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

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 ! Open clusters 100’s of stars Globular clusters 100,000 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! There must be an internal energy source: nuclear fusion reactions However, the Sun lives 4.6 billion years

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 star mass (solar masses) Lifetime (years) 60 3 million 30 11 million 10 32 million 3 370 million 1.5 3 billion 1 10 billion 0.1 1000's billions 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 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 Neutron star if the core mass is less than ~ 3 solar masses Black hole otherwise Black hole otherwise

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

26. 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

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.

30. p. 192

31. p. 193

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

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

37. 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!

38. 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!

39. White dwarfs in a globular cluster

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

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

45. Crab nebula: the remnants of supernova 1054

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

47. Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density:  ~ 10 14 g/cm 3  Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!

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

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 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 They should have huge magnetic field due to conservation of the magnetic flux in the collapse

51. Jocelyn Bell Discovery of pulsars: Bell and Hewish, 1967

53. When the core is too massive, nothing can prevent collapse into a black hole

55. Schwarzschild radius: event horizon for a nonrotating body To make a black hole from a body of mass M, one needs to squeeze it below its Schwarzschild’s radius RsRs Gravitational collapse: the body squeezes below its event horizon

57. Black holes are NOT big cosmic bathtub drains! Far from a black hole R >> R s weak field Approaching a black hole R  R s (strong field): gravity pull very strong If our Sun collapses into a black hole, we won’t see any difference in the gravitational pull (but it will be VERY cold)

60. How to observe a black hole if it does not emit any radiation? Good news: most stars are in binary systems Good news: most stars are in binary systems

61. The Universe in gamma-ray eyes

62. Gamma-ray bursts

65. Our Earth and our bodies are made of atoms that were synthesized in previous generations of stars