1. Star evolution Chapters 17 & 18 (Yes, we skip chap. 16, star birth)

2. Goals & Learning Objectives Learn some simple astronomical terminology Develop a sense of what scientists know about the overall universe, its constituents, and our location Describe stellar evolution Contrast the life history of a low-mass star with the life history of a high-mass star. Explain how black holes are formed and their effect on their surrounding environment.

3. 3 star groups (p. 565) 3 categories of stars: – Low mass (<2 M sun ) – Intermediate mass (2  8 M sun ) – High mass (>8 M sun ) Intermediate similar to both high and low mass. Book focuses more on similarities with high mass (in section 17.1). One major difference: high mass stars die very differently!

4. Which star group has the highest core pressure? 1.Low mass 2.Intermediate mass 3.High mass 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

5. Which star group has the hottest core temperature? 1.Low mass 2.Intermediate mass 3.High mass So what can you conclude about the fusion rate? Luminosity? Which stars live longer? Why? 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

6. The end of the Sun Eventually core runs out of hydrogen. What did the core need fusion for? What will happen to it as a result of losing fusion? What happens to gas balls when they shrink? What happens to the temperature of the material surrounding the core? CLICKER QUESTION (next slide). What are the surrounding layers made of? What can happen if they get hot enough? For Sun, this takes hundreds of millions of years.

7. Is there Hydrogen outside the Sun’s core? 1.Yes 2.No 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

8. Shell “burning” In fact, the outer layers get hotter than 15 million K. What does that tell us about hydrogen fusion rate? What should we observe as a result? CLICKER The light “gets stuck” and pushes the outer layers out. What happens to gas when you expand it? Color of outside? What kind of star do we have? What is the core made of? What is the structure? See fig. 17.4 page 568

9. Star becomes ______ luminous 1.More 2.Less 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

10. What’s happening to the mass of the HELIUM core as the shell “burns”? 1.Increasing 2.Decreasing 3.Staying the same 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

11. Inside the core… Core shrinks Core gets hotter More hot helium dumped onto core Something must stop the core from shrinking. – Low mass stars: degeneracy pressure Read section 16.3, page 557 and S4.4 pp. 481-483 Mosh pit – Intermediate & High mass: fusion causing thermal & gas pressure. Helium Fusion turns on at 100 million K – Low mass: whole core starts fusing simultaneously: helium “flash” – Intermediate & high mass: “regular” fusion

12. Next phase Structure of the star now? Figure 17.5 This lasts until … What happens to the core? – Low & intermediate mass: core shrinks until degeneracy pressure stops it. Focus on that now. – [for High mass: next fusion turns on] Back to low mass: What’s the core made of? Shrinks to size of Earth. What happens outside the core? – Temp, composition

13. Double shell burning Not stable Outer layers pulsate Outer layers come off See pictures around the planetarium – Cat’s eye, Butterfly, Ring: all “planetary nebula” See also figure 17.7 – more examples NOT related to planets What’s in the center of a planetary nebula? End of low & intermediate mass stars… Show interactive figure 17.4

14. Do low mass stars like the Sun fuse Carbon into anything? 1.Yes 2.No 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

15. If the universe contained only low mass stars, would there be elements heavier than carbon? 1.Yes 2.No 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

16. High mass star differences Degeneracy pressure never turns on – Gas & thermal pressure always stronger Can fuse carbon with helium into Oxygen Can fuse Oxygen with helium into neon Etc. (magnesium, silicon, sulfur) When core hot enough, can fuse carbon with carbon, carbon with oxygen … Etc. Big picture: carbon and stuff fuses until you get to a core made of … Iron (Fe on the periodic table, #26, middle section, top row, see page A-13, Appendix C)

17. Iron Most stable nucleus Can’t release energy by fusing it – Fusion USES energy (uses instead of ___________) True for everything heavier than iron, too. – Fission USES energy True for most things lighter than iron, too. Iron is the last element made in stable reactions in stars Look at the periodic table on page A-13 – Find iron – Gold = Au. Mercury = Hg. Xenon = Xe. Are these made in stable stars?

18. What we see See figure 17.12, page 575 for onion skin model See HR diagram on p. 575 (fig. 17.13) – Runs out of core fuel, goes right – Next fuel turns on, goes back left – Repeat until core is made of Iron

19. After the Iron core forms Iron core shrinks Gravity is stronger than Electron degeneracy pressure Electrons squeezed more than they can tolerate Electrons merge with protons Result: neutrons – And neutrinos! – (Fly straight out! We observe them first!) No more electron degeneracy pressure support. Rapidly shrinks: Earth-size shrinks to town-size in 1 second! Lots of energy released. Turn on neutron degeneracy pressure. Core bounces. Demo Supernova explosion. Leaves behind core Core is made of … Called … Interactive figure 17.12 & 17.17 (crab nebula in 1054) (If the core is too heavy for neutron degeneracy pressure…)

20. Production of Elements High mass stars make up to Iron Everything heavier made DURING the supernova – Lots of neutrons around – They merge with nuclei quickly (r-process) – Eventually nucleus decays to something stable – Like Gold, Silver, Platinum, Lead, Mercury, etc.

21. Stellar remnants End states for stars – Low mass stars become … – Intermediate mass also become … (Oxygen) – & high mass stars become … – The highest mass stars (O & B) become …

22. Which stars should begin with the most heavy elements inside them? 1.The stars that formed earliest 2.The most recently formed stars 1234567891011121314151617181920 2122232425262728293031323334353637383940 41424344454647484950

23. Summary of star death When fusion runs out, core ____ & _____ Shell fusing occurs. Many shells possible. Core fusion can turn on. What’s different for low mass & high mass? Which elements get made in low & high? What’s special about iron? Degeneracy pressure (electron & neutron) – What, where, why Possible end states; which stars make them – RG  PN  WD, RG  SN  NS or BH

24. Chapter 18: Stellar remnants The next few slides are material from chap 18.

25. White dwarfs Radius – Earth sized (4000 miles) What kind of pressure resists gravity? – Electron degeneracy pressure Temperature – Start hot. [Clicker question] – Cool down (black dwarf eventually) Composition: – Usually carbon – sometimes oxygen (intermediate mass) or helium (very low mass) Gravity: teaspoon weighs 5 tons!

26. What kind of light would a white dwarf emit most when it is first detectable? 1.X-rays 2.Visible light 3.Infrared 4.Radio waves 1234567891011121314151617181920 2122232425262728293031323334353637383940 4142434445464748495051525354555657585960 6162636465666768697071727374757677787980

27. White dwarf limit Observed around 1 M sun Can be up to 1.4 M sun If heavier, electrons can’t push out strongly enough to resist gravity. [they’d have to move faster than c] What happens if you add mass to a 1.4 M sun white dwarf? – Where could extra mass come from? – Supernova explosion! – “White dwarf supernova” (“Type 1a”) Are a “standard candle”. What’s that? – Leaves NOTHING behind, unlike massive star supernovae – LESS VIOLENT: Nova if add small amount of stuff to lower mass WD.

28. X-ray image & visible image superimposed Sirius binary system What you’d see through a telescope Ignore the spikes

29. Neutron stars Composition? – Gigantic nuclei. – No empty space like in atoms (99.999% empty) Paper clip of neutrons weighs as much as a mountain! Dropping brick: energy = an atom bomb! – As stuff falls onto a neutron star, releases X-rays! Mass – Observed: 1-1.4 M sun – Can be up to 2-3 M sun (we don’t know exact upper limit) – Any heavier & neutrons can’t push out strongly enough to resist gravity. Radius: City sized (6 miles). WD = 4000 miles! What kind of pressure resists gravity? – Neutron degeneracy pressure Neat trivia: Escape speed = ½ c. (Gravity very strong!)

30. Pulsars See figures 18.7 & 18.8 Jocelyn Bell Should’ve won the Nobel Prize Rapidly spinning neutron stars 1800 known pulsars, pulsing radio, but some also emit other types: visible + X-rays and sometimes gamma. – 1 pulsar, discovered in October 2008 emits only gamma See figure 18.9 Is it possible to be a neutron star that’s not a pulsar? How about vice versa? [2 clicker Q’s] Spin up to 600 times per SECOND! (Show movie!) – Larger objects would break apart

31. Is it possible to be a neutron star but not a pulsar, as seen on Earth? 1.Yes 2.No 1234567891011121314151617181920 2122232425262728293031323334353637383940 4142434445464748495051525354555657585960 6162636465666768697071727374757677787980

32. Is it possible to be a pulsar but not a neutron star, as seen on Earth? 1.Yes 2.No 1234567891011121314151617181920 2122232425262728293031323334353637383940 4142434445464748495051525354555657585960 6162636465666768697071727374757677787980

33. Black holes Black holes don’t “suck” – Strong gravity. Things FALL in; don’t get SUCKED Event horizon / escape speed – What happens if further away than event horizon? Schwarzschild radius: 3km per solar mass. Falling in – Redshift – Time dilation; time “travel” – Tidal stretching – Friends won’t see you die if fall into high mass How do we know they exist? – Cygnus X-1, XRB, accretion disks – Looking for BH collisions emitting gravitational waves, LIGO. – Gravitational lenses (MACHOs) Hawking radiation – black hole evaporation

34. Chap. 18, #18: If a black hole 10 times as massive as our Sun were lurking just beyond Pluto’s orbit, we’d have no way of knowing it was there. 1.True 2.False 1234567891011121314151617181920 2122232425262728293031323334353637383940 4142434445464748495051525354555657585960 6162636465666768697071727374757677787980

35. Summary of stellar “graveyard” White dwarf properties: mass, radius, pressure White dwarf limit, results of exceeding it Neutron star properties Pulsars Black holes – Falling in – Gravity far away – How we can find them