1. Star Stuff Chapter 17

2. Leaving the Main Sequence
The core begins to collapse
H shell heats up and H fusion begins there
there is less gravity from above to balance this pressure
so the outer layers of the star expand
the star is now in the subgiant phase of its life

3. Stellar Mass and Fusion
The mass of a main-sequence star determines its core pressure and temperature.
Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived.
Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes.
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4. High-Mass Stars
> 8MSun
Intermediate-Mass Stars
Low-Mass Stars
< 2MSun
Brown Dwarfs

5. When core hydrogen fusion ceases, a main-sequence star becomes a giant
When hydrogen in the core is no longer fusing into helium, the star can no longer support its weight The enormous weight from the outer layers compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion. This extra internal heat causes the outer layers to expand into a giant star.

7. Helium fusion begins at the center of a giant
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 and oxygen
core helium fusion
3 He  C + energy and C + He  O + energy
occurs rapidly - called the Helium Flash

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Helium Flash
The thermostat of a low-mass red giant is broken because degeneracy pressure supports the core.
Core temperature rises rapidly when helium fusion begins.
Helium fusion rate skyrockets until thermal pressure takes over and expands the core again.
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9. Life Track after Helium Flash
Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

10. Life Track after Helium Flash
Observations of star clusters agree with those models.
Helium-burning stars are found on a horizontal branch on the H-R diagram.

11. Red Giants The He core collapses until it heats to 108 K
He fusion begins ( He  C)
sometimes called the “triple- process”
The star, called a Red Giant, is once again stable.
gravity vs. pressure from He fusion reactions
red giants create and release most of the Carbon from which organic molecules (and life) are made

12. Red Giants
Helium-burning stars neither shrink nor grow because core thermostat is temporarily fixed.

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Double Shell Burning
After core helium fusion stops, helium fuses into carbon in a shell around the carbon core, and hydrogen fuses to helium in a shell around the helium layer.
This double shell–burning stage never reaches equilibrium—fusion rate periodically spikes upward in a series of thermal pulses.
With each spike, convection dredges carbon up from core and transports it to surface.
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14. Planetary Nebulae When the Red Giant exhausts its He fuel
the C core collapses
Low mass stars don’t have enough gravitational energy to heat to 6 x 108 K (temperature where Carbon fuses)
The He & H burning shells overcome gravity
the outer envelope of the star is gently blown away
this forms a planetary nebula
This animation is from the Hubble Space Telescope archive and is in the public domain.
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Movie. Click to play.

15. Planetary Nebulae

16. Planetary Nebulae
Cat’s Eye Nebula
Twin Jet Nebula

17. Planetary Nebulae Ring Nebula Hourglass Nebula
The collapsing Carbon core becomes a White Dwarf

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End of Fusion
Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some helium fuses to carbon to make oxygen).
Degeneracy pressure supports the white dwarf against gravity.
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Earth's Fate
The Sun's luminosity will rise to 1000 times its current level—too hot for life on Earth.
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Earth's Fate
The Sun's radius will grow to near current radius of Earth's orbit.
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21. Someday it will all be over for Earth…

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What have we learned?
What are the life stages of a low-mass star?
Hydrogen fusion in core (main sequence)
Hydrogen fusion in shell around contracting core (red giant)
Helium fusion in core (horizontal branch)
Double shell burning (red giant)
How does a low-mass star die?
Ejection of hydrogen and helium in a planetary nebula leaves behind an inert white dwarf.
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23. 17.3 Life as a High-Mass Star
Our goals for learning:
What are the life stages of a high-mass star?
How do high-mass stars make the elements necessary for life?
How does a high-mass star die?
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24. High Mass Main Sequence Stars
The CNO cycle is another nuclear fusion reaction which converts Hydrogen into Helium by using Carbon as a catalyst.
Effectively 4 H nuclei go IN and 1 He nucleus comes OUT.

25. High Mass Main Sequence Stars
CNO cycle begins at 15 million degrees and becomes more dominant at higher temperatures.
The C nucleus has a (+6) charge, so the incoming proton must be moving even faster to overcome the electromagnetic repulsion!!
The Sun (G2) -- CNO generates 10% of its energy
F0 dwarf CNO generates 50% of its energy
O & B dwarfs -- CNO generates most of the energy

26. Life Stages of High-Mass Stars
Late life stages of high-mass stars are similar to those of low-mass stars:
Hydrogen core fusion (main sequence)
Hydrogen shell burning (supergiant)
Helium core fusion (supergiant)
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27. Supergiants
What happens to the high mass stars when they exhaust their He fuel?
They have enough gravitational energy to heat up to 6 x 108 K.
C fuses into O
C is exhausted, core collapses until O fuses.
The cycle repeats itself.
O  Ne  Mg  Si  Fe

29. Multiple Shell Burning
Advanced nuclear burning proceeds in a series of nested shells.

30. Supergiants on the H-R Diagram
As the shells of fusion around the core increase in number:
thermal pressure overbalances the lower gravity in the outer layers
the surface of the star expands
the surface of the star cools
The star moves toward the upper right of H-R Diagram
it becomes a red supergiant
example: Betelgeuse
For the most massive stars:
the core evolves too quickly for the outer layers to respond
they explode before even becoming a red supergiant

31. The Iron (Fe) Problem
The supergiant has an inert Fe core which collapses & heats
Fe can not fuse
It has the lowest mass per nuclear particle of any element
It can not fuse into another element without creating mass
So the Fe core continues to collapse until it is stopped by electron degeneracy.
(like a White Dwarf)

32. Supernova
BUT… the force of gravity increases as the mass of the Fe core increases
Gravity overcomes electron degeneracy
Electrons are smashed into protons  neutrons
The neutron core collapses until abruptly stopped by neutron degeneracy
this takes only seconds
The core recoils and sends the rest of the star flying into space

33. Supernova
The amount of energy released is so great, that most of the elements heavier than Fe are instantly created
In the last millennium, four supernovae have been observed in our part of the Milky Way Galaxy: in 1006, 1054, 1572, & 1604
Crab Nebula in Taurus
supernova exploded in 1054

34. Tycho’s Supernova (X-rays)
Supernovae
Tycho’s Supernova (X-rays)
exploded in 1572
Veil Nebula

35. Summary of the Differences between High and Low Mass Stars
Compared to low-mass stars, high-mass stars:
live much shorter lives
have convective cores, but no other convective layers
while low-mass stars have convection layers at their surfaces
have a significant amount of pressure supplied by radiation
fuse Hydrogen via the CNO cycle instead of the p-p chain
die as a supernova; low-mass stars die as a planetary nebula
can fuse elements heavier than Carbon
may leave either a neutron star or black hole behind
low-mass stars leave a white dwarf behind
are far less numerous

36. Close Binary Stars
Most stars are not single – they occur in binary or multiple systems.
binary stars complicate our model of stellar evolution
Remember that mass determines the life path of a star.
two stars in a binary system can be close enough to transfer mass from one to the other
gaining or losing mass will change the life path of a star
For example, consider the star Algol in the constellation Perseus.
Algol is a close, eclipsing binary star consisting of…
a main sequence star with mass = 3.7 M & a subgiant with mass = 0.8 M
since they are in a binary, both stars were born at the same time
yet the less massive star, which should have evolved more slowly, is in a more advanced stage of life
This apparent contradiction to our model of stellar evolution is known as the Algol Paradox.

37. The Algol Paradox Explained
This paradox can be explained by mass exchange.
The 0.8 M subgiant star used to be the more massive of the two stars.
When the Algol binary formed:
it was a 3 M main sequence star…
with a 1.5 M main sequence companion
As the 3 M star evolved into a red giant
tidal forces began to deform the star
the surface got close enough to the other star so that gravity…
pulled matter from it onto the other star
As a result of mass exchange, today…
the giant lost 2.2 M and shrunk into a subgiant star
the companion is now a 3.7 M MS star

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Role of Mass
A star's mass determines its entire life story because it determines its core temperature.
High-mass stars with > 8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.
Low-mass stars with < 2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.
Intermediate-mass stars can make elements heavier than carbon but end as white dwarfs.
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