1. 12 April 2005AST 2010: Chapter 211 Stars: From Adolescence to Old Age

3. 12 April 2005AST 2010: Chapter 213 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

4. 12 April 2005AST 2010: Chapter 214 Main Sequence: Lifetime vs. Mass

5. 12 April 2005AST 2010: Chapter 215 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

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 (10 7 ) years long for low-mass stars up to 10 billion (10 10 ) 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. 12 April 2005AST 2010: Chapter 2113 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

14. 12 April 2005AST 2010: Chapter 2114 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)

15. 12 April 2005AST 2010: Chapter 2115 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

16. 12 April 2005AST 2010: Chapter 2116 Stage 6: Helium Fusion 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 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 4 He  12 C

17. stage 6: helium flash  yellow giant

18. star mass (solar masses) time (years)Spectral type 603 millionO3 3011 millionO7 1032 millionB4 3370 millionA5 1.53 billionF5 110 billionG2 (Sun) 0.11000's billionsM7

19. 12 April 2005AST 2010: Chapter 2119 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

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

21. 12 April 2005AST 2010: Chapter 2121 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

22. Upper Main-Sequence Stars

25. 12 April 2005AST 2010: Chapter 2125 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

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

27. 12 April 2005AST 2010: Chapter 2127

28. 12 April 2005AST 2010: Chapter 2128 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!

31. 12 April 2005AST 2010: Chapter 2131

33. 12 April 2005AST 2010: Chapter 2133 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

34. 12 April 2005AST 2010: Chapter 2134 End of Stellar Life-Cycle: the Corpse Stage 9: Core Remnant -- remains of the core after outer layers are ejected White Dwarf –Mass less than 1.4 solar masses –Electrons prevent further collapse of the core -- degenerate electron gas Neutron Star –Mass between 1.4 and 3 solar masses –Neutrons prevent further collapse of the core -- degenerate neutron gas Black Hole –Greater than 3 solar masses -- star collapses to a point –Escape velocity around the point mass is greater than the speed of light –Event horizon--the distance at which the escape velocity equals the speed of light is called the event horizon