatures than hydrogen fusion—larger
charge leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three He nuclei to make carbon
Helium Flash
• He core
– Eventually the core gets hot
enough to fuse Helium into
Carbon.
– This causes the temperature to
increase rapidly to 300 million K
and there’s a sudden flash when
a large part of the Helium gets
burned all at once.
– We don’t see this flash because
it’s buried inside the Sun.
• H layer
• Envelope
Movement on HR diagram
Movement on HR diagram
Helium Flash
• He core
– Eventually the core gets
hot enough to fuse Helium
into Carbon.
– The Helium in the core is
so dense that it becomes
a degenerate gas.
• H layer
• Envelope
Red Giant after Helium Ignition
• He burning core
– Fusion burns He into C, O
• He rich core
– No fusion
• H burning shell
– Fusion burns H into He
• Envelope
– Expands because of
increased energy
production
Sun moves onto horizontal branch
Sun burns He
into Carbon
and Oxygen
Sun becomes
hotter and
smaller
What happens
next?
What happens when the star’s core
runs out of helium?
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The star explodes
Carbon fusion begins
The core starts cooling off
Helium fuses in a shell around the core
Helium burning in the core stops
H burning is continuous
He burning happens in
“thermal pulses”
Core is degenerate
Sun moves
onto
Asymptotic
Giant
Branch
(AGB)
Sun looses mass via winds
• Creates a “planetary nebula”
• Leaves behind core of carbon and oxygen
surrounded by thin shell of hydrogen
• Hydrogen continues to burn
Planetary nebula
Planetary nebula
Planetary nebula
Hourglass
nebula
When on the horizontal branch, a
solar-mass star
1.
2.
3.
4.
Burns H in its core.
Burns He in its core.
Burns C and O in its core.
Burns He in a shell around the core.
White dwarf
• Star burns up rest of hydrogen
• Nothing remains but degenerate core of Oxygen
and Carbon
• “White dwarf” cools but does not contract
because core is degenerate
• No energy from fusion, no energy from
gravitational contraction
• White dwarf slowly fades away…
Evolution on HR diagram
Time line for Sun’s evolution
In which order will a single star of one solar
mass progress through the various stages of
stellar evolution?
1. Planetary nebula, main-sequence star, white
dwarf, black hole
2. Proto-star, main-sequence star, planetary
nebula, white dwarf
3. Proto-star, red giant, supernova, planetary
nebula
4. Proto-star, red giant, supernova, black hole
Pulsating stars
• Hydrodynamic equilibrium
• Pulsating stars
• Distance indicators
If a star is neither expanding nor contracting, we
may assume that throughout the star there is a
balance between pressure and
1.
2.
3.
4.
temperature
density
luminosity
gravity
Do mass on spring demo
Pulsating stars
Pulsating stars
• This should happen in all stars
• We already know there are small
oscillations visible on the surface of the sun
that represent sound waves that travel deep
into the interior
• In most stars, the pulsations damp out
Pulsating stars
• To have large amplitudes need driven oscillations
• Pulsations in Cepheids and RR Lyrae stars are driven by opacity
changes:
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Layer near surface is heated to ~40,000 K which ionizes He+ to He++
Freed electrons scatter, opacity shoots up
Base of opaque layer absorbs light, increasing temperature and pressure
Increased pressure makes layer expand
Expansion leads to cooling, He++ recombines to He+
Opacity drops, trapped photons leave layer
Layer contracts
Oscillations grow to large amplitude because period for opacity
changes matches period of acoustic waves
Pulsation
cycle
Rate of fusion in
the core stays
constant.
Transport of
energy through
outer layers of
star oscillates.
Pulsating stars
Pulsating stars
Why is this useful?
Flux versus luminosity relation
Flux A Luminosity

Flux B Luminosity
A
B
 Distance B 


 Distance A 
2
We can figure out the luminosity of a
pulsating star by timing the pulsations.
Since, we can measure its flux, we can
then find the distance to the star.
A Cepheid has the same pulsation
period, but is 1/16 the brightness of
another Cepheid known to be at a
distance of 2 kpc. How far away is the
dimmer star?
1.
2.
3.
4.
5.
2 kpc
4 kpc
8 kpc
16 kpc
32 kpc
Death of stars
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Final evolution of the Sun
Determining the age of a star cluster
Evolution of high mass stars
Where were the elements in your body made?
Higher mass protostars contract faster
Hotter
Higher mass stars spend less time on
the main sequence
Determining the age of a star cluster
• Imagine we have a cluster of stars that were
all formed at the same time, but have a
variety of different masses
• Using what we know about stellar evolution
is there a way to determine the age of the
star cluster?
Turn-off point of cluster reveals age
The HR diagram for a cluster of stars shows stars
with spectral types A through K on the main
sequence and stars of type O and B on the (super)
giant branch. What is the approximate age of the
cluster?
1.
2.
3.
4.
1 Myr
10 Myr
100 Myr
1 Gyr
Higher mass stars do
not have helium flash
Nuclear burning
continues past
Helium
1. Hydrogen burning: 10 Myr
2. Helium burning: 1 Myr
3. Carbon burning: 1000 years
4. Neon burning: ~10 years
5. Oxygen burning: ~1 year
6. Silicon burning: ~1 day
Finally builds up an inert Iron core
Multiple Shell Burning
• Advanced nuclear
burning proceeds in
a series of nested
shells
Why does fusion stop at Iron?
Fusion versus Fission
Advanced reactions in stars make elements like Si, S, Ca, Fe
Supernova Explosion
• Core degeneracy
pressure goes away
because electrons
combine with
protons, making
neutrons and
neutrinos
• Neutrons collapse to
the center, forming a
neutron star
Core collapse
• Iron core is degenerate
• Core grows until it is too heavy to support itself
• Core collapses, density increases, normal iron
nuclei are converted into neutrons with the
emission of neutrinos
• Core collapse stops, neutron star is formed
• Rest of the star collapses in on the core, but
bounces off the new neutron star (also pushed
outwards by the neutrinos)
If I drop a ball, will it bounce
higher than it began?
Do 8B10.50 - Supernova Core Bounce
Supernova explosion
Crab nebula
Cas A
In 1987 a nearby supernova gave us a
close-up look at the death of a
massive star
Neutrinos from SN1987A
Where do the elements in your
body come from?
• Solar mass star produce elements up to Carbon
and Oxygen – these are ejected into planetary
nebula and then recycled into new stars and
planets
• Supernova produce all of the heavier elements
– Elements up to Iron can be produced by fusion
– Elements heavier than Iron are produced by the
neutrons and neutrinos interacting with nuclei in the
supernova explosion
Energy and neutrons released in supernova explosion enable elements
heavier than iron to form, including Au and U
Types of Supernovae
• Type I – no hydrogen absorption lines
– Ia – no hydrogen lines, no helium lines, late in decay
strongest lines are iron
– Ib – strong helium lines, still no hydrogen
• Type II – hydrogen absorption lines
• Collapse of massive stars (previous slides) leads
to type II and Ib, only difference is whether or
not star sheds outer hydrogen layer before
exploding
Type Ia supernova
• Thought to be result of a white dwarf under
going fusion of carbon and oxygen into
iron. Star is evaporated and no remnant
remains.
• Occurs either when matter accretes onto a
white dwarf or two white dwarfs collide.
Review Questions
• What are the evolutionary stages of a one solar
mass star?
• How does the evolution of a high mass star
differ from that of a low mass star?
• How can the age of a cluster of stars, all formed
at the same time, be determined?
• Why does fusion stop at Iron?
• How are heavy elements produced?