Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Astro 10-Lecture 10:Stellar Structure and EvolutionNow that we know the properties of stars, lets talk about how stars work.Important Dates!•April 8: Spring Break•April 15: Exam #2. Homework #4 due•April 16: Chabot field trip.•April 22: Project #1 dueChabot Trip•Friday, April 16•Meet at a Ashby BART station 6:30 pm.–Need volunteers to drive from there.•Admission $11+$6 for planetarium show•Planetarium show (7:30) and telescope viewingStar Structure and FormationThe Big PictureStars are big balls of hot gasStars exist because of gravityThey shine because they are hotThey continuously generate energy through nuclear fusionThey balance gravity with pressureThey form because gravity makes interstellar clouds of gas contractThey die when they have no more fuel, and gravity takes overStellar Structure – MS StarsMain Sequence STARS Gravitational (hydrostatic) equilibrium:Gravity and Pressure balanceDEMO Temperature and Pressure related When Temp increases, so does P (DEMO)Temperature-Pressure relationPressure inside balances Pressure outsideBalloon cools, molecules inside slow down, pressure inside decreasesBalloon shrinks until inside and outside pressures again balanceHydrostatic EquilibriumStellar Structure (2)Stellar structure analogy Star is emitting light/heat, so losing energy!It should cool => P decrease => collapseBUT Sun has been stable for 5 billion years!What’s the energy source to maintain equilibrium? FUSIONEnergy Generation in the SunEnergy is generated through nuclear fusion in the CENTER of the Sun! What are the conditions there?How hot are stars?Sun: 5800 K But this was a SURFACE TEMPERATURECentral Temperature of SunWhat’s the “surface”?Stars densest and hottest in the centersThis is why we see dark absorption lines in stellar spectraEnergy generated in the center, and flows outwardLight emitted blocked by material sitting on top“Surface” = layer where star still dense enough to emit lots of light, but not so dense that light is blockedSun’s surface Temp = 5800 KSun’s central Temp = 1.5 x 107 KAll atoms are fully ionized, electrons stripped awayConcepTestWould a star that is more massive than the Sun have a (A) higher (B) lower or (C) same central pressure? Or (D) can you not tell from the information given?What about central temperature?FUSIONFusion: transforms elements themselvesUnlike chemical reactionCenter of sun: have just nuclei and electrons, separatedNucleus made up of protons/neutrons: why don’t positive protons repel one another by electrostatic repulsion?Strong nuclear force binds them togetherAtoms/NucleiFusion (2)“Strong nuclear force” – only acts when protons/neutrons are VERY close together“Electrostatic repulsion” – positive charges repelIF you can overcome the repulsion and get 2 protons close enough, then they will STAY together (details deferred) H ion = 1 proton, 4H = 4pHe ion = 2 p + 2 nFUSION: 4H He + ENERGY!Why is energy released?FUSION – Releasing EnergyDue to the strong nuclear force, it takes ENERGY to pull apart a nucleus: This is a BINDING ENERGYIt takes more energy to pull apart a Helium nucleus (2p + 2n) than a Deuterium nucleus (1p + 1n).Most tightly bound element is Iron (Fe)2p + 2n in a He nucleus He nucleus in lower energy state than 4 protons in 4 H nuclei4H He + energy!He more tightly bound than 4H, so energy is emittedEnergy and mass are really the same thing. (Einstein)A He nucleus weighs 0.7% less than 4 P (because of the binding energy difference)E=mc2 (Einstein!)Compare 4 x proton mass to mass of He nucleus, and convert the difference to EnergyEnergy released = 0.43 x 10-11 JFusion and the SunSun transforms 5 TONS of matter into energy EVERY SECOND!Over lifetime of sun, 1/10 of Sun’s H is converted to He, so 0.07% of Sun’s mass will keep it shining 10 billion yearsWhy isn’t fusion happening all the time?ONLY happens when Temp high enough to overcome electrostatic repulsion between +ve ionsDEMONOT a chemical reaction! TRANSFORMS the elements.Stellar Structure•How do we figure out the structure of a star? We can only see the surface.•Scientific Models:•BUILD a model based on our “rulebook”•COMPARE calculated properties with observed properties•IF they DIFFER, adjust the “rulebook”•What’s the “rulebook”?•Imagine a star is divided into a series of shells•Apply conservation of mass, conservation of energy, physical laws of energy transportStellar Structure - Ingredients1) Total Mass = sum of masses in each layer2) Amount of Energy flowing out of a shell = energy coming in + energy generated there3) Hydrostatic Equilibrium4) Energy Travels from hot to cool regions by Conduction, Radiation, and ConvectionStellar Structure – cont’d•The devil is in the details:•How much energy is generated in each layer?•Depends on the conditions in the layer•How is the energy transported? How quickly? By what mechanism? (More later on the mechanisms)ConcepTestIn nuclear fusion, energy is released:A) when an electron changes energy levels within an atomB) when an atomic nucleus splitsC) when 2 atomic nuclei combineD) in a chemical reactionStellar Structure:Testing our ModelModels like this correctly “predict” the radius, age, and other surface properties of the sun (luminosity, surface Temp…)Not every detail is exactly right, but these indicate we’re on the right trackThe results of the computer model are only as good as the assumptions that went in!We must test Predictions of the Model with ObservationsStellar Structure: Testing the Model in the Sun’s InteriorComparing surface predictions with surface properties doesn’t test the model at it’s heart – the center of the Sun where energy generation occurs!Key to testing the model’s description of the conditions INSIDE
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