Star evolution

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 Msun) • Intermediate mass (2  8 Msun) • High mass (>8 Msun) • 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? • Low mass • Intermediate mass • High mass

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

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? • Yes • No

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 • More • Less

10. What’s happening to the mass of the HELIUM core as the shell “burns”? • Increasing • Decreasing • Staying the same

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? • Yes • No

15. If the universe contained only low mass stars, would there be elements heavier than carbon? • Yes • No

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? • The stars that formed earliest • The most recently formed stars

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? • X-rays • Visible light • Infrared • Radio waves

27. White dwarf limit • Observed around 1 Msun • Can be up to 1.4 Msun • 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 Msun 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. What you’d see through a telescope Ignore the spikes X-ray image & visible image superimposed Sirius binary system

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 Msun • Can be up to 2-3 Msun (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? • Yes • No

32. Is it possible to be a pulsar but not a neutron star, as seen on Earth? • Yes • No

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. • True • False

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