The Death of a Low-Mass Star: Planetary Nebula The Helix Nebula Remnants of stars with ~ 1 – a few M sun Radii: R ~ 0.2 - 3 light years Expanding at ~10 – 20 km/s ( Doppler shifts) Last < 10,000 years Have nothing to do with planets!
The Formation of Planetary Nebulae The Ring Nebula in Lyra Two-stage process: Slow wind from a red giant blows away cool, outer layers of the star Fast wind from hot, inner layers of the star overtakes the slow wind and heats it => Planetary Nebula
Earth’s Fate Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth
Earth’s Fate Sun’s radius will grow to near current radius of Earth’s orbit
High mass stars : CNO Cycle H fusion is faster because C, N and O act as catalysts Same net result: 4 H become 1 He. No total gain or loss of C, N, O
Life Stages of High-Mass Stars high-mass stars are similar to low-mass stars: –Hydrogen core fusion (main sequence) –Hydrogen shell burning (supergiant) –Helium core fusion (subgiant) They are also different.. –H-->He via CNO cycle not p-p chain –Core much hotter –fuse C, O into heavier elements –He core is not degenerate –no He flash! –Lose a lot of mass
High-mass stars make the elements necessary for life!
Big Bang made 90% H, 10% He – stars make everything else
Helium fusion can make only carbon in low-mass stars
Helium Capture occurs only in high-mass stars High core temperatures allow helium to fuse with heavier elements
Helium capture builds C into O, Ne, Mg, … Total # of P+N = Multiples of 4! Why?
Evidence for helium capture: Higher abundances of elements with even numbers of protons
Advanced Nuclear Burning Core temperatures in stars with >8M Sun allow fusion of elements up to iron
Si, S, Ca, Fe, etc. can only be made in high-mass stars
Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells
Iron is “ash” of fusion: nuclear reactions involving iron do not release energy Iron-56 has lowest mass per nuclear particle Highest “binding energy” of all the elements Fusion releases energy only when the mass of the products < mass of the reactants