How Stars Evolve
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Transcript of How Stars Evolve
![Page 1: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/1.jpg)
How Stars Evolve
• Pressure and temperature– Normal gases
– Degenerate gases
• The fate of the Sun– Red giant phase
– Horizontal branch
– Asymptotic branch
– Planetary nebula
– White dwarf
![Page 2: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/2.jpg)
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
![Page 3: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/3.jpg)
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
![Page 4: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/4.jpg)
Fermi exclusion principle
• No two electrons can occupy the same quantum state
• Quantum state = energy level + spin
• Electron spin = up or down
![Page 5: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/5.jpg)
Electron orbits
Only two electrons (one up, one down) can go into each energy level
![Page 6: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/6.jpg)
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.
![Page 7: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/7.jpg)
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.
![Page 8: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/8.jpg)
The Fate of the Sun
• How will the Sun evolve over time?
• What will be its eventual fate?
![Page 9: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/9.jpg)
Sun’s Structure
• Core– Where nuclear fusion
occurs
• Envelope– Supplies gravity to keep
core hot and dense
![Page 10: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/10.jpg)
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
![Page 11: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/11.jpg)
Gradual change in size of Sun
Now 40% brighter, 6% larger, 5% hotter
![Page 12: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/12.jpg)
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
![Page 13: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/13.jpg)
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
![Page 14: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/14.jpg)
Sun’s Red Giant Phase
![Page 15: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/15.jpg)
HR diagram
Giant phase is when core has been fully converted to Helium
![Page 16: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/16.jpg)
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.
![Page 17: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/17.jpg)
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
![Page 18: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/18.jpg)
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 fusion
![Page 19: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/19.jpg)
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
![Page 20: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/20.jpg)
Movement on HR diagram
![Page 21: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/21.jpg)
Movement on HR diagram
![Page 22: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/22.jpg)
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
![Page 23: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/23.jpg)
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
![Page 24: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/24.jpg)
Sun moves onto horizontal branch
Sun burns He into Carbon and Oxygen
Sun becomes hotter and smaller
What happens next?
![Page 25: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/25.jpg)
What happens when the star’s core runs out of helium?
– The star explodes– Carbon fusion begins– The core starts cooling off– Helium fuses in a shell around the core
![Page 26: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/26.jpg)
Helium burning in the core stops
H burning is continuous
He burning happens in “thermal pulses”
Core is degenerate
![Page 27: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/27.jpg)
Sun moves onto
Asymptotic Giant
Branch (AGB)
![Page 28: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/28.jpg)
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
![Page 29: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/29.jpg)
![Page 30: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/30.jpg)
Planetary nebula
![Page 31: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/31.jpg)
Planetary nebula
![Page 32: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/32.jpg)
Planetary nebula
![Page 33: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/33.jpg)
Hourglass nebula
![Page 34: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/34.jpg)
When on the horizontal branch, a solar-mass star
1. Burns H in its core.
2. Burns He in its core.
3. Burns C and O in its core.
4. Burns He in a shell around the core.
![Page 35: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/35.jpg)
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…
![Page 36: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/36.jpg)
Evolution on HR diagram
![Page 37: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/37.jpg)
Time line for Sun’s evolution
![Page 38: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/38.jpg)
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
![Page 39: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/39.jpg)
Pulsating stars
• Hydrodynamic equilibrium
• Pulsating stars
• Distance indicators
![Page 40: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/40.jpg)
If a star is neither expanding nor contracting, we may assume that throughout the star there is a balance between pressure and
1. temperature
2. density
3. luminosity
4. gravity
Do mass on spring demo
![Page 41: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/41.jpg)
Pulsating stars
![Page 42: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/42.jpg)
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
![Page 43: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/43.jpg)
Pulsating stars• To have large amplitudes need driven oscillations
• Pulsations in Cepheids and RR Lyrae stars are driven by opacity changes:– 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
![Page 44: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/44.jpg)
Pulsation cycle
Rate of fusion in the core stays constant.
Transport of energy through outer layers of star oscillates.
![Page 45: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/45.jpg)
Pulsating stars
![Page 46: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/46.jpg)
Pulsating stars
![Page 47: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/47.jpg)
Why is this useful?
Flux versus luminosity relation2
A
B
B
A
B
A
Distance
Distance
Luminosity
Luminosity
Flux
Flux
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.
![Page 48: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/48.jpg)
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 kpc
2. 4 kpc
3. 8 kpc
4. 16 kpc
5. 32 kpc
![Page 49: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/49.jpg)
Death of stars
• 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?
![Page 50: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/50.jpg)
Higher mass protostars contract fasterHotter
![Page 51: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/51.jpg)
Higher mass stars spend less time on the main sequence
![Page 52: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/52.jpg)
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?
![Page 53: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/53.jpg)
Turn-off point of cluster reveals age
![Page 54: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/54.jpg)
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. 1 Myr
2. 10 Myr
3. 100 Myr
4. 1 Gyr
![Page 55: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/55.jpg)
Higher mass stars do not have helium flash
![Page 56: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/56.jpg)
Nuclear burning continues past
Helium
1. Hydrogen burning: 10 Myr2. Helium burning: 1 Myr3. Carbon burning: 1000 years4. Neon burning: ~10 years5. Oxygen burning: ~1 year6. Silicon burning: ~1 day Finally builds up an inert Iron core
![Page 57: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/57.jpg)
Multiple Shell Burning• Advanced nuclear
burning proceeds in a series of nested shells
![Page 58: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/58.jpg)
Why does fusion stop at Iron?
![Page 59: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/59.jpg)
Fusion versus Fission
![Page 60: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/60.jpg)
Advanced reactions in stars make elements like Si, S, Ca, Fe
![Page 61: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/61.jpg)
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
![Page 62: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/62.jpg)
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)
![Page 63: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/63.jpg)
If I drop a ball, will it bounce higher than it began?
Do 8B10.50 - Supernova Core Bounce
![Page 64: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/64.jpg)
Supernova explosion
![Page 65: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/65.jpg)
Crab nebula
![Page 66: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/66.jpg)
Cas A
![Page 67: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/67.jpg)
In 1987 a nearby supernova gave us a close-up look at the death of a
massive star
![Page 68: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/68.jpg)
Neutrinos from SN1987A
![Page 69: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/69.jpg)
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
![Page 70: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/70.jpg)
Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U
![Page 71: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/71.jpg)
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
![Page 72: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/72.jpg)
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.
![Page 73: How Stars Evolve](https://reader036.fdocuments.net/reader036/viewer/2022062718/56812fee550346895d9564a7/html5/thumbnails/73.jpg)
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?