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ASTR 100 Lecture 22: Life of a high-mass star Keep reading about star death, Next week, start reading about Our Galaxy: Ch. 12, 13, 14 in “Essential” This/next week’s material: Going to track stars over their entire lifetime, fill out the rest of the H-R diagram: Cradle to grave Black holes and Einstein’s version of gravity Summary: We know about how stars are formed We know larger stars live fast and die young Talk about what happens to high mass stars from formation to “death” Last weird branch of the HR diagram + neutron stars, pulsars, black holes, core-collapse supernova Chemical enrichment of the Universe Star formation…. “The Initial Mass Function”: For every O star, there are ~200 K and M stars Updated picture of collapse of a star-forming cloud: In a star-forming cloud, what’s pulling inwards? What’s pushing outwards? Protostellar/protoplanetary disk forms. Jet powered by magnetic fields transports lots of material. Gravity in. Thermal pressure out. Cloud to main sequence, we understand: Powered by gravitational contraction, burn brighter than fusion, but just for a few 104-5 yrs Some comparative star anatomy: On the main sequence, stars merrily fuse Hydrogen to Helium for however long …Then what Today, finish talking about the remnants from “low mass” stars High mass stars: (3-100 Solar masses) What’s happening on the main sequence? More and more Helium, less and less Hydrogen Runs out and Hydrogen fusion stops…. Radiation pressure stops, Gravitational contraction… Subgiant: No fusion, but gravitational contraction increases luminosity of star, making star expand…. Non-burning, Helium core gets smaller, rest of star gets bigger Hydrogen around Helium core also gets crushed and eventually starts fusing: Shell-burning Solar wind increases Solar mass stars have enough pressure for: Helium to Carbon in core, Hydrogen to Helium in the shell This story agrees with observations from cluster HR diagrams and simulations in Solar mass stars. Much harder to see observationally in high-mass stars, too fast, too few. A lot of this story is theory/simulation. Composite image of X-ray and visible light For solar-mass stars, we stopped at Carbon. Not enough gravitational pressure to go further. E = mc2 A high mass star (Supergiant) has enough mass and gravitational pressure to burn past carbon: Multiple shells of fusing material Differentiated. But it stops at iron. All sorts of crazy stuff going on in the cores. Flowchart of stellar nucleosynthesis pathways: Memorize by Monday’s quiz! Just kidding. The ballet of contraction/shell burning occupies the entire Supergiant branch of the HR diagram Towards Iron this isn’t really a lot of power…. Law of diminishing returns. The Silicon shell, only 5 days. High mass stars have enough gravitational pressure to get out to Iron. Crazy. Now what? What’s keeping the star from crushing? The iron isn’t fusing (no radiation pressure), but it’s solid. “Electron degeneracy pressure” = electrons staying in orbit Solid isn’t so solid (the nucleus is 106x too large in that pic): In fact, we’ve seen “Electron degeneracy pressure” before. It’s what keeps the Earth and White Dwarfs from collapsing …and fruit bowls on kitchen tables This is this fact from chemistry with “electron shells”, no two electrons can be in the same place, so they stack up like bricks. Actually. You can crush an atom. These were some of the steps we never talked about in making hydrogen. p + e + (v) = N A proton, electron, (and anti-neutrino) can get smooshed together to make a neutron No more electron, no more electron degeneracy pressure. (Neutrons can decay into a proton, electron, and anti-neutrino too….) The iron core gets crushed so that it’s no longer iron, the electrons and protons combine into neutrons, the volume of the core reduces by a factor of 1018 Outer core falls in at about 25% of the speed of light, the core temp rises to ~1011K Option 1) The core (formerly iron) is now a stack of neutrons none of which can occupy the same state. This “neutron degeneracy pressure” fends off the falling outer layers Option 1) Summary of core-collapse (Type II) Supernova Option 2) neutron degeneracy pressure isn’t enough to resist the gravitational inflow…. A hole in spacetime…. Option 1) Computer simulations verify speed and power of core-collapse Supernovae Option 1) Planetary nebulae on steroids…. Supernova remnants: A large fraction of the star’s mass, chemically enriched with elements up to and BEYOND iron, back into space. SN1006 about 60ly across. Lots of historical records of “guest stars” In stellar mass death, we had Planetary nebula (cloud) and dense core remnant (White dwarf) Core remnant of just neutrons: A Neutron “star” ~1 Solar mass, ~20km, spin ~1000x per second “Pulsars” are Neutron stars whose radiation we can see. Like lighthouses…. Pulsars are the directions we gave to our Solar system on the Voyager/Pioneer records/plaques Where does the Uranium come from? E = mc2 mc2 = E Lots of energy from the gravitational collapse http://www.youtube.com/watch?v=Xaj407ofjNE The Life and Times of a high-mass star (pics not to scale) 1) 2) 3) 4) 5) 6) 7) Protostar ~104yrs Main sequence star ~106yrs Red Supergiant (shell burning) ~105yrs Helium Burning ~106yrs Multiple shell burning ~104yrs Supernova a few months BH/pulsar/neutron star …forever Story of stars is sort of the story of pressure vs. gravity…. Star-forming clouds: Thermal pressure vs. gravity Main sequence stars: Radiation pressure vs. gravity White dwarfs (and all solid matter): Electron degeneracy pressure vs. gravity Neutron stars/pulsars: Neutron degeneracy pressure vs. gravity Black holes: …. Gravity wins. Covered entire HR diagram now!!! Key terms: neutron “star”, pulsars, electron degeneracy pressure, neutron degeneracy pressure, core-collapse (Type II) supernova Key Ideas: What are the main stages in a high mass star’s life? What happens in the core of a high mass star at the end of its life? Why does fusion stop at Iron in high mass stars? Where do elements heavier than Iron come from? What are the two possibilities when the electron degeneracy pressure in a high mass star’s Iron core fails? How do we know about core-collapse supernova? What are the two parts of a core-collapse supernova remnant and what are their properties?