1. How Stars Evolve Pressure and temperature The fate of the Sun
Normal gases
Degenerate gases
The fate of the Sun
Red giant phase
Horizontal branch
Asymptotic branch
Planetary nebula
White dwarf

2. 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

3. 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

4. Fermi exclusion principle
No two electrons can occupy the same quantum state
Quantum state = energy level + spin
Electron spin = up or down

5. Electron orbits
Only two electrons (one up, one down) can go into each energy level

6. 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.

7. Which of the following is a key difference between the pressure in a normal gas and in a degenerate gas?
Degenerate pressure exists whether matter is present or not.
In a degenerate gas pressure varies rapidly with time.
In a degenerate gas, pressure does not depend on temperature.
In a degenerate gas, pressure does not depend on density.

8. The Fate of the Sun How will the Sun evolve over time?
What will be its eventual fate?

9. Sun’s Structure Core Envelope Where nuclear fusion occurs
Supplies gravity to keep core hot and dense

10. 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

11. Gradual change in size of Sun
Now 40% brighter, 6% larger, 5% hotter

12. 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

13. Red Giant Phase He core H layer Envelope No nuclear fusion
Gravitational contraction produces energy
H layer
Nuclear fusion
Envelope
Expands because of increased energy production
Cools because of increased surface area

14. Sun’s Red Giant Phase

15. HR diagram
Giant phase is when core has been fully converted to Helium

16. A star moves into the giant phase when:
It eats three magic beans
The core becomes helium and fusion in the core stops.
Fusion begins in the core
The core becomes helium and all fusion in the star stops.

17. 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

18. 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

19. Helium Flash He core H layer Envelope
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

20. Movement on HR diagram

21. Movement on HR diagram

22. Helium Flash He core H layer Envelope
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

23. 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

24. Sun moves onto horizontal branch
Sun burns He into Carbon and Oxygen
Sun becomes hotter and smaller
What happens next?

25. 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

26. Helium burning in the core stops
H burning is continuous
He burning happens in “thermal pulses”
Core is degenerate

27. Sun moves onto Asymptotic Giant Branch (AGB)

28. 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

30. Planetary nebula

31. Planetary nebula

32. Planetary nebula

33. Hourglass nebula

34. When on the horizontal branch, a solar-mass star
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.

35. 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…

36. Evolution on HR diagram

37. Time line for Sun’s evolution

38. In which order will a single star of one solar mass progress through the various stages of stellar evolution?
Planetary nebula, main-sequence star, white dwarf, black hole
Proto-star, main-sequence star, planetary nebula, white dwarf
Proto-star, red giant, supernova, planetary nebula
Proto-star, red giant, supernova, black hole

39. 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?

40. Higher mass protostars contract faster
Hotter

41. Higher mass stars spend less time on the main sequence

42. 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?

43. Turn-off point of cluster reveals age

44. 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 Myr
10 Myr
100 Myr
1 Gyr

45. Higher mass stars do not have helium flash

46. 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

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

48. Why does fusion stop at Iron?

49. Fusion versus Fission

50. Advanced reactions in stars make elements like Si, S, Ca, Fe

51. 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

52. 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)

53. If I drop a ball, will it bounce higher than it began?
Do 8B Supernova Core Bounce

54. Supernova explosion

55. Crab nebula

56. Cas A

57. In 1987 a nearby supernova gave us a close-up look at the death of a massive star

58. Neutrinos from SN1987A

59. 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

60. Energy and neutrons released in supernova explosion enable elements heavier than iron to form, including Au and U

61. 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?