Stars: From Adolescence to Old Age

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

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 AST 2010: Chapter 21

4. Main Sequence: Lifetime vs. Mass AST 2010: Chapter 21

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 AST 2010: Chapter 21

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 main red giant stage • short for massive stars • as low as 10 million (107) years • long for low-mass stars • up to 10 billion (1010) years

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. 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 different stars in a cluster provide a test for theories of stellar evolution • Three types of clusters: • Globular clusters -- only contain very old stars • Open clusters -- contain relatively young stars • Stellar associations -- small groups of young stars AST 2010: Chapter 21

14. Testing the Theory: Relatively Young Stars • Comparison of the model prediction for the stars of a 3-million-year-old cluster (left) with measurements of the stars in cluster NGC 2264 (right) AST 2010: Chapter 21

15. Testing the Theory: An Older Cluster • Comparison of the model prediction for a 4.24-billion-year-old cluster (left) with measurements of stars in 47 Tucanae (right) • Note the different scales AST 2010: Chapter 21

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 AST 2010: Chapter 21

17. stage 6: helium flash  yellow giant

18. 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

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 AST 2010: Chapter 21

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

21. 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 AST 2010: Chapter 21

22. Upper Main-Sequence Stars

25. Stage 7: Red Giant or Supergiant • When 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 AST 2010: Chapter 21

26. Red Supergiant Betelgeuse • core radius earth-sized • heavy element fusion in shells • envelope 5 AU

27. AST 2010: Chapter 21

28. 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! AST 2010: Chapter 21

31. AST 2010: Chapter 21

33. 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 are 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 AST 2010: Chapter 21