Prepared by Lisa M. Will, San Diego City College

1. Lecture Slides CHAPTER 12: Evolution of Low-Mass Stars Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College

2. Evolution of Low-Mass Stars • Understand the role of stellar mass in the evolution of a star.The goal is to understand this process. • Explain the future evolution of the Sun. • Utilize the H-R diagram to determine the evolution of stars.

3. Role of Stellar Mass: Star’s Life Main sequence stars generate energy by converting hydrogen to helium in their cores. Eventually the fusion sources change, then halt. A star’s life depends primarily on mass and composition (to a lesser extent). Low-massstars and high-mass stars evolve differently. Low-mass stars: M < 8 M

4. Role of Stellar Mass: Main-Sequence Lifetimes • Higher mass leads to higher temperature and pressure in the core. • Higher core temperature means faster nuclear fusion => Stars with higher masses burn their fuel more quickly.

5. Role of Stellar Mass: Main Sequence Star • Recall that a protostar becomes a star when nuclear fusion begins. => That is when it becomes a main sequence star! • Mass establishes a star’s evolutionary track.

6. Role of Stellar Mass: Changes in Structure • The star’s structure will change as it uses fuel. => Must maintain balance between pressure and gravity.

7. Fusion Reactions • Main-sequence stars fuse hydrogen to helium in their cores. • Eventually, much of the hydrogen in the core is converted to helium. • A core of non-fusing helium builds up.

8. Fusion Reactions (Cont.)

9. Fusion Reactions (Cont.)

10. Fusion Reactions (Cont.)

11. Class Question Prediction: When hydrogen fusion in the core stops, its temperature ______ and its thermal pressure _______ so the size of the core ______. • Decreases, decreases, decreases. • Increases, increases, increases. • Decreases, increases, decreases. • Increases, decreases, increases.

12. Fusion Reactions: Electron-Degenerate At this point, hydrogen fusion only takes place in a shell around the helium core. Because the helium is not fusing, gravity begins to win over the pressure, causing the helium core to shrink. The core becomes more dense, and becomes electron-degenerate. This means pressure is due to a quantum mechanical effect: there’s a limit to how tightly electrons can be packed together.

13. Red Giant: Hydrogen Shell Burning • When the core shrinks, its gravitational pull gets stronger. • Stronger gravity => higher pressure => faster nuclear reactions in the hydrogen burning shell => more energy produced!

14. Red Giant: Hydrogen Shell Burning (Cont.)

15. Red Giant: Hydrogen Shell Burning (Cont.)

16. Increase in pressure and energy production results in larger size and lower surface temperature. Star: larger, more luminous, cooler, redder => Red Giant! Red Giant: A Larger Star

17. Red Giant: A Larger Star (Cont.)

18. Red Giant: A Larger Star (Cont.)

19. What causes a low-mass star, like the Sun, to evolve away from the main sequence? When hydrogen is exhausted in the core. When all of the hydrogen becomes helium. When carbon fusion begins. Class Question

20. Red Giant: Branch

21. Red Giant: Branch (Cont.)

22. Red Giant: Branch (Cont.)

23. Helium Fusion • As the helium core shrinks, its density and temperature increase. • When hot and dense enough, helium fusion begins. • Helium fuses to carbon via the triple-alphaprocess.

24. Helium Fusion: The Helium Flash • Within seconds of helium ignition, the thermal pressure to the point that the helium core literally explodes. • This explosion is called the helium flash.

25. After the helium flash, the stars are on the horizontal branch of the H-R diagram. Helium fuses to carbon in the core, while hydrogen fuses to helium in a shell around the core. Helium Fusion: Horizontal Branch Star

26. After helium is used up in the core, hydrogen and helium fusion continues in shells around non-fusing carbon core. Outer layers expand and cool => asymptotic giant branch of the H-R diagram. Helium Fusion: Asymptotic Giant Branch

27. As the star expands, some of its material leaves as a stellar wind. This mass loss means the star cannot hold onto the outer layers easily. Eventually the outer layers are ejected into space. Planetary Nebula

28. The ejected material creates a planetary nebula. Planetary nebulae having nothing to do with planets! The remaining star shrinks and becomes hotter, moving rapidly from right to left across the H-R diagram. Planetary Nebula: The Ejected Material

29. The star ionizes the gas in the expanding outer layers, causing the planetary nebula that we can observe. Planetary nebulae do not last forever – eventually the gas disperses. Planetary Nebula: Lifespan

30. Planetary Nebula: Facts • The first observed planetary nebulae had circular appearances (hence the name), but now we observe examples with much more complex structure.

31. Leftover core of star remains as white dwarf. Masses 0.6–1.4 M, size like Earth. They are hot, but not very luminous due to small size. White dwarfs cool off because no nuclear fusion is occurring. White Dwarf

32. Our Sun’s evolution

33. Star Clusters • Star clusters are bound groups of stars, all made at the same time.

34. Star Clusters (Cont.)

35. Star Clusters (Cont.)

36. Star Clusters (Cont.)

37. Star Clusters (Cont.)

38. Star Clusters (Cont.)

39. Star Clusters (Cont.)

40. Star Clusters: Young and Old • Young clusters still have high-mass stars on main-sequence. • In older clusters, high-mass stars have already died. • Location of main-sequence turnoff gives cluster age.

41. We compare our models to the observed H-R diagrams of star clusters. Agreement shows that our models of stellar evolution are on the right track! Star Clusters: Models of Stellar Evolution

42. Recall that most stars are in binary systems. In each pair of low-mass stars, the more massive star evolves first. It can only expand so much before it begins to lose material. Evolution in Close Binary Systems

43. Evolution in Close Binary Systems (Cont.)

44. Evolution in Close Binary Systems (Cont.)

45. Material can flow from the giant star to the companion. This is called mass transfer. The giant becomes a white dwarf. When the second star is a giant, it can dump material onto the white dwarf. Evolution in Close Binary Systems: Mass Transfer

46. Evolution in Close Binary Systems: Mass Transfer (Cont.)

47. Evolution in Close Binary Systems: Mass Transfer (Cont.)

48. As hydrogen collects on the white dwarf, nuclear reactions can start on the surface => gets much brighter temporarily => nova. For a few hours, it can be a half-million times more luminous than the Sun. Evolution in Close Binary Systems: Evolution of Nova

49. Evolution in Close Binary Systems: Evolution of Nova (Cont.)

50. Evolution in Close Binary Systems: Evolution of Nova (Cont.)