Life story of a star

1. Life story of a star Micro-world Macro-world Lecture 20

2. Life Cycle of Stars • Recycling • Supernovae produce • - heavy elements • neutron stars • black holes • Martin Rees - Our Cosmic Habitat

3. Our favorite star: The Sun • R๏ =696,000 km • (109 x Rearth) • M=2x1030kg • (3x105 x Mearth) • Rotation period: • 25 days(equator) • 30days (poles) • Composition: • 70% Hydrogen • 28% Helium

4. Stars have different colors Stars have different colors • B: blue – hottest • A: green – warm • C: red - cool Infer temperature of a star from the peak wavelength of its black body radiation

5. Color, Brightness + Count them Sun BelindaWilkes

6. Solar fusion processes + 1.4 MeV + 5.5 MeV + 12.9 MeV

7. Neutrinos come directly from solar core

8. Superkamiokande

9. Sun as seen by a neutrino detector

10. What happens when the Sun’s Hydrogen is all used up?

11. Evolution of a Star Red Giant (Sun)

13. Main Sequence Evolution • Core starts with same fraction of hydrogen as whole star • Fusion changes H  He • Core gradually shrinks and Sun gets hotter and more luminous

14. Evolution of the Sun • Fusion changes H  He • Core depletes of H • Eventually there is not enough H to maintain energy generation in the core • Core starts to collapse

15. The Sun will become a Red Giant

16. The Sun 5 Billion years from now Earth

17. The Sun Engulfs the Inner Planets

18. Red Giant Phase • He core • No nuclear fusion • Gravitational contraction produces energy • H layer • Nuclear fusion • Envelope • Expands because of increased energy production • Cools because of increased surface area

19. Helium fusion Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

20. Helium Flash • He core • Eventually the core gets hot enough to fuse Helium into Carbon. • This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. • We don’t see this flash because it’s buried inside the Sun. • H layer • Envelope

21. Red Giant after Helium Ignition • He burning core • Fusion burns He into C, O • He rich core • No fusion • H burning shell • Fusion burns H into He • Envelope • Expands because of increased energy production

22. What happens when the star’s core runs out of helium? • The star explodes • Carbon fusion begins • The core starts cooling off • Helium fuses in a shell around the core

23. Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate

25. Sun looses mass via winds • Creates a “planetary nebula” • Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen a “white dwarf star”

27. Planetary nebula

28. Planetary nebula

29. Planetary nebula

30. Hourglass nebula

31. White dwarf • Star burns up rest of hydrogen • Nothing remains but degenerate core of Oxygen and Carbon • “White dwarf” cools • No energy from fusion, no energy from gravitational contraction • White dwarf slowly fades away…

32. Time line for Sun’s evolution

33. Brightest Star – Sirius A – (Sirius B is a white dwarf) Orion Constellation ( Nebula) Sirius Betelgeuse (Red Giant) Comet Hale-Bop Sirius B

34. This is a Hubble Space Telescope image - the first direct picture of the surface of a star other than the Sun. • While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger. If placed at the center of our Solar System, it would extend past the orbit of Jupiter. • Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familiar constellation of Orion, the Hunter. • The name Betelgeuse is Arabic in origin. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova. In this historic image, a bright hotspot is revealed on the star's surface. Betelgeuse is a red supergiant star about 600 light years distant

35. The Sun Engulfs the Inner Planets

36. The Sun becomes a White Dwarf Composition: Carbon & Oxygen

37. What about M>1.4 M๏ stars?

39. Nuclear burning continues past Helium 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core

40. Multiple Shell Burning • Advanced nuclear burning proceeds in a series of nested shells

41. Fusion stops at Iron

42. Fusion versus Fission

43. Advanced reactions in stars make elements like Si, S, Ca, Fe

44. Atomic collapse Supernova Explosion • Core pressure goes away because atoms collapse: electrons combine with protons, making neutrons and neutrinos • Neutrons collapse to the center, forming a neutron star

45. Atomic Collapse Ordinary matter ~few grams/cm3 White Dwarfs ~1 ton/cm3 Neutron star ~108 ton/cm3

46. Core collapse • Iron core grows until it is too heavy to support itself • Atoms in the core collapse, density increases, normal iron nuclei are converted into neutrons with the emission of neutrinos • Core collapse stops, neutron star is formed • Rest of the star collapses in on the core, but bounces off the new neutron star (also pushed outwards by the neutrinos)

47. Supernova explosion

48. SN1987A Tarantula Nebula in LMC Neutrinos are detected Feb 23, 1987 Feb 22, 1987

49. Previously observed Supernova“Kepler’s Supernova” Oct 8, 1604 ChosunSilok Kepler’s Supernova today

50. Light curve from Kepler’s Supernova