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The Death of Stars

The Death of Stars. Stellar Recycling. Topics. Low Mass Stars Red Giants Planetary Nebulae White Dwarfs Novae High Mass Stars Red Super Giants Supernovae Neutron Stars Summary. Low Mass Stars. Red Giants

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The Death of Stars

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  1. The Death of Stars Stellar Recycling

  2. Topics • Low Mass Stars • Red Giants • Planetary Nebulae • White Dwarfs • Novae • High Mass Stars • Red Super Giants • Supernovae • Neutron Stars • Summary

  3. Low Mass Stars • Red Giants • Hydrogen fusion occurs in a shell around the helium core, the heat from which causes the outer layers of the star to expand. • This increases the luminosity of the star. • As the outer layers expand they cool and the star becomes a red giant.

  4. Planetary Nebulae Helix Nebula (450 ly) Ring Nebula (2300 ly)

  5. White Dwarfs • White Dwarfs • End-point of low mass stars like the Sun. • Properties • Radius about 104 km, that is, about the size of the Earth. • Mass < 1.4 times that of the Sun. • Density about 106 g/cm3. One teaspoon of white dwarf matter weighs as much as a car!

  6. Fig. 13-5, p.263

  7. Novae • Binary Stars • Binary star systems are very common. • So it is possible to have a white dwarf star paired with another star. • In such a binary system, the white dwarf can strip hydrogen from the companion star. • This process forms an accretion disk about the white dwarf.   

  8. Novae – II • Nova • The hydrogen heats up as it slams into the surface of the white dwarf. If the temperature exceeds about 10 million K, a gigantic thermonuclear explosion can occur, called a nova, on the star’s surface. • Recurrent Novae • Sometimes this process can occur repeatedly. The binary system is then referred to as a recurrent nova.

  9. Supernovae – Type I • Chandrasekhar Limit • In 1928, the Indian graduate student Subrahmanyan Chandrasekhar calculated that  a star heavier than about 1.4 solar masses cannot remain stable.  This limit is called the Chandrasekhar Limit. • If the amount of hydrogen accreted onto a white dwarf causes the star's mass to exceed 1.4 solar masses the star will disintegrate explosively, leaving no remnant. • This is called a Type I supernova.

  10. High Mass Stars • Red Super Giants • When core has been converted to helium the weight of the outer layers squeezes it thereby making it hotter. • When the core reaches ~ 100 million K helium to carbon fusion begins as the star grows to a red super-giant.

  11. 4He 4He 8Be 4He 12C → → + + We are Made of Stardust • Helium to Carbon Fusion • 4He + 4He → 8Be • 4He + 8Be →12C Beryllium lasts only ~ 10-12 s

  12. 7.6 MeV 4He 4He 8Be 4He 12C → → + + Stardust – II The total energy of the beryllium and helium nuclei, which includes themassof beryllium + helium + theirkinetic energy, is about4%greater, on average, than the7.6 MeV(million electron-volt) energy level of carbon. Because of this coincidence the rate of beryllium/helium fusion to carbon is increased enormously.

  13. Life of a 20 Solar Mass Super Giant • Hydrogen fusion • ~ 10 million years • Helium fusion • ~ 1 million years • Carbon fusion • ~ 300 years • Oxygen fusion • ~ 9 months • Silicon fusion • ~ 2 days http://cassfos02.ucsd.edu/public/tutorial/SN.html

  14. Betelgeuse • In massive stars, like Betelgeuse, nuclear synthesis of heavier and heavier elements occurs up to the element iron. • The reaction that creates iron is endothermic (it cools the surroundings), which decreases the core’s pressure.

  15. Supernovae – Type II • Type II • At some point the pressure in the core is unable to balance the gravitational compression, and the iron core collapses, leaving a void. • The outer layers of the star crash onto the core, crushing it further and heating it to extreme temperatures. • This triggers one of the most powerful thermonuclear explosions in the Universe: A Type II supernova.

  16. Supernovae – Type II,… • Core Collapse • The collapse of the iron core fuses each proton with an electron to form a neutron and a neutrino. • Neutrino Shockwave • The mass of the collapsed core is of order 1030 kg. • Each proton has a mass of order 10-27 kg. • So the number of protons converted to neutrons is about 1030 / 10-27 = 1057, equal to the number of neutrinos that rush outwards from the collapsed core at the speed of light.

  17. Supernovae – Type II,… • Neutrino Shockwave • As noted earlier, neutrinos interact so weakly with matter that, unlike photons, they stream out of the Sun unimpeded in a matter of seconds. • However, the number of neutrinos created in a Type II supernova is so enormous that the tiny fraction that does interact with the in-falling matter is enough to push the matter outwards, reheating it and making it glow brighter than an entire galaxy of stars.

  18. Supernovae – Type II,… • Neutron Star • As the outer layers expand the neutron core continues to be crushed. What is left behind is an unimaginably dense object called a neutron star. • Black Hole • But even this is not the most extreme remnant. For the most massive stars, the neutron star is unstable and is predicted to collapse to a black hole.

  19. SN1987A • 1987 Supernova • In 1987 a fantastic milestone occurred in astronomy. For the first time in history scientists were able to record the neutrino shockwave from a Type II supernova. • The supernova occurred in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way, that lies about 160,000 light years from us.

  20. Fig. 13-13a, p.268

  21. Fig. 13-13b, p.268

  22. Neutron Stars and Pulsars • Calculated Properties • Radius about 10 km. • Density from 104 g/cm3  at the surface to about 1015 g/cm3 at the center. This is about a billion times denser than a white dwarf! • Structure the star is a very smooth, rapidly spinning, rigid metallic shell filled with a neutron super-fluid, a fluid that moves without resistance.

  23. Neutron Star Fig. 13-18, p.271

  24. Jocelyn Bell Discovered first pulsar (1967) Fig. 13-19, p.271

  25. Crab Nebula Fig. 13-11a, p.267

  26. Pulsar Model A rapidly rotating neutron star Fig. 13-22, p.273

  27. Summary • Low Mass Stars • End-point is a white dwarf star, roughly the size of the Earth but the mass of the Sun. • A white dwarf in a binary system can accrete matter from its partner and become a nova. If the mass exceeds 1.4 solar masses the white dwarf is destroyed in a Type I supernova. • High Mass Stars • End-point either a neutron star or a black hole after suffering a Type II supernova.

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