ASTR100 (Spring 2008) Introduction to Astronomy Life as a Low-mass Star

1. ASTR100 (Spring 2008) Introduction to AstronomyLife as a Low-mass Star Prof. D.C. Richardson Sections 0101-0106

2. What are the life stages of a low-mass star?

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

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.

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. 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.

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

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

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

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

23. Helix Nebula

24. Cat’s Eye Nebula

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

26. Life stages of a low-mass star like the Sun.

27. Life Track of a Sun-like Star

28. ASTR100 (Spring 2008) Introduction to AstronomyLife as a High-mass Star Prof. D.C. Richardson Sections 0101-0106

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

30. 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.

31. 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).

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

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

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

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

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

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

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

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

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

41. Iron is a dead end for fusion because nuclear reactions involving iron do not release energy. (Fe has lowest masspernuclearparticle.)

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

43. How does a high-mass star die?

44. Iron builds up in core until degeneracy pressure can no longer resist gravity. Core then suddenly collapses, creating supernova explosion.

45. Supernova Simulation

46. 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.

47. Energy and neutrons released in supernova explosion enable elements heavier than iron to form, e.g. Au, U.

48. Elements made during supernova explosions.

49. 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. 1054. Multiwavelength Crab Nebula

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