Life Cycles of Stars

1. Life Cycles of Stars

2. The Hertzsprung-Russell Diagram

4. How Stars Form • Collapsing gas and dust cloud • Protostar - mostly infrared

5. Main Sequence Stars • Brown Dwarf (L, T, Y) • Red Dwarf (M) • Normal Star (O, B, A, F, G, K)

6. All Objects Exist Because of a Balance Between Gravity and Some Other Force • People, Planets-Interatomic Forces • Normal Stars-Radiation • White Dwarfs-Electron Repulsion • Neutron Stars-Nuclear Forces • Quark Stars? • Black Holes-No Known Force

7. Mass, Luminosity, Lifetime • Luminosity = Mass3.5 (Solar Units) • Lifetime = Mass/Luminosity = 1/Mass2.5 • Mass = .1: Lifetime = 316 (3160 b.y.) • Mass = .5: Lifetime = 5.7 (57 b.y.) • Mass = 1: Lifetime = 1 (10 b.y.) • Mass = 10: Lifetime = .003 (3 m.y) • Mass = 50: Lifetime = .000057 (570,000 yr)

8. Mass, Luminosity, Lifetime

9. Before Stars Form • Pre-stellar cores • Protostars • Pre-main sequence star (PMS) • Planet system formation. 

10. Protostars or Young Stellar Objects (YSO’s) • Class 0 (T <70K) Emits in microwave range because of opaque surrounding cloud • Class I (T = 70-650K) Emits in infrared. Star still invisible but can detect warm material around it. • Class II (T = 650-2880K) T Tauri stars. Massive expulsion of material        • Class III(T > 2880K) PMS stars

11. Early Stars and Planets • (Class 0) Early main accretion phase • (Class I) Late accretion phase • (Class II) PMS stars with protoplanetary disks • (Class III) PMS stars with debris disks

12. Super-Massive Stars • Stars beyond a certain limit radiate so much that they expel their outer layers • W stars (Wolf-Rayet stars) are doing this: T Tauri on steroids • Upper limit about 100 solar masses • More massive stars can form by merger but don’t last long

13. Wolf-Rayet Star

14. How Stars Die • Main Sequence Stars Brighten With Age • The More Massive a Star, the Faster it Uses Fuel • Giant Phase • White Dwarf • Supernova • Neutron Star - Pulsar • Black Hole

15. Leaving the Main Sequence • Helium accumulates in core of star • Fusion shuts down • Star begins to contract under gravity • Core becomes denser and hotter • Nuclear fusion resumes around helium core • Outer layers puff up enormously but cool down • Star becomes redder and larger (Red Giant)

16. “Live hard, die young, leave a good looking corpse”

17. Peeling off to the Giant Phase

18. Later Lives of Giants • Inert helium core begins to fuse helium to carbon and oxygen • Contraction of core stops • Outer envelope contracts and heats up • Red Giant becomes Yellow Giant • Helium core runs out of fuel • Helium fusion shell on outside of core, hydrogen fusion above • Star loops between red and yellow on H-R plot

19. Making the Elements • Heavy nuclei: Energy from Fission • Light Nuclei: Energy from Fusion • Both end at Iron: Most stable nucleus • Stars can generate H-Fe through Fusion • How do we get beyond Fe? • Two processes • S-Process (Slow) in Red Giants • R-Process (Rapid!) in Supernovae

20. Beyond Helium • He + particle = mass 5: not stable • He + He = Mass 8: Not Stable • The Mass 5-8 Bottleneck • Sometimes three He collide to make C • Li, Be, B rare in Universe • Destroyed in Stars • Created by spallation - knocking pieces off heavier atoms

21. Iron and Beyond • Build from C to Fe by fusing successively heavier atoms • Can’t Build Beyond Fe by Adding Protons • Repulsion of nuclei = Charge1 x Charge2 • He + C = O: Repulsion = 2 x 6 = 12 • Fe + p = Co: Repulsion = 26 x 1 = 26 • Can Add Neutrons Until Atoms Become Unstable • n  p + e (Beta Decay)

22. The S-Process

23. Building Atoms

24. The R-Process • There are nuclei the s-process can’t make • The process is slow • Precursors break down before next neutron hits • Stops at Bi and Pb. Where do U and Th come from? • The r-process piles neutrons on faster than atoms can decay • Occurs in Supernovae

25. The End Fate of Medium-Size Stars • Core reaches limits of its ability to sustain fusion • Fusion shells sputter and become unstable • Star expels outermost layers as Planetary Nebulae • Inert core left as white dwarf • Dwarf has such tiny surface area it takes billions of years to cool • Coolest (oldest?) known: 3900 K

26. Tiny Stars • Red Dwarfs are tiny but have huge sunspots and violent flares • They have convection throughout their interiors • Interiors uniform in composition • Do not accumulate helium in core • Can use much more of their hydrogen up • Never fuse He to C • Lifetimes longer than age of Universe

27. Exploding Stars • Nova • White dwarf attracts matter from neighboring star • Nuclear fusion resumes on surface of star • Many novae repeat at decade or longer intervals • Type I Supernova • White dwarf attracts matter from neighboring star • White dwarf core resumes fusion • Type II Supernova • Collapse of massive single star

28. Shell Structure of Massive Star • 4H –> He • 3He –> C • He + C –> O, Ne • Ne + He, C –> O, Mg • 2O –> Si • 2Si –> Fe

29. Core Collapse • Fe core collapses to neutron star in milliseconds • Remaining star material falls in at up to 0.1c • Nuclei beyond Plutonium created • Star blows off outer layers • We see the thermonuclear core of the star • Much of the light is from radioactive nickel

30. Historical Supernovae • 185 - Chinese • 1006 - Chinese, one European record • 1054 - Chinese, European, Anasazi? • 1572 - Tycho’s Star • 1604 - Kepler’s Star • 1885 – Andromeda Galaxy • 1987 - Small Magellanic Cloud (170,000 l.y.)

31. Remains of SN 1054 (Crab Nebula)

32. Life (Briefly!) Near a Supernova • Sun’s Energy Output = 90 billion megatons/second • Let’s relate that to human scales. What would that be at one kilometer distance? • 90 x 1015 tons/(150 x 106km)2 = 4 tons • Picture a truckload of explosives a km away giving off a one-second burst of heat and light to rival the Sun

33. Now Assume the Sun Goes Supernova • Brightens by 10 billion times • 1010 = 25 magnitudes • Our 4 tons of explosive becomes 40,000 megatons • Equivalent to entire Earth’s nuclear arsenal going off one km away - every second • This energy output would last for days

34. Neutron Stars and Pulsars • Mass of sun but diameter of a few km • Rotate at high speed • Sun 1,400,000 km –> 10 km • Rotation speeds up 140,000 x • 28 days –> 17 seconds • Pulsars: infalling matter emits jets of radiation • Millisecond pulsars: probably “spun up” by accretion, or merger of neutron stars

35. How a Pulsar Works

36. Black Holes • Singularity: gravity but no size • Event horizon (Schwarzschild radius): no information can escape • Detectable from infalling matter, which emits X-rays • Quantum (atom-sized) black holes may exist • Cores of galaxies have supermassive black holes

37. Black Hole

38. Probably Not

39. Planetary Systems • Protoplanetary Disks • Accretion of Planets • Expulsion and Migration of Planets • About 400 extrasolar planets known • Our Solar System may be unusual?

40. Protoplanetary Disks in Orion