1. The Birth, Life and Death of Stars
Prasad
U6_StarLife

2. How can we learn about the lives of stars when little changes except on timescales much longer than all of human history?
Suppose you had never seen a tree before, and you were given one minute in a forest to determine the life cycle of trees. Could you piece together the story without ever seeing a tree grow?
This is about the equivalent of a human lifetime to the lifetime of the Sun.
Fruit flies view of humans.
What characterizes human life cycle from incidental characteristics?
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U6_StarLife

3. Stellar “Forest”
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U6_StarLife

4. The Stellar Cycle
New (dirty) molecular clouds are left behind by the supernova debris.
Cool molecular clouds gravitationally collapse to form clusters of stars
Molecular cloud
The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion.
Stars generate helium, carbon and iron through stellar nucleosynthesis
Prasad
U6_StarLife

5. Star Birth Cold gas clouds contract and form groups of stars.
When O and B stars begin to shine, surrounding gas is ionized
The stars in a cluster are all about the same age.
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U6_StarLife

6. Cloud Collapses to Form Stars
Radiation from protostars arises from the conversion of gravitational energy to heat.

7. Pre-Main Sequence Contraction
Protostars contract until core reaches HHe fusion temperature.
Low mass protostars contract more slowly.
Nature makes more low-mass stars than high-mass stars.
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U6_StarLife

8. Anatomy of a Main Sequence Star
Hydrogen burning core
shell
Hydrogen fuel
Helium “ash”
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9. Up the red giant branch
As hydrogen in the core is being used up, it starts to contract, raising temperature in the surrounding. Eventually, hydrogen will burn only in a shell. There is less gravity from above to balance this pressure. The Sun will then swell to enormous size and luminosity, and its surface temperature will drop,  a red giant.
Sun in ~5 Gyr
Sun today
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U6_StarLife

10. Helium fusion at the center of a giant
While the exterior layers expand, the helium core continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbon
Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.
For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion.
In very massive stars, elements can be fused into Fe.
U6_StarLife

11. The Sun will expand and cool again, becoming a red giant
The Sun will expand and cool again, becoming a red giant. Earth will be engulfed and vaporized within the Sun. The Sun’s core will consist mostly of carbon.
Red Giants create most of the Carbon in the universe (from which organic molecules—and life—are made)
Prasad
U6_StarLife

12. H, He, C burning
Since fusing atomic nuclei repel each other because of their electric charge, the order of easiest to hardest to fuse must be
H, He, C
C, He, H
H, C, He
He, C, H
Carbon-triple alpha process
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U6_StarLife

13. The Sun’s Path
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U6_StarLife

14. Planetary Nebula Formation
When the Red Giant exhausts its He fuel
the C core collapses  white dwarf
No fusion going on inside … this is a dead star.
He & H burning shells overcome gravity
the outer envelope of the star is blown outward  a planetary nebula
This animation is from the Hubble Space Telescope archive and is in the public domain.
For policy statement see:
Prasad
U6_StarLife

15. What holds the white dwarf from collapsing?
As matter compresses, it becomes denser.
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).
Indistinguishable particles are not allowed to stay in the same quantum state.
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U6_StarLife

16. What holds the white dwarf from collapsing?
The resulting outward pressure which keeps the electrons apart is called electron degeneracy pressure – this is what balances the weight.
Only if more energy drives the electrons into higher energy states, can the density increase.
Adding mass can drive electrons to higher energies so star shrinks.
At 1.4 solar masses—the Chandrasekhar Limit—a star with no other support will collapse, which will rapidly heat carbon to fusion temperature.
Prasad
U6_StarLife

17. WD has a size slightly less than that of the earth
WD has a size slightly less than that of the earth. It is so dense, one teaspoon weights 15 tons! WD from an isolated star will simply cool, temperature dropping until it is no longer visible and becomes a “black dwarf”.
1 teaspoon = 1 elephant
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U6_StarLife

18. Sun’s life Prasad U6_StarLife
This simulation shows where the Sun lies and moves on the HR diagram as it evolves over time.
Prasad
U6_StarLife

19. What is a planetary nebula?
A large swarm of planets surrounding a star.
A disk of gas and dust around a young star.
Glowing gas in Earth’s upper atmosphere.
Ionized gas around a white dwarf star.
Prasad
U6_StarLife

20. The lead-up to disaster
In massive stars (M > 8 Msun), elements can be fused into Fe.
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.
Now the electron degeneracy pressure disappears.
What comes next … is core collapse.

21. Supernova! Type II (Core-Collapse)
The core implodes, but no fuel there, so it collapses until neutron degeneracy pressure kicks in.
Core “bounces” when it hits neutron limit; huge neutrino release; unspent fuel outside core fuses…
Outer parts of star are blasted outward.
A tiny “neutron star” or a black hole remains at the center.
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U6_StarLife

22. Supernova 1987a before/after
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23. Production of Heavy Elements
(There is evidence that the universe began with nothing but hydrogen and helium.)
To make elements heavier than iron extra energy must be provided.
Supernova temperatures drive nuclei into each other at such high speeds that heavy elements can be made.
Gold, Silver, etc., -- any element heavier than iron, were all made during a supernova.
We were all once fuel for a stellar furnace. Parts of us were formed in a supernova!
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U6_StarLife

24. All of the Heavy Elements are Made During Supernovae
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U6_StarLife

25. Prasad
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26. Life of a 15 solar mass star
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U6_StarLife

27. Stellar Evolution in a Nutshell
Mass controls the evolution of a star!
0.5 MSun < M < 8 MSun
M > 8 MSun
Mcore < 3MSun
Mcore > 3MSun
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U6_StarLife

28. The H-R diagram
Which of these star is the hottest?
What are Sun-like stars (0.5 Msun < M < 8 Msun) in common?
What about red dwarfs (0.08 Msun < M < 0.5 Msun) ?
Where do stars spend most of their time?
Which is the faintest? the sun, an O star, a white dwarf, or a red giant? O
Stars with mass < 0.5 Msun do not need to go through the RG phase. They also have lifetime longer than the age of the U
Stars with M < 0.08 Msun  Brown dwarf (fusion never starts)
Prasad
Answers: 1. O star, 2. end as a WD, 3. no RG phase, lifetime longer than the age of the Universe, 4. MS, 5. WD
U6_StarLife

29. The evolution of 10,000 stars
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U6_StarLife

30. If we came back in 10 billion years, the Sun will have a remaining mass about half of its current mass. Where did the other half go?
It was lost in a supernova explosion
It flows outward in a planetary nebula
It is converted into energy by nuclear fusion
The core of the Sun gravitationally collapses, absorbing the mass
Prasad
U6_StarLife

31. A star cluster containing _____ would be MOST likely to be a few billion years old.
luminous red stars
hot ionized gas
infrared sources inside dark clouds
luminous blue stars
Prasad
U6_StarLife