The Lives of Stars

1. The Lives of Stars Chapter 12

2. Life on Main-Sequence • Zero-Age Main Sequence (ZAMS) • main sequence location where stars are born • Bottom/left edge of main sequence • H fusion begins • As star ages • Energy source is H fusion • composition changes • H -> He • Location in H-R diagram slowly changes • begins to move away (right/up) from ZAMS • broadens (smears out) main sequence

3. Stellar Lifetimes • 90% of star’s life spent in main sequence • Lifetime depends on mass

4. Main Sequence to Red Giant • H in core used up • He “ash” in core • no more fuel for energy • Gravity begins to win • core contracts, gets hotter • start H fusion in shell surrounding core • outer layers expand • Star becomes Red Giant

5. Further Evolution • As core contracts • temperature increases • becomes hot enough • Begins to fuse He into C • Energy production • stops core collapse • star is stable again

6. Beginning of the End • When He exhausted, star out of fuel again • core collapse resumes • He shell burning begins • outer layers expand • Star becomes Red Supergiant • strong mass loss occurs via stellar wind

7. Stellar Evolution

8. Evolution of Massive Stars • Up to C-O core, evolution same for all stars • From then on, different paths • Low-mass stars: • no C burning • core energy generation complete • star dies • High-mass stars: • C burning begins in core • Eventually fuse heavier and heavier elements

9. Making Heavy Elements • High-mass stars fuse heavier elements in cores C ->Ne -> O -> Si -> Fe • at each step, core collapses further • This nucleosynthesis produces most elements up to iron (Fe)

10. Evolutionary Tracks

11. Star Clusters • Star clusters • Stars born same location, same time • contains stars with different masses • permits study of stellar evolution • Age of cluster • determined by which stars have departed main sequence

12. Globular Clusters • spherical “ball” of stars • concentrated toward center • 10,000 - 100,000 stars • about 150 around our Galaxy • very distant from Sun (>10,000 LY) • sizes 50-300 LY diameter

13. Open Clusters • 100’s of stars (up to 1000) • smaller than Globular clusters • no central concentration • Found within the Galaxy • 1000’s known • diameters < 30 LY

14. H-R Diagrams of Clusters • Cluster ages are different • globular clusters oldest • open clusters relatively young • H-R diagrams indicate age • interpret using stellar evolution theory

15. Cluster Evolution

16. Estimating Cluster Ages • Make H-R diagram for cluster • Have all stars arrived at ZAMS? • if not, cluster extremely young • Have some stars departed Main Sequence? • cluster is older • main sequence turn-off point • determines cluster age • the farther down the turn-off, the older the cluster

17. Theoretical NGC 2264 Theoretical 47 Tuc

18. Ages of Clusters • Globular clusters • only lowest part of main sequence is present • typical age: 15 billion yrs • Open clusters • much younger than globulars • all ages: 1 million yrs up to a few billion yrs

19. Stellar Death

20. Death of Stars • Two possibilities • Low mass stars < 5 Msun • end is planetary nebula  WHITE DWARF • High mass stars • end is type II supernova  either: NEUTRON STARorBLACK HOLE {

21. Fate of Low Mass Stars • During end of red supergiant phase • large mass loss • star loses entire envelope, revealing core • core becomes white dwarf • white dwarf slowly cools • eventually becomes “black dwarf”

22. Planetary Nebulae • During transition to white dwarf • outer layers expanding • exposes hot core; • shell material heated; begins to glow • Result is a planetary nebula • tens of thousands known in our Galaxy

23. Planetary Nebulae

24. Evolution to White Dwarf • Low mass stars • cannot fuse carbon • lose energy source • gravity wins • core contracts • Core contraction • produces very high density • electron degeneracy pressure • stops core collapse • remnant core becomes white dwarf

25. White Dwarf Stars • Properties • diameter ~ same as Earth • very dense (1 tsp = several tons!) • very hot on surface • Chandrasekhar limit • Maximum mass = 1.4 Msun • larger stars collapse “Diamond stars”??

26. Fate of Massive Stars • High-mass stars fuse heavier elements in cores C ->Ne -> O -> Si -> Fe • at each step, core shrinks further • fusion stops when iron (Fe) produced • This nucleosynthesis produces most elements up to iron

27. Collapse and Explosion • When core mass exceeds 1.4 Msun • collapse continues unabated • all protons converted into neutrons • collapse abruptly halted by neutron degeneracy pressure • results in shock wave & explosion • Produces type II supernova • Some material falls onto core • M < 2.5 Msunneutron starremains • M > 2.5 Msunblack holeproduced

28. Supernovae • Supernovae as bright as entire galaxy • Ejection velocities • millions of miles/hr (~10,000 km/s) • Supernova explosion • heavy elements (C, N, O, Fe) returned to interstellar medium for recycling • also produces elements heavier than iron • elements such as gold, silver, uranium

29. Pulsars • Pulsating radio sources • Periods .001-10 seconds • Very regular • also observed in optical (crab nebula) • Pulsars = spinning neutron stars • fast period requires very small objects • neutron stars only possibility • Radiation and particles beamed out from magnetic poles • spinning lighthouse effect results in observed “pulses”

30. Novae • Novae NOT same as Supernova • less energetic; not as bright • Binary system with mass transfer onto WD • material accumulates on WD surface • eventually nuclear detonation occurs • result is a nova

31. White Dwarf Supernovae • As mass accumulates, WD exceeds Chandrasekhar limit • rapid core collapse occurs • Resulting explosion = Type I supernova • Properties somewhat different than Type II SN (caused by massive star explosions)