The Death of Stars

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.