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UT AST 309L - Cosmic Evolution

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Cosmic EvolutionPart 1: Protons to heavy elementsBig Bang occurred 13.7 Billion yrs ago (13.7 x 109 yr)Only fundamental particles existed for first few minutesMass1.7 × 10–24 g1.7 × 10–24 g 1 × 10–27 g 0~ 0 (very small)Charge+0–00SymbolpneγνNameProtonNeutronElectronPhotonNeutrinoBuilding blocks of nuclei but only one kind of nucleusProton = nucleus of HydrogenEnergy of Motion (Kinetic Energy)E = mv2(if v not close to c)Gas at Temperature T,Avg. Energy E = kTSo avg. v : mv2 = kTv = 3kT ≡ 3kT m m123232√12Higher T → Higher v, E on avg.12Early Universe so hot that collisions broke apartany complex things that might have formedAs Universe expanded, T droppedat ~ 3 min, T ~ 109 KA few nuclei form (nucleosynthesis)at ~ 30 min, T ~ 3 × 108 K end of nucleosynthesisComposition of Universe at 30 min.~ 94% proton~ 6% alpha particle (and electrons)At 380,000 years T ~ 3000 KNuclei + electron → AtomsHHeH HeFirst Generation StarsExpanding UniverseBut, Gravity collected matter into StarsStars now 0.1 to 100 M; first stars more massiveLater into Galaxies (~ 1011 to 1012 M)Oldest stars in disk ~ 1010 y oldFirst generation stars → No C, O, N, …⇒ No life No Si, Fe ⇒ No Earthlike planetsBut they made some “heavy” elementsSo later stars could have solid planets, lifeGravitational Potential EnergyFor example: Reservoir of water behind damLower GPEHigherGPEEnergy releasedas water fallsConsider a clump of Gas collapsing to form a starGPE → HeatAtoms move faster Temperature is higherApply to collapsing gasPause for DemonstrationBack to Formation of First StarsCollapse released Gravitational Potential EnergyThe gas heats upThe Temperature in core reaches 107 k Nuclear reactions beginCollapse stops Why?Nuclear Potential EnergyFour basic forces: gravity, electromagnetic,weak and strong nuclear forceEach has potential energy. Nuclear potentialenergy can be released by nuclear reactions.e.g. 1st step: p + p → d + e+ + νd = deuteron = proton + neutrone+ = positron (antiparticle of electron)Nuclear Potential EnergyNuclearPotentialEnergyd + e+ + νp + pEnergyReleased↓Kinetic Energy ↓HeatSeparation of two protonsThe energy released by nuclear reactionssupplies heat → pressureResists gravity ⇒ stable starElectromagnetic BarrierWhy do we need high T (~ 107 K)?Protons have positive electric chargeLike Charges RepelAs protons approach, repulsion grows,corresponds to climbing hill of electromagneticpotential energyElectromagnetic BarrierPotentialEnergyd + e+ + νp + pSeparationBarrier is really much higher than k · 107 KVery few can get over barrier⇒ Stars live a long time rather than explodingBarrier32Questions• Why do nuclear reactions produce a long-lived system in star, but an explosion in abomb?• What will happen when a star’s fuel runs out?p + p → d + e+ + ν→→3He + 3He → 4He + 2p 4He = 2p + 2n→proton pneutron npositron e+neutrino νphoton γd + p → 3He + γNucleosynthesisAgainHow to get past helium? We need C, O, N, P, S, …4He + 4He → 8Be 8Be = 4p + 4nProblem: 8Be has more nuclear potential energy than parts;It is unstable (radioactive).To get carbon, we need another 4He to hit 8Be before 8Be falls apart8Be + 4He → 12C + γ →4He + 12C → 16O 16O = 8p + 8n4He + 4He → 8Be→ 8Be = 4p + 4n12C = 6p + 6n16O + 16O → 32S + γ Sulfur 16O + 16O → 31P + p Phosphorus 16O + 16O → 28Si + 4He SiliconQuestions• What was needed to make the bioelements?• Are any missing?• How do the bioelements get out of the star?SummaryHeavy elements needed for life were created byearly generations of massive stars.Except for H, we are made of star debrisNatural forces (Gravity, EM, Nuclear) producedfirst evolution of matter from simple to complex(protons → heavy


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UT AST 309L - Cosmic Evolution

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