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 57Slide 58Slide 59Slide 60Slide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Slide 68Slide 69Slide 70Slide 71Lecture 11Stellar EvolutionHow are stars born?How do they die?Exam #2Avg: 34.9Median: 34.9Curve: 45- A 635-45 B 925-35 C 915-25 D 60-15 F 02 no-show, 1 excused absenceQuiz today.One question1. What is your name?Temperature-Pressure relationPressure inside balances Pressure outsideBalloon cools, molecules inside slow down, pressure inside decreasesBalloon shrinks until inside and outside pressures again balanceHydrostatic EquilibriumPressure-Temperature ThermostatIn a star, inward pull of gravity balanced by the internal pressureAs the star loses energy, the T and P would drop, except nuclear fusion is generating just enough energy to maintain the balanceIf reactions begin to produce too much energy, this extra energy raises T, which raises P, so star expands, which cools it slightly. This slows the nuclear reactions.If reactions slow, then inner T drops, lowering P. Gravity compresses the star slightly. Compression of gas raises T & P increasing nuclear fusion rate.M-L Relation explainedRemember that most massive MS stars are also the most luminous?Explained by GRAVITATIONAL EQUILIBRIUMInterstellar MediumDark NebulaeDark CloudDark Cloud / ClusterInterstellar Medium - Gas•Narrow absorption lines in stellar spectra–Line from the atmosphere of the star are broad due to “doppler broadening.” (Remember temperature is motion of atoms).–Cool interstellar gas (not much motion) results in narrow lines.•Emission nebulae•Usually pink/red because of energies of electrons transitionsEmission NebulaeReflection Nebulae•Look Blue!Collapse of a Protostar•Stars form from the collapse of “dense” (~1000 atoms/cm3) molecular clouds–Cloud has few 100,000 or a million solar masses of material–Temperature ~ 10K (COLD!)•Why do they collapse? GRAVITY–Sitting in gravitational equilibrium, compressed slightly, gravity takes over!–Converts gravitational potential energy to THERMAL energy (infalling material heats up)–Cloud fragments as it collapses – each fragment becomes a PROTOSTAR, emitting radiation because it is hotCollapsing Interstellar CloudFrom Protostar to Star•What slows and eventually stops the collapse? PRESSURE –Gas falls in, heats up – As the temperature rises, so does the pressure!–Three kinds of pressure:•Thermal pressure (Temp-Pressure related)•Radiation Pressure (due to photons)•Degeneracy Pressure (later)•When the temperature rises high enough, FUSION (OH) begins, and A STAR IS BORN!–Surrounding gas/dust get blown awayBirths of Stars•Where on the HR diagram do new stars lie?•THE MAIN SEQUENCE•“Tracks” follow position of single star during its life (models)Formation Timescales•Most massive stars form the most quickly–Gravity collapses the cloud fragment more quickly in these casesEvidence of Star Formation Theory•I should present EVIDENCE to support this theory–Not much time – see text for more details–See objects that match our expectationsHot young stars evaporate surrounding material, revealing the cores where other stars are formingStellar Evolution•“Evolution” means what happens to a star DURING its lifetime –(not over generations of stars)–How can we see this, since we don’t see any single star evolve significantly during our lifetime?–Observe many different stars of different ages and try to piece together the story –Like taking a snapshot of the human population and figuring out how humans ageUse theory to model the evolutionAnd compare with the observed populationRules of Stellar Evolution•Births of stars governed by balance between gravity and pressure•Structure of Main-Sequence stars governed by the same gravitational equilibrium (OH 81)•EVERYTHING that happens to a star, from birth to death, is governed by a competition between gravity and pressure!Modelling Stellar Evolution•Apply the same rules that we did before to model MS stars–Gravitational Equilibrium–Energy Generation–Energy Transport–Energy conservationMain Sequence Lifetimes•Once a star is born, how long does it live on the Main Sequence? (OH: Table 9-2)–Stays on the main sequence while fusing H to He–Eventually, runs out of H in its center (core)–Energy generation changes => Leaves MS–Stars spend 90% of lifetime on MS –More massive stars use up fuel more quickly, so run out of H FIRST! They spend LESS time on the MS! That’s one reason why there are more dim red MS stars than luminous blue MS starsConcepTest discussion•Would you expect to find intelligent life on planets orbiting hot, blue, luminous main-sequence stars?Post-Main-Sequence Stars•What about the other stars on the HR diagram?–These stars have run out H in their cores–Core out of H => He “ash” in core => no energy generated there => T,P drop–H is still fusing (“burning”) in a shell–Gravity collapses core => T,P rise at center => He begins to fuse to Carbon“Post-MS” – He fusion–Core collapses, heats and Helium fusion begins due to higher Temperature –Luminosity goes up (larger volume of material involved in fusion)•Surface temperature increase results in pressure increase•Since force of gravity (mass) hasn't changed, increased pressure causes outer layers to expand.•Expansion causes outer layers to cool•Star gets larger and cooler!Red Giants•We now have a bigger, cooler star, so where does it fall on the HR diagram?•Sun-like stars will be red giants, very massive ones will be red supergiantsRed Giants – Now What?•Eventually, will run out of He in core!–He burning won’t last as long as H “burning” because He generates less Energy per unit mass (OH 60)•MASS controls what happens next!HST image of BetelgeuseFate of “Low-Mass” Stars•Stars like the Sun are considered “low-mass”•When they run out of He in core:–Left with Carbon “ash”–Core contracts–Outer layers blown away by stellar
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