1. Slide 1 Death of Stars “All hope abandon, ye who enter here” Dante

2. Slide 2 The Dumbbell nebula (M76): a dying sun-like star

3. Slide 3 Life of stars: Gravity is everything Stars are born due to gravitational collapse of gas clouds Star’s life is a battle between thermal pressure generated by nuclear reactions and gravity Eventually, a star loses this battle, and gravity overwhelms

4. Slide 4 What happens when all hydrogen is converted into helium in the core?? Mass defines the fate of the star

5. Slide 5 Evolution on the Main Sequence Luminosity L ~ M 3.5 A star’s life time T ~ energy reservoir / luminosity T ~ M/L ~ 1/M 2.5 Energy reservoir ~ M Massive stars have short lives!

6. Slide 6 Evolution on the Main Sequence Zero-Age Main Sequence (ZAMS) Main-Sequence stars live by fusing H to He. Finite supply of H => finite life time. MS evolution

7. Slide 7 1. Accumulation of helium ash in the core Helium burning requires higher temperatures The star loses the ability to generate nuclear energy

8. Slide 8 2. Core collapses and gets hotter, while the envelope expands and cools down Note the hydrogen fusion in a shell surrounding the core! It is now hot enough there.

9. Slide 9 Evolution off the Main Sequence: Expansion into a Red Giant Hydrogen in the core completely converted into He: H burning continues in a shell around the core. He Core + H-burning shell produce more energy than needed for pressure support Expansion and cooling of the outer layers of the star  Red Giant  “Hydrogen burning” (i.e. fusion of H into He) ceases in the core.

10. Slide 10 Red Giant Evolution 4 H → He He H-burning shell keeps dumping He onto the core. He-core gets denser and hotter until the next stage of nuclear burning can begin in the core: He fusion through the “Triple-Alpha Process” 4 He + 4 He  8 Be +  8 Be + 4 He  12 C + 

11. Slide 11 If M > 0.4 M sun, the temperature reaches 100 million K Nuclear fusion of helium and heavier elements starts Carbon and then oxygen are produced

12. Slide 12 Red Dwarfs: no red giant phase Stars with less than ~ 0.4 solar masses are completely convective.  Hydrogen and helium remain well mixed throughout the entire star.  No phase of shell “burning” with expansion to giant. Star not hot enough to ignite He burning. Mass

13. Slide 13 Sunlike Stars Sunlike stars (~ 0.4 – 4 solar masses) develop a helium core.  Expansion to red giant during H burning shell phase  Ignition of He burning in the He core  Formation of a degenerate C,O core Mass

14. Slide 14 In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.

15. Slide 15 Outer layers expand due to radiation pressure from a hot core The star becomes a Red Giant Surface temperature drops by a factor of ~ 2 The radius increases by a factor of ~ 100 Luminosity increases ~ R 2 T 4 ~ 100-1000 times

16. Slide 16 Future of the Sun (SLIDESHOW MODE ONLY)

17. Slide 17 Supergiant

18. Slide 18 A star leaves the main sequence and becomes a red giant

19. Slide 19 Simulated evolution of the main sequence

20. Slide 20 While outer layers are expanded, inner helium core contracts, and its temperature rises

21. Slide 21 Helium Fusion When pressure and temperature in the He core become high enough, He nuclei can fuse to build heavier elements:

22. Slide 22 For stars with M > 4 M sun, helium fusion proceeds gradually as the core heats up. For stars with 0.4 M sun < M < 4 M sun, electrons in helium core becomes degenerate and their pressure does not increase with temperature. Helium fusion results in runaway explosion – helium flash. In any case, carbon ash accumulates in the core. Core contracts and becomes degenerate. Then helium burns in the shell. Subsequent evolution depends on the core mass. Eventually all reactions stop and the star dies.

23. Slide 23 Inactive He C, O Red Giant Evolution (5 solar-mass star)

24. Slide 24 Fusion Into Heavier Elements Fusion into heavier elements than C, O: requires very high temperatures; occurs only in very massive stars (more than 8 solar masses).

25. Slide 25 The Life “Clock” of a Massive Star (> 8 M sun ) Let’s compress a massive star’s life into one day… 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 Life on the Main Sequence + Expansion to Red Giant: 22 h, 24 min. H burning H  He He  C, O He burning: (Red Giant Phase) 1 h, 35 min, 53 s

26. Slide 26 The Life “Clock” of a Massive Star (2) H  He He  C, O C  Ne, Na, Mg, O Ne  O, Mg H  He He  C, O C  Ne, Na, Mg, O 12 1 2 3 4 5 6 7 8 9 10 11 C burning: 6.99 s Ne burning: 6 ms 23:59:59.996

27. Slide 27 The Life “Clock” of a Massive Star (3) H  He He  C, O C  Ne, Na, Mg, O Ne  O, Mg O burning: 3.97 ms 23:59:59.99997 O  Si, S, P H  He He  C, O C  Ne, Na, Mg, O Ne  O, Mg Si burning: 0.03 ms The final 0.03 msec!! O  Si, S, P Si  Fe, Co, Ni

28. Slide 28 Inside Stars (SLIDESHOW MODE ONLY)

29. Slide 29 The Fate of Our Sun and the End of Earth Sun will expand to a Red giant in ~ 5 billion years Expands to ~ Earth’s radius Earth will then be incinerated! Sun may form a planetary nebula (but uncertain) Sun’s C,O core will become a white dwarf

30. Slide 30 The Final Breaths of Sun-Like Stars: Planetary Nebulae The Helix Nebula Remnants of stars with ~ 1 – a few M sun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts) Less than 10,000 years old Have nothing to do with planets! 

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32. Slide 32 The Dumbbell Nebula in Hydrogen and Oxygen Line Emission

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39. Slide 39 What is left?? A stellar remnant: white dwarf, composed mainly of carbon and oxygen

40. Slide 40 Detecting the presence of an unseen companion, Sirius B by its gravitational influence on the primary star, Sirius A. Wobbling motion of Sirius A 1850: a strange star was discovered

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42. Slide 42 Sirius B is very hot: surface temperature 25,000 K Yet, it is 10,000 times fainter than Sirius A It should be very small: R ~ 2 R earth Its mass M ~ 1 M sun It should be extremely dense! M/R 3 ~ 10 6 g/cm 3

43. Slide 43 White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! White Dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Chunk of WD material the size of a beach ball would outweigh an ocean liner!

44. Slide 44 All atoms are smashed and the object is supported by pressure of degenerate electrons

45. Slide 45 Degenerate gas -The core is compressed until the inter-particle distance = de Broglie wavelength -One particle occupies finite volume in space and in momentum space - we can decrease the particle size by increasing its velocity (energy) - Pauli exclusion principle permits only one particle per each state

46. Slide 46 Perhaps one of the key questions when Bohr offered his quantized orbits as an explanation to the UV catastrophe and spectral lines is, why does an electron follow quantized orbits? The response to this question arrived from the Ph.D. thesis of Louis de Broglie in 1923. de Broglie argued that since light can display wave and particle properties, then perhaps matter can also be a particle and a wave too. Energy and momentum of a particle are related to wavelength: Wave-particle duality Wave packet

47. Slide 47 Your de Broglie wavelength: de Broglie wavelength for the electron in an atom: Note the velocity dependence!

48. Slide 48 Degenerate matter N particles in volume V Density of particles = N/V Share of volume per particle:  V = V/N = 1/n Distance between particles = d  V ~ d 3 ; therefore d ~ (  V) 1/3 = n -1/3 d Particle density When particles are so close that d ~ deBroiglie, gas becomes degenerate

49. Slide 49 Pauli exclusion principle: Only one electron per quantum state With increasing density, particles occupy states with higher velocity and kinetic energy Pressure increases; however the temperature does not increase!

50. Slide 50 Equation of State: Molecular weight  =1 for pure hydrogen, 4 for helium, etc. Classical gas

51. Slide 51 Equation of State: Degenerate gas Low density, small kinetic energy High density, large kinetic energy

52. Slide 52 Strange properties of degenerate matter It strongly resists compression: P ~  5/3 Pressure does not depend on temperature Gas becomes more ideal with increasing density

53. Slide 53 Evolution of sun-like stars on H-R diagram

54. Slide 54 White Dwarfs (2) Low luminosity; high temperature => White dwarfs are found in the lower left corner of the Hertzsprung-Russell diagram.

55. Slide 55 White dwarfs in a globular cluster

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58. Slide 58 As it cools, carbon crystallizes into diamond lattice. Imagine single diamond of mass 10 30 kg! Don’t rush, you would weigh 15,000 tons there!

59. Slide 59 Summary of Post Main-Sequence Evolution of Stars M > 8 M sun M < 4 M sun Evolution of 4 - 8 M sun stars is still uncertain. Fusion stops at formation of C,O core. Mass loss in stellar winds may reduce them all to < 4 M sun stars. Red dwarfs: He burning never ignites M < 0.4 M sun Supernova Fusion proceeds; formation of Fe core.