Lives and Deaths of Stars

1. Lives and Deaths of Stars

2. Milky Way Galaxy 28,000 light years 200 billion stars

3. 100 billion galaxies in the observable Universe 10 day exposure photo! Over 1500 galaxies in a spot 1/30 the diameter of the Moon Farthest and oldest objects are 12-13 billion light years away!

4. Proxima Centauri (Alpha Centauri C) Closest star (4.2 light-years from the Sun) Voyager 1: 12 light-hours from the Sun (90 AU) Launched in 1977 The most distant human-made object in space

6. How can we learn about the life of stars?? • Our life span is ~ 80 years • Human civilization exists ~ 5000 years • Our Sun exists at least 4.6 billion years!

7. Star Clusters – “School Classes” for Stars They consist of stars of the same age ! Globular clusters 100,000 of stars Open clusters 100’s of stars

8. Giant molecular clouds – stellar nurseries

9. Great Orion Nebula

10. The Horsehead Nebula

11. Star Forming Region RCW 38

12. Coldest spots in the universe: • T ~ 10 K • Composition: • Mainly molecular hydrogen • 1% dust

14. Protostars: warm clumps of gas surrounded by infalling matter Disks: planet formation?!

16. The matter stops falling on the star • A star becomes hot enough to sustain the pressure of gravity

17. Contraction stops when the gravity is balanced by thermal pressure Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity.

18. Surface temperature 6000 K Temperature at the center 14,000,000 K!

19. A puzzle: the Sun and other stars radiate away huge amounts of energy. They should lose all their heat in less than a million years! However, the Sun lives 4.6 billion years There must be an internal energy source: nuclear fusion reactions

20. “Planetary” model of atom Proton mass: 1.7x10-27 kg Electron mass: 9x10-30 kg

21. Nuclear reactions • Fission: decay of heavy nuclei into lighter fragments • Fusion: synthesis of light nuclei into a heavier nucleus

22. A star will live until all hydrogen is exhausted in its core Our Sun will live 5 billion years more

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

24. Fate of the collapsed core • White dwarf if the remnant is below the Chandrasekhar limit 1.4 solar mass • Neutron star if the core mass is less than ~ 3 solar masses • Black hole otherwise

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

26. Outer layers expand due to radiation pressure from a hot core • Surface temperature drops by a factor of ~ 2 • The radius increases by a factor of ~ 100 • Luminosity increases ~ R2 T4 ~ 100-1000 times The star becomes a Red Giant

27. 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.

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

36. White dwarf It is extremely dense All atoms are smashed and the star is supported by pressure of free electrons

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

38. As it cools, carbon crystallizes into diamond lattice. Imagine single diamond of mass 1030 kg! Don’t rush, you would weigh 15,000 tons there!

39. White dwarfs in a globular cluster

40. Death of a massive star (SLIDESHOW MODE ONLY)

41. The iron core of a giant star cannot sustain the pressure of gravity. It collapses inward in less than a second. The shock wave blows away outer layers of a star, creating a SUPERNOVA EXPLOSION!

42. For several weeks the supernova outshines the whole galaxy

43. Eta Carinae: will explode soon Distance 7500 ly

44. Supernova Remnants X-rays The Crab Nebula: Remnant of a supernova observed in a.d. 1054 Cassiopeia A The Veil Nebula Optical The Cygnus Loop

45. Crab nebula: the remnants of supernova 1054

46. Formation of Neutron Stars Compact objects more massive than the Chandrasekhar Limit (1.4 Msun) collapse further. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object: p + e-n + ne Neutron Star

47. Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 Msun Density: r ~ 1014 g/cm3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!

49. Neutron stars have been theoretically predicted in 30s. Landau, Oppenheimer, Zwicky, Baade Isolated neutron stars are extremely hard to observe

50. However, there are two facts that can help: • Neutron stars should rotate extremely fast due to conservation of the angular momentum in the collapse • They should have huge magnetic field due to conservation of the magnetic flux in the collapse