Astr 1020 1st Edition Lecture 12 Current LectureLow mass: < 2times the Sun (last time)High mass: > 8 times the Sun (today)REVIEW:Q: After a supernova, what is left behind?A: A neutron star or a black holeReview of Atoms- Atomic number Z = # of protons /electrons /kind of element- Mass number A = total number of nucleons (protons + neutrons)- Periodic Table lists each element in increasing atomic number (Z=1-113+)Nucleosynthesis =creating nuclei- Big Bang produced H and He- Low-mass stars can fuse He  CPatterns: - Very little Li, Be, B (low dip with few abundance and low atomic #)- Light elements much more common than heavy ones- Lots of Fe (spike)- Odd/even pattern as atomic number increasesFission and Fusion overview- Fission =split apartThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Fuse = join together- Strong force (attractive) holds nuclei together- Electromagnetism (repulsive) pushes protons apart - must overcome energy barrier to fuse/fissionFusion –Nuclei fuse together to form a heavier nucleusProton-proton(smaller things to bigger things) /Triple-alpha (3 He  C + energy)Fission –heavy nucleus splits into lighter nuclei (bigger things into smaller things)Strong force attraction overcome until EM repulsion takes overNuclear Energy- Different nuclei are more tightly bound (less energy) - Difference in binding energy between two nuclei comes out as heat: /\E = /\m c^2Summary: Life of a Low-Mass StarMain Sequence StarRed GiantHelium Burning StarDouble-shell Burning Red Giant Planetary NebulaHIGH MASS STARS PRINCIPLES ARE THE SAME:Battle between Pressure and GravityHigh mass stars do everything faster including fusion /MS lifetimes are much shorterEarly stages after main sequenceFurther burningCore contracts, heats againStar’s massive enough to get temperatures >600m K despite electron degeneracy pressureNext stage of fusion: C + He  O  Mg  etcRGB –Red Giant BranchHB –Horizontal BranchAGB –Asymptotic Giant Branch - Most elements are formed via “Helium Capture”o A helium (2 protons) nucleus is fused, energy is released- The elements are created going up the periodic table in steps of 2 protons (or 4 mass units)Onion model of High-Mass Stars~Concentric shells of increasing temp and pressure produce heavier + heavier elements- Energy created by each new shell is enough to balance gravity, until….Various elements = 56^FeTimescales- Successive shells are hotter, denser, and “burn out” in shorter timesFor a 20 solar mass star:FUSION: HHe TEMP: 37 million DURATION: 8.1 MyrFUSION: HeC, O TEMP: 190 million DURATION: 1.2 MyrThe further down the chain you go the shorter the duration/faster fusionIRON (Fe) !!- Iron nucleus is most tightly bound possible- Iron does not release energy through fusion or fissiono There is no way iron can produce any energy to push back against the crush of gravity in the star’s coreThe core is accumulating iron as layers above it fuse- No energy from iron fusion = no thermal pressure- Electron degeneracy pressure supports the core until the mass of iron gets heavierWhen pressure is large enough, iron atoms get compressed into pure neutrons Protons + electrons  neutrons + neutrinos - This takes less than .01 seconds for the ENTIRE core- Electron degeneracy pressure –GONE ! (Core collapses)Eventually neutron degeneracy pressure stops the collapse abruptlySUPERNOVA- The lightweight atmosphere impacts on the heavy core and is “bounced” off in a huge explosion (energy release from neutrinos- Exploding remnant of massive star disperses heavy elements through the galaxy- Inside may be a neutron star –a remnant core of pure neutronsHeavier elements? Can’t fuse them from lighter elements without adding energyHow did we get heavy metals?- Elements heavier than iron don’t generate energy through fusion..but you can fuse them if you add enough energy (endothermic reaction)- Supernovae create a LOT of energy- All elements heavier than Fe originated in a supernova –we are star stufPatterns of fusion- Atoms heavier that iron created very rapidly during the SN by neutron capture:o Neutrons blasted into iron heavy elements up to uraniumo Much less common than lighter metalsDeep Thoughts:All heavy elements are created and dispersed though the galaxy by starsWithout supernovae, nothing heavier than carbonWE ARE STAR STUFFOur atoms were once parts of stars the exploded more the 4.6 billion years ago, whose remains wereswept up into the cloud out of which our Sun(+solar system) formedHIGH-MASSLarge star red supergiant  supernova  neutron star/black holeLOW-MASSSmall StarRed GiantPlanetary NebulaWhite