1. Chapter 22 The Death of Stars Will a star die with a Bang or a whimper ?

2. Death of Low-mass stars Low mass stars go through two red-giant stages. First when the core hydrogen is depleted, the core shrinks and shell hydrogen burning starts. This causes the outer layers to expand and become a red-giants.

3. Death of Low-mass stars Once the core gets heated to about 100 million Kelvin core helium burning starts, the core expands and the outer layers shrink - No longer a red-giant. When the core helium is depleted, once again the core shrinks, shell helium burning starts, and the outer layers expand, and the star is once again a red-giant.

5. Post-main-sequence evolution of a low mass star Red-Giant Branch After the star leaves the main-sequence, the core shrinks and the outer layers expand. Luminosity increases and the surface temperature drops. The star moves up and to the right in the H-R diagram

6. Post-main-sequence evolution of a low- mass star Horizontal Branch Core helium burning and shell hydrogen burning. outer layers shrink. The surface temp. goes up, the luminosity goes down slightly. Star moves to the left & slightly down. Remains in the branch approximately 100 mill. yrs.

7. Post-main-sequence evolution of a low- mass star Asymptotic Giant Branch (AGB) Core helium depleted. Shell Helium burning. Outer layers expand and cool. The surface temp. goes down, the luminosity goes up due to increasing size Ascends to the red-giant region in the H-R diagram for the second time.

8. The structure of an AGB star near the end of its life. Asymptotic Giant Branch Star

9. Planetary Nebulae Dying low-mass stars gently eject their outer layers. In the Sun, convection is responsible for energy transport only in the outer layers. –This involves the up-and -down movement of gasses During the final stages the convection zone can reach all the way down to the core.

10. Planetary Nebulae During this time the convection currents can “dredge-up” the heavy elements (carbon) produced in and around the core to the surface. During the last stage of an AGB star it ejects shells of mater in to space in a series of bursts.

11. An aging 1M  star looses as much as 40% of its mass. As the outer layers are ejected, the hot core (about 100,000 K) of the dying star is exposed. This hot core emits UV radiation and that excites and ionizes the ejected gas. The gas then glows, producing planetary nebulae. Planetary Nebulae

12. Helix Nebula: the closest planetary nebula to us. Planetary nebulae are very common - there are estimated 20,000 - 50,000 in our Galaxy.

13. Planetary Nebulae Planetary nebula NGC 7027.

14. White Dwarf Stars Sirius and its white dwarf companion.

16. White dwarfs Final state of low mass stars Mass: less than 1.4 M  - Chandrasekhar limit Size about same as Earth Temperature: typically 25,000 K (after cooling from 100,000K) No energy source - glows from residual heat Cools to about 20,000 K in 10 million years Eventually becomes a black dwarf

18. Evolution from Giants to White Dwarfs TrackGiant starejected nebulaWhite dwarf A3.01.81.2 B1.50.70.8 C 0.20.6 Mass in M 

19. White dwarfs (cont.) Density: 10 6 g/cm 3 - extremely dense Structure of white dwarfs: Gravity not balanced by heat pressure, but by degeneracy of its electrons - due to Pauli exclusion principle. Adding mass causes radius to shrink!

20. Degeneracy: the Chandrasekhar limit Degeneracy means electrons are free to move about the whole white dwarf (not just stuck in single atom) –Caused by quantum mechanical Pauli exclusion principle –The gravity is balanced by degeneracy electron pressure

21. Degeneracy: the Chandrasekhar limit Adding mass causes radius to shrink. Eventually forces electrons to join with protons to become neutrons. White dwarf can collapse into neutron star. Max. mass of white dwarf = Chandrasekhar limit = 1.4M 

22. White dwarf binary systems A white dwarf can be a companion star in a binary system. When the other star evolves into red giant and if the stars are close enough, matter from the other star can fall into the white dwarf and increase its mass. –Example: Sirius A and B

23. White dwarf binary systems Companion

24. The Mass-Radius Relationship for white dwarfs. Higher the mass, smaller the radius due to gravitational collapse. White dwarfs (cont.)

25. Nova and Type I supernova Adding mass to white dwarf results in –Nova (“new star”): hydrogen ignites on surface in large explosion that does NOT destroy white dwarf. Can be recurring. –Type I supernova: White dwarf increases mass over Chandrasekhar limit (perhaps after many novae) and collapses into neutron star and may explode in supernova - runaway carbon burning occurs. Results in formation of large amount of radioactive Ni-56, which eventually decays to Fe-56.

27. Characteristics of Type I supernova Type I: –precursor: white dwarf –cause: mass increases beyond Chandrasekhar limit –result: destruction of star, formation of large amount of iron –identified by gamma ray spectrum caused by radioactive Ni-56 decay and lack of hydrogen emission or absorption lines. –lack of hydrogen lines in the spectrum because most of the hydrogen in the outer layers of the star has already been expelled into space in the form planetary nebula.

28. Death of massive stars In massive stars (M > 4 M  ), once the core helium burning is over, the degeneracy electron pressure is not great enough to stop the core from collapsing and heating. When the core temp. reaches 600 million Kelvin, core carbon burning begins. This scenario will proceed through neon burning, oxygen burning and finally silicon burning. Between each stage, the star will go through red- giant phases and the H-R diagram track will make several back and forth gyrations.

29. Death of massive stars At each stage the outer layers expand more and more. - The final result is a supergiant star : Betelgeuse and Rigel in Orion constellation.

30. The structure of an high-mass star near the end of its life.

31. Death of massive stars Stars whose mass is less then 8 M  eject most of their mass in the form of planetary nebulae. For stars with M > 8 M , the end comes with a spectacular explosion. Once such a star gets to the iron core stage, (and since iron cannot fuse), the core contracts rapidly and the temperature jumps to a whopping 5 billion Kelvin. This will disintegrate the iron into helium ions in a fraction of second.

32. Death of massive stars The pressure becomes so high that the electrons are forced to combine with protons to form neutrons. This process takes only another tenth of a second. e - + p +  n + Core becomes very stiff, and the collapse suddenly stops. The plunging outer layers bounces off the extremely stiff core back and into to the surrounding space.

33. Death of massive stars The energy released in this catastrophic event is more than all the energy emitted by our Sun in the past 4.6 billion yrs. The Star has become a Supernova. This type of a Supernova is called a Type II supernova. The material being ejected by such processes are so compressed that there are waves of thermonuclear processes that take place and these create all the elements heavier than iron.

35. Death of massive stars

36. Supernova of 1987: SN 1987A On February 23, 1987, a supernova was discovered in our companion galaxy - the Large Magellanic Cloud(LMC) - 50,000 pc from Earth. Peak luminosity was so large (10 8 L  ) that it could be seen with the naked eye. SN 1987A was the first supernova after 400 years that was visible to the naked eye.

37. Supernova of 1987: SN 1987A Before and after. Progenitor star was a B3 I supergiant.

38. Supernova of 1987 True color view of Hubble space telescope.

39. Supernova of 1987 Possible origin of the ring.

40. Characteristics of type II supernova Type II: –precursor: massive star –cause: nuclear fuel spent, iron core no longer able to hold up weight of star, collapses, rest of star bounces off of shrunken neutron core. –result: destruction of star, formation of elements heavier than iron, neutron star or black hole remains –identified by presence of hydrogen emission or absorption lines.

41. Type I Type II

42. The final fate of stars