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CU-Boulder ASTR 1120 - Lecture Notes

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ASTR 1120ASTR 1120General Astronomy:General Astronomy:Stars & GalaxiesStars & Galaxies Fiske planetarium: ”The Birth of Stars” by Prof. John Bally - TH 09/24-FRI 09/25, 7:30pmHOMEWORK #3 due NEXT TUE, 09/29, by 5pmAstronomerAstronomer’’s Toolbox:s Toolbox:What do we know how to doWhat do we know how to donow?now?• Measure DistanceDistance:– parallax…good to nearby stars but not beyond• Measure Luminosity:Luminosity:– measure apparent brightness and distance, inferluminosity• Measure Temperature:Temperature:– Wien’s law, or, better yet, take spectra and usespectral classification.spectral classification.Next: MassMassMasses are much harder thanMasses are much harder thandistance, luminosity, or temperaturedistance, luminosity, or temperature• Since we are only ever seeing a pointsource, it is hard to determine howmuch mass is contained.– If we could see another nearby object(another star maybe?) we could use thegravity between the objects as a measureof the mass.Binary Stars to the Rescue!!Binary Stars to the Rescue!!• Types of binary star systems:– Visual Binary– Eclipsing Binary– Spectroscopic BinaryAbout halfhalf of all stars are in binary systemsVisual BinaryVisual BinaryWe can directly observe the orbital motions of thesestarsEclipsing BinaryEclipsing BinaryWe can measure periodic eclipsesSpectroscopic BinarySpectroscopic BinaryWe determine the orbit by measuring Doppler shiftsAnimation from http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Movies/spanim.gifIsaac NewtonDirect mass measurements arepossible only for stars in binarystar systemsOnce we know: p = period a = average separationWe can solve Newton’s equationsfor mass (M)NewtonNewton’’s Laws of gravitys Laws of gravityprovide the massprovide the massAstronomerAstronomer’’s Toolbox:s Toolbox:What do we know how to doWhat do we know how to donow?now?• Measure DistanceDistance:– parallax…good to nearby stars but not beyond• Measure Luminosity:Luminosity:– measure apparent brightness and distance, inferluminosity• Measure Temperature:Temperature:– Wien’s law, or, better yet, take spectra and usespectral classification.• Measure Mass: Mass:––For stars in binaryFor stars in binary orbits, if we can get theirorbits, if we can get their orbitalorbitalparameters, we can figure outparameters, we can figure out their masstheir massWide range ofluminosities, temperatures and massesAny correlationcorrelationamong thesequantities?TheThe Hertzsprung Hertzsprung RussellRussellDiagramDiagram• THIS IS AN IMPORTANT DIAGRAMTO UNDERSTAND.• Basics:– Plots Stellar LuminosityStellar Luminosity (not apparentbrightness)Vs––Temperature Temperature or ColorColor or Spectral ClassSpectral ClassStudy this plot!Study this plot!Are the variables plotted hereAre the variables plotted hererelated to each other?related to each other?A. Yes, they show arelationshipB. You can’t be sure – youdon’t know what they are!C. They are related to eachother or else both arerelated to a third variableD. A or CE. None of the aboveClicker QuestionClicker QuestionThey DO show a relationship!They DO show a relationship!!-R diagra"!-R diagra"Emitted power per unit area=Total luminosity from astar of radius R:! "T4For the sametemperature, moreluminous stars havelarger radii! "= 5.67x10#5ergK#4cm#2s#1where! L = 4"R2#T4TemperatureLuminosityMain sequence starsMain sequence stars• Burning hydrogenhydrogen in theircores• Stellar masses decrease downward• Temperatures are hotterhotter formore massivemassive stars (moregravitational pressure !higher T, rememberEquation of State)• More luminousluminous (higher T ! much higher emitted power)Available hydrogen fuel is greaterfor the most massive stars…But luminosity (rate at whichhydrogen is fused) is MUCHMUCHMUCHMUCH higher! More massive (more luminous)main sequence stars run out offuel soonerExample: Most massive O star:M = 100 MSunL = 106 LSunM/L = 102 /106 = 10-4 of the SunLifeO-Star=1010 yrs * 10-4 = 101066 yrs yrsGeo Metro#$%ar life&mes along 'e main sequenc(#$%ar life&mes along 'e main sequenc(Lifetimes of Main SequenceLifetimes of Main SequenceStarsStars• Rock-star analogy:more massivemassive,hotterhotter, moreluminousluminous stars burnthrough theavailable fuel faster,leading to earlyburnoutLifetimes on Main Sequence (MS)Lifetimes on Main Sequence (MS)••Stars spend 90% of their lives on MSStars spend 90% of their lives on MS••Lifetime on MSLifetime on MS = amount of time star= amount of time starfuses fuses hydrogen (gradually) in its corehydrogen (gradually) in its core••ForFor Sun (G)Sun (G),, this is about 10 billion yearsthis is about 10 billion years••ForFor more massive starsmore massive stars (OBAF), lifetime is(OBAF), lifetime is(much) shorter(much) shorter••ForFor less massive starsless massive stars (KM), lifetime is(KM), lifetime islongerlongerGeorge and Abe are two mainGeorge and Abe are two mainsequence stars; George is an M starsequence stars; George is an M starand Abe is a B star. Which is moreand Abe is a B star. Which is moremassive? Which is redder in color?massive? Which is redder in color?A. George is more massive and redderB. Abe is more massive and redderC. George is more massive; Abe is redderD. Abe is more massive; George is redderE. They are both main sequence, they’re thesame mass and same color.Clicker QuestionClicker QuestionGeorge and Abe are two mainGeorge and Abe are two mainsequence stars; George is an M starsequence stars; George is an M starand Abe is a B star. Which is moreand Abe is a B star. Which is moremassive? Which is redder in color?massive? Which is redder in color?A. George is more massive and redderB. Abe is more massive and redderC. George is more massive; Abe is redderD. Abe is more massive; George is redderE. They are both main sequence, they’re thesame mass and same color.Clicker QuestionClicker QuestionMain-Sequence Star SummaryHighHigh MassMass:: High Luminosity Short-Lived Large Radius Hot BlueLowLow MassMass:: Low Luminosity Long-Lived Small Radius Cool RedTemperatureLuminosityWhat about the other objects onthe H-R diagram? As stars run out of hydrogen fuel their properties change (generally they turn into red giants- more on why next week)• Top end of mainsequence starts to“peel off”• Pleiades star clustershown ! no more Oand B starsMain-sequenceturnoff pointof a clustertells


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