Chapter 21: Stars: From Adolescence to Old Age

1. Chapter 21: Stars: From Adolescence to Old Age Astronomy 2010

2. Recall: the H-R Diagram Astronomy 2010

3. Mass Determines Life Stages • Mass determines stages stars go through and how long they last in each stage • with just little bit of dependence on composition • Massive stars evolve faster than small stars • Relationship between the luminosity and mass determined by how compressed gases behave. • Small increase in mass produces a large increase in the luminosity of a star. Astronomy 2010

4. Lifetime vs. Mass star mass (solar masses) time (years) Spectral type 60 3 million O3 30 11 million O7 10 32 million B4 3 370 million A5 1.5 3 billion F5 1 10 billion G2 (Sun) 0.1 1000's billions M7 main sequence: more mass  hotter and shorter life Astronomy 2010

5. Old Age: Main Sequence to Red Giant Stage 5: Red Giant collapse: fusion stops when the hydrogen in the core runs out shell burning: hydrogen shell surrounding the core ignites star expands and becomes a subgiant, then a red giant Stage 6: Helium Fusion helium fusion begins in the core star passes through a yellow giant phase equilibrates as a red giant or supergiant Stage 7: Stellar Nucleosynthesis – fusion of heavier elements (up to iron) core fuel in stage 6 runs out and collapse resumes fusion of heavier elements may ignite if star is sufficiently massive Astronomy 2010

6. Stage 5, part 1: Collapse • main sequence: inward gravity balanced by the outward pressure • pressure due to fusion in core • hydrogen in the core eventually converted to helium  nuclear reactions stop! • gravity takes over and the core shrinks • outside layers also collapse • layers closer to the center collapse faster than those near the surface. • As the layers collapses, the gas compresses and heats up

7. Stage 5, part 2: Shell Burning • shell layer outside the core becomes hot and dense enough for fusion to start • fusion in the layer just outside the core is called shell burning • shell fusion is very rapid because the shell layer is still compressing and increasing in temperature • luminosity of the star increases from its main sequence value • Gas surrounding the core puffs outward under the action of the extra outward pressure • The star expands and becomes a subgiant and then a red giant. • surface has a red color because star is puffed out and cooler • red giant is very luminous because of its huge surface area

8. time to reach red giant stage • short for big stars • as low as 10 million (107) years • long for little stars • up to 10 billion (1010) years for low mass

10. stage 5: shell burning  red giant

12. End of Life on Earth … • When the Sun becomes a red giant, it will swallow Mercury,Venus and perhaps the Earth too. • Or conditions on Earth’s surface will become impossible for life to exist. • Water oceans and atmosphere will evaporate away.

13. 21.2 Star Clusters • We saw that stars tend to form in clusters • The stars in the cluster have different masses but about the same age. • The evolution of the stars in a cluster should be consistent with the theory. • Three types of clusters: • Globular clusters -- only contain very old stars • Open clusters -- contain relatively young stars • Stellar associations -- small groups of young stars Astronomy 2010

14. 21.3 Checking Out the Theory • Comparison of the prediction for the stars of a 3 million year old cluster (left) with measurements of the stars in cluster NGC 2264 (right). Astronomy 2010

15. An Older Cluster • Comparison of the model for a 4.24 billion year old cluster (left) with measurements of stars in 47 Tucanae (right). Note the different scales. Astronomy 2010

16. Stage 6: Helium Fusion • red giant: dead helium core plus hydrogen burning shell • gravity plus inward pressure from burning shell heats core • helium fusion starts at 100 million K • triple alpha process: three 4He  12C • helium flash: onset of helium fusion produces a burst of energy • reaction rate settles down • Fusion in the core releases more energy/second than core fusion in main sequence • star is smaller and hotter, but stable! • hydrostatic equilibrium holds until the core fuel runs out. Astronomy 2010

17. stage 6: helium flash  yellow giant

19. Stage 6: Helium Fusion • hydrostatic equilibrium holds until the core fuel runs out • star is a yellow/orange giant • dead carbon core shrinks under its weight • gravity  pressure and heat • heats helium shell surrounding core • fusion of hydrogen surrounding helium shell • star again puffs out to red giant • Sun-like or smaller stars: terminal stage • heavier stars: • helium shell flashes • pulsation (as in Cephied variable stars) • heavier elements fuse Astronomy 2010

20. stage 6: yellow giant  red giant or supergiant

22. Pulsating Stars • In ordinary stars hydrostatic equilibrium works to dampen (diminish) the pulsations. • But stars entering and leaving stage 6 can briefly (in terms of star lifetimes!) create conditions where the pressure and gravity are out of sync and the pulsations continue for a time. • Larger, more luminous stars will pulsate with longer periods than the smaller, fainter stars • because gravity takes longer to pull the more extended outer layers of the larger stars back. • The period-luminosity relation can be used to determine the distances of these luminous stars from the inverse square law of light brightness. Astronomy 2010

24. Upper main-sequence star Astronomy 2010

28. Stage 7: Red Giant or Supergiant • When the core fuel runs out again, the core resumes its collapse. • If the star is massive enough, it will repeat stage 5. • The number of times a star can cycle through stages 5 to 7 depends on the mass of the star. • Each time through the cycle, the star creates new heavier elements from the ash of fusion reactions in the previous cycle. Astronomy 2010

29. Stellar Nucleosynthesis Fusion creates heavier elements from lighter elements • very massive stars produce elements up to iron in the core • nuclear fusion releases energy for elements lighter than iron • past iron, fusion absorbs energy • stars like our Sun produce elements up to carbon and oxygen • heavier elements produced in supernova explosions of very massive stars • density gets so great that protons and electrons are combined to form neutrons (+ neutrinos) • outer layers are ejected in a huge supernova explosion • elements heavier than iron are formed and ejected Astronomy 2010

30. red supergiant Betelgeuse • core radius earth-sized • heavy element fusion in shells • envelope 5 AU

31. Stage 5 begins: collapse of the star generates heat post-main sequence evolution of a 5 solar mass star hydrogen fuel in core runs out

32. Stage 5 continues: star collapses further as shell burning begins • Star adjusts as collapse continues • further collapse of core • exterior of star cools

33. Stage 5-6: Young Red Giant forms, Shell Burning starts • Shell burning starts • outer layers of star expand • core continues to contract

34. Stage 6: Core Burning drives further expansion • strong new heat source from helium fusion • hotter  more yellow

35. Stage 6-7: Red Giant matures mature red giant region (AGB) young red giant region (RGB)

36. Planetary Nebula • Planetary nebula got their name because some looked like round, green planets in early telescopes. • Now known to be formed when old, low-mass stars are unable to fuse heavier elements, and their cores collapse. • The outer layer of the star is ejected by wind. • About one or more light years across • much larger than our solar system! Astronomy 2010

39. The Helix Nebula

41. Astronomy 2010