The Death of Stars

1. The Death of Stars

2. Parts of a MS star What holds a star up while it is on the MS? On the Main Sequence How does energy get out? Radiation & Convection May take a million years to reach the surface.

3. The smallest stars Brown Dwarfs : Stars with core mass < .08 Msun (failed stars). Brown Dwarfs do not get hot enough to fuse H, but they do fuse Deuterium for a very short time. Deuterium is an isotope of H, with a neutron. About 1,000 Brown Dwarfs have been found. They radiate in the infrared wavelength.

4. Red Dwarfs : Stars with a core mass of .08 to 0.4 solar mass Coolest and dimmest of all MS stars. They remain on MS hundreds of billions of years. When all the H is converted to He fusion ceases, they cool down, moving down and to the right in the H-R diagram.

5. Red Dwarfs are very low mass stars with no more than 40% of the mass of the Sun and represent the majority of the stars. They have relatively low temperatures in their cores; red dwarfs transport energy from the core to the surface by convection. . A low-mass main-sequence star of spectral classes M and L. Red dwarf stars range from about 0.6 solar mass at class M0 down to 0.08 solar mass in cool M

6. Death of Low Mass Star Final State Core Mass 0.5 - 1.4 White Dwarf

7. Thermodynamics When Fusion stops, core shrinks & temperature of core rises. When the Envelope expands its temperature cools down.

8. Evolution of Low-Mass Stars 0.5 - 1.4 Msun Main Sequence Phase Energy Source: H fusion in the core Using P-P cycle H fuses to He Slowly builds up an inert He core He Fusing Envelope

9. When the H in the core is almost completely converted into He,H fusion stops in core. The left over H is pushed out into a shell ring around the He core. The core collapses & heats up. The increasing temp will cause the H shell to fuse forming He that will join the core Outer layer expands and cool forming a Red Giant

10. (On the H_R Diagram) • The star gets brighter and redder, climbs up the Giant Branch. (Takes 1 Byr) At the top of the Giant Branch, the star’s envelope is about the size of Venus’ orbit

11. The core will contract until it gets hot enough to fuse the He in the core into Carbon & Oxygen. When the fusion begins, the burning occurs rapidly because of the H shell burning and the He burning in the core. This is called the “Helium Flash”. Some of the outer layers are blown outward causing the star to loose mass.

12. The star gets hotter, and moves onto the Horizontal Branch

13. Once the He fuses and forms C & O, the core contracts. • C-O core collapses and heats up • He burning shell outside the C-O core • H burning shell outside the He burning shell • The core never gets hot enough to fuse the Carbon • & Oxygen                                     Outside: Envelope swells &cools because of H & He burning Climbs the Asymptotic Giant Branch

14. Climbs the Giant Branch again, slightly to the left , and higher, becoming a super red giant. .

15. Core and Envelope separate, takes ~100,000 yr C-O core continues to contract: With weight of envelope taken off, core never reaches Carbon fusion temp of 600 Million K Outer envelope gets slowly ejected . This is a non-violent ejection; a series of puffs or burps. Expanding envelope forms a ring nebula around the contracting C-O core.

16. A Planetary Nebula forms Hot C-O core is exposed, moves to the left Becomes a White Dwarf

17. Planetary Nebulae

18. Butterfly Nebula Fig. 13.16c

19. Planetary Nebula Ejection not explosive Nebula shell expands Outer shells of red supergiant “puffed off” The nebula is ionized, and heated by the. Ultra-violet radiation from the hot star • After ~ 50,000 years, the nebula spreads so far that the nebulosity simply fades from view. Hot dwarf left behind Cools down to form a WD

20. Contraction of the core is stopped by electron degeneracy. The electrons repel each other as they are pressed closer together and a White Dwarf forms. White Dwarfs have a mass that is less than 1.4 Mo They will shine for a long time but no fusion is taking place. • One teaspoon weighs about 5 tons.

21. Electron energy levels • Only two electrons (one up, one down) can go into each energy level. • In a degenerate gas, all low energy levels are filled.

22. White Dwarfs are planetary in size, but have a stellar mass Radius (a little smaller than Earth!) Temp. – anywhere from 100,000 to 2500 K. White dwarfs shine by leftover heat, no fusion. WD will cool off and fade away slowly, becoming a "Black Dwarf“. White Dwarf’s mass < than the Chandrasekhar mass(1.4 Solar Mass) Takes ~10 Tyr to cool off , so none exists yet.

23. White Dwarfs are so small, that they can only be seen if close-by, or in a binary systems. Sirius B The most famous W.D. is Sirius’ companion . Sirius B Temp. 25,000 K Size: 92% Earth's diameter Mass: 1.2 solar masses Sirius B The mass of a star, in the size of a planet.

24. A lone white dwarf is a cooling corpse but a white dwarf in a binary system can be revived

25. There is more !! A White Dwarf in a binary system… I White Dwarf Evolving (dying) star Roche Lobes II Evolving (dying) star White Dwarf Accretion Disk III Evolving (dying) star Roche Lobe filled

26. W.D. can take on material but, if the W.D. exceeds 1.4 solar masses (Chandrasekar limit) powerful explosions take place and they could happen more than once. The star will get down below 1.4 solar mass. Type 1asuper NOVA!!

27. Since the Type 1a supernova is always a white dwarf they can be used to judge very great distances (using the inverse square law). Type Ia: No hydrogen lines in the spectrum Type II: Hydrogen lines in the spectrum There is a further subdivision of I into Ia, Ib, Ic

28. Sun Low Mass Stars Becomes Red Giant when H is almost gone Envelope separates from core and forms a planetary nebula Orbit out to almost Venus Red Giant If the White Dwarf is a binary star, a Supernova type 1a can form, if its mass becomes greater than 1 ¼ solar masses Red Super Giant Becomes a Red Super Giant Core forms a WhiteDwarf White Dwarf becomes a Black Dwarf (dead star) Only H , He in shells, C & O in core left C & O do not fuse

29. Wanted Of course you know the relationship is just going to end in a Type 1a supernovae...but I suppose its better to have transferred mass and exploded than to have never transferred mass at all...

30. Stellar Graveyard High Mass Stars Final Core Mass Final State 1.4 < M < 3.0 Neutron Star

31. Evolution of Massive Stars Massive stars have the same internal changes as we saw in low mass stars , except : massive stars evolve more rapidly due to rapid nuclear burning, and massive stars produce heavier elements

32. Evolution of High-Mass Stars • High-Mass Stars • O & B Stars core mass >1.4 and <3 Msun • Burn Hot • Live Fast • Die Young • Main Sequence Phase: • Burn H to He in core using the CNO cycle • Build up a He core, like low-mass stars • But this lasts for only ~ 10 Myr

33. Red Supergiant Phase • After H core exhaustion: • Inert He core contracts & heats up the • H burning in a shell . • Envelope expands due to the burning H shell and cools • Envelope ~ size of • orbit of Jupiter

34. Moves horizontally across the H-R diagram, becoming a Red Supergiant star Takes about 1 Myr to cross the H-R diagram.

35. Core Temperature reaches 170 Million K Helium Flash : Helium ignites This Helium flash is not as explosive as the one for low mass stars. Helium Fusion produces C & O in core: Star heats up and becomes a Yellow Supergiant.

36. Star becomes a YellowSupergiant. Yellow

37. When He exhausted in core • Inert C-O core collapses & heats up the • H & He burning in shells. Star expands • and becomes a Red Supergiant again

38. C-O Core collapses until: Tcore> 600 MillionK • Carbon in the Core ignites. C fuses to form : Ne , and O • Core at the end of • Carbon Burning • Phase:

39. Nuclear burning continues past Helium Things happen fast! 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core End of the line!!

40. Massive star at the end of Silicon Burning: Onion Skin of nested nuclear burning shells

41. Protons & electrons form neutrons & neutrinos. Collapse is final. • At the start of Iron Core collapse: • Radius ~ 6000 km (~Rearth) • Density ~ 108 g/cc • A second later!! , the properties are: • Radius ~50 km • Density ~1014 g/cc • Collapse Speed ~0.25 c !

42. Supernova explosion Neutron degeneracy pressure halts the collapse Material falling inwards rebounds. Outer layers of the atmosphere, including shells, are blown off in a violent explosion called a supernova. The star will outshine all the other stars in the galaxy combined.

43. Elements heavier than Lead are produced in the explosion and ejected into space. Stars do recycle. The ejected material often attain speeds of 100,000 km/sec. Close to 150 supernova remnants have been detected in the Milky Way. There are smaller numbers of massive stars and so smaller amount of explosions.

44. The Famous Supernova SN 1987A type II Supernova At maximum Before

45. Supernova remnants Cygnus Loop (HST): green=H, red=S+, blue=O++ Cas A in x-rays (Chandra) Vela Remnant of SN386, with central pulsar (Chandra) SN1998bu

46. The rings of SN 1987A are from previous mass loss

47. 1a is binary with a White Dwarf Type II : Hydrogen lines in the spectrum

48. Supernova explosion 1, The iron core collapses 2. Neutrons stop the collapse 3. The rebound of the core sends shock waves causing an explosion that blows the outer atmosphere into space as a super nova

49. The Crab Nebula. A supernova that, according to the Chinese, exploded in 1054. Despite a distance of ~ 7,000 light-years, the supernova was brighter than Venus for weeks before fading from view after nearly two years. • Even today, the nebula • is still expanding at • more than 3 million • miles per hour.

50. Structure of a Neutron Star • Diameter~ 12 km in diameter • Mass -about 1.4 times that of our Sun. • One teaspoonful of material would weigh a billion tons! Rotation Rate: 1 to 100 rotations/sec