1. Life as a Low-mass Star
Image: Eagle Nebula in 3 wavebands (Kitt Peak 0.9 m).

2. What are the life stages of a low-mass star?
Low-mass: < 2 M_Sun; high-mass: > 8 M_Sun.

3. A star remains on the main sequence as long as it can fuse hydrogen into helium in its core.
main-seq_lifetime_and_mass.swf

4. Thought Question
What happens when a star can no longer fuse hydrogen to helium in its core?
Core cools off.
Core shrinks and heats up.
Core expands and heats up.
Helium fusion begins immediately.

5. Thought Question
What happens when a star can no longer fuse hydrogen to helium in its core?
Core cools off.
Core shrinks and heats up.
Core expands and heats up.
Helium fusion begins immediately.

6. Life Track After Main Sequence
Observations of star clusters show star becomes larger, redder, and more luminous after its time on the main sequence is over.

7. Broken Thermostat
As core contracts, H begins fusing to He in a shell around core.
Luminosity rises because core thermostat broken: increasing fusion rate in shell does not stop core from contracting.
Shell burning is vigorous and adds more inert He to the core, increasing contraction and heating shell even more: red giant swells.

8. He-fusion requires higher temperatures than H-fusion because larger charge leads to greater repulsion.
Fusion of two He nuclei doesn’t work, so He-fusion must combine three He nuclei to make carbon (C).

9. Thought Question
What happens in a low-mass star when core temperature rises enough for helium fusion to begin?
Helium fusion slowly starts up.
Hydrogen fusion stops.
Helium fusion rises very rapidly.
Hint: degeneracy pressure is the main form of pressure in the inert helium core.

10. Thought Question
What happens in a low-mass star when core temperature rises enough for helium fusion to begin?
Helium fusion slowly starts up.
Hydrogen fusion stops.
Helium fusion rises very rapidly.

11. Helium Flash
Thermostat broken in low-mass red giant because degeneracy pressure supports core.
Core temperature rises rapidly when helium fusion begins.
Helium fusion rate skyrockets until thermal pressure takes over and expands core again.

12. This is the “horizontal branch” (HB).
Helium-burning stars neither shrink nor grow because thermostat is temporarily fixed.

13. Life Track After Helium Flash
Models show that a red giant should shrink and become less luminous after helium fusion begins in the core.

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

15. How does a low-mass star die?

16. Thought Question What happens when a star’s core runs out of helium?
The star explodes.
Carbon fusion begins.
The core cools off.
Helium fuses in a shell around the core.

17. Thought Question What happens when a star’s core runs out of helium?
The star explodes.
Carbon fusion begins.
The core cools off.
Helium fuses in a shell around the core.

18. Double Shell Burning Late in its life, a star like our Sun will have…
An inert carbon core…
…surrounded by a shell of fusing helium…
…surrounded by a shell of fusing hydrogen.
The star swells enormously in size, even bigger than before.
But the core never gets hot enough to fuse carbon.
Double shell burning never reaches equilibrium: fusion rate spikes periodically in a series of thermal pulses, dredging carbon up from the core.

19. Only a white dwarf is left behind.
A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind.
Great planetary-nebula movies can be downloaded from Press Release (Helix Nebula) of the Space Telescope Science Institute (see hubble.stsci.edu)

20. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind.

21. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind.

22. A star like our sun dies by puffing off its outer layers, creating a planetary nebula.
Only a white dwarf is left behind.

23. White Dwarf
No fusion, and it cannot contract (due to degeneracy pressure).
So a white dwarf just cools off forever, fading away…

24. Life Track of a Sun-like Star
Fig

25. Life as a High-mass Star
Order: I1-63 HYDROGEN EXPLOSION
Image: Kepler’s SNR (Chandra, Hubble, Spitzer composite)

26. What are the life stages of a high-mass star?

27. CNO Cycle
High-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts.
A greater core temperature enables H nuclei to overcome greater repulsion.

28. 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).
High-mass stars become supergiants: luminosity doesn’t change much, but radius expands a lot.

29. How do high-mass stars make the elements necessary for life?

30. Big Bang made 75% H, 25% He — stars make everything else.

31. Helium fusion can make carbon in low-mass stars.

32. CNO cycle in high-mass stars can change C  N & O.

33. Helium Capture
High core temperatures allow helium to fuse with heavier elements.

34. Helium capture builds C into O, Ne, Mg, …

35. Advanced Nuclear Burning
Core temperatures in stars > 8 MSun allow fusion of elements as heavy as iron.

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

37. Multiple-Shell Burning
Advanced nuclear burning proceeds in a series of nested shells.
The Death Sequence of a High-Mass Star

38. Iron is a dead end for fusion because nuclear reactions involving iron do not release energy.
(Fe has lowest mass per nuclear particle.)

39. Evidence for helium capture:
Higher abundances of elements with even numbers of protons.

40. How does a high-mass star die?

41. Core then suddenly collapses, creating supernova explosion.
Iron builds up in core until degeneracy pressure can no longer resist gravity.
Core then suddenly collapses, creating supernova explosion.
death_seq_of_high-mass_star.swf
Download a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)

42. 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.

43. Side reactions involving neutrons can make nuclei with odd numbers of protons, etc.
Energy and neutrons released in supernova explosion enable elements heavier than iron to form, e.g. Au, U.

44. Elements made during supernova explosions.

45. Supernova Remnant
Energy released by the collapse of the core drives outer layers into space.
The Crab Nebula is the remnant of the supernova seen in A.D
Multiwavelength Crab Nebula

46. Supernova 1987A
The closest supernova in the last four centuries was seen in 1987.

47. The next nearby supernova?

48. How does a star’s mass determine its life story?

49. Role of Mass
A star’s mass determines its entire life story because it determines its core temperature.
High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.
Low-mass stars have long lives, never becoming hot enough to fuse carbon nuclei, and end as white dwarfs.

50. Low-mass Star Summary
Main Sequence: H fuses to He in core.
Red Giant: H fuses to He in shell around He core.
Helium Core Burning: He fuses to C in core while H fuses to He in shell.
Double Shell Burning: H and He both fuse in shells.
5. Planetary Nebula leaves White Dwarf behind.
Not to scale!

51. Reasons for Life Stages
Core shrinks and heats until it’s hot enough for fusion.
Nuclei with larger charge require higher temperature for fusion.
Core thermostat is broken while core is not hot enough for fusion (shell burning).
Core fusion can’t happen if degeneracy pressure keeps core from shrinking.
Not to scale!

52. High-mass Star Summary
Main Sequence: H fuses to He in core.
Red Supergiant: H fuses to He in shell around He core.
Helium Core Burning: He fuses to C in core while H fuses to He in shell.
Multiple Shell Burning: Many elements fuse in shells.
5. Supernova leaves Neutron Star behind.
Not to scale!

53. How are the lives of stars with close companions different?

54. Thought Question
The binary star Algol consists of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant star…
What’s strange about this pairing?
How did it come about?

55. Stars in Algol are close enough that matter can flow from the subgiant onto the main-sequence star.

56. The star that is now a subgiant was originally more massive.
As it reached the end of its life and started to grow, it began to transfer mass to its companion (mass exchange).
Now the companion star is more massive.