ASTR 1120General Astronomy:Stars & Galaxies!NNOUNCEMENTS• HOMEWORK #6 DUE TODAY, by 5pm• HOMEWORK #7 DUE Nov. 10, by 5pmDark matter halo for galaxies• Dark matter extendsbeyond visible part ofthe galaxy -- mass is~10x stars and gas!• Probably not normalmass that we know of(protons, neutrons,electrons).• Most likely subatomicparticles, as yetunidentified (weaklyinteracting massiveparticles – WIMPs?)REVIEWA Case for a Supermassive BlackHole at the Galactic Center• Doppler shiftmeasurementsof spiraling stars andgas suggest 3million MSun blackhole• Still too far out toexclude otherpossibilities…We need to be able to see closer in to reallyprove there is a supermasive black hole!Distance from Sgr A* (pc)Enclosed mass (Mo)Genzel 1996REVIEWStars appear to beorbiting somethingmassive butinvisible … ablack hole!Orbits of starsindicate a mass ofabout 3-4 millionMsun within 600RSchwarzchildREVIEWState of Affairs at the GalacticCenter• Stellar orbits have made our Galaxy one of thebest proofs of supermassive black holes at thecenter of most galaxies.– Millions to billions of times the mass of our Sun.Must be created by something entirely differentthan a massive star supernova.• Flares are often observed in X-rays and IR– Observations of occasional flares are interpreted asthe result of occasional “swallowing” of a gas clumpor a star by the giant black hole.REVIEWIrregular GalaxiesHubbleUltraDeepFieldSpiral GalaxyElliptical GalaxyElliptical GalaxyREVIEWWhich type of galaxycontains a low percentage ofcool gas and dust?A. SpiralB. EllipticalC. IrregularD. Barred spiralE. Everyone but BClicker QuestionWhich type of galaxycontains a low percentage ofcool gas and dust?A. SpiralB. EllipticalC. IrregularD. Barred spiralE. Everyone but BClicker QuestionHubble classification of galaxy typesBarred spiralSpiralsEllipticals• Spirals:mostly ingroups(3-10galaxies)Where do spirals and ellipticals live?HST: Hickson CG 44• Ellipticals -most often indense clustersof galaxies(involve 100’sto 1000’s)HST: Abell 1689The Big Picture: Universe is filled withnetwork of galaxies in groups and clusters~100 billion galaxies!Pattern of galaxies (3 million+),15o portion of skyBrighter =more galaxiesWhich of the following is NOT aclassification of a type of galaxy?A. KeplerianB. SpiralC. LenticularD. EllipticalE. Irregular Clicker QuestionWhich of the following is NOT aclassification of a type of galaxy?A. KeplerianB. SpiralC. LenticularD. EllipticalE. Irregular Clicker QuestionOur “Local Group” of galaxies3 spirals:Andromeda (M31) 3/2 MMWMilky Way 1 MMWTriangulum (M33) 1/5 MMW2 irregulars:LMC 1/8 MMWSMC 1/30 MMW16+ dwarfs~21 GalaxiesBiggest is Andromeda (Sb - M33)• Andromeda is~3 million lightyears away (or ~30MW diameters), has~1.5 mass of MW• We see it as it was3 million years ago,not as it is today! –this is lookback time• Oops! It may crashinto MW in about 2billion yearsTriangulum(M33)• 1/5 mass of MW, spiralclassified as Sc• Several bright (pink)star forming regionsLarge & Small Magellanic CloudsLMCSMCLMC has 30Doradus, homeof SN 1987AWhat are theMagellanic Clouds?A. Two nebulae in disk of Milky Way visible only insouthern hemisphereB. Clouds of dust and gas in many placesthroughout the Milky Way galaxyC. Two small galaxies in the same group as theMilky WayD. Star-forming clouds in constellation OrionClicker QuestionWhat are theMagellanic Clouds?A. Two nebulae in disk of Milky Way visible only insouthern hemisphereB. Clouds of dust and gas in many placesthroughout the Milky Way galaxyC. Two small galaxies in the same group as theMilky WayD. Star-forming clouds in constellation OrionClicker QuestionHow do we get distances tothings far outside our Galaxy? Mapping the Universe: Weneed Distances to Galaxies!Methods we are familiar with:Radar and Stellar parallaxThe problem: orOnly useful inside the Solar System A few thousand lyNew Methods:Bootstrap our way• Identify (and calibrate) objects that could serveas “STANDARD CANDLES” -- beyond directmeasurement1. Make some measure of an object whichidentifies its luminosity2. Use this luminosity and measure apparentbrightness to infer distance to itMain-Sequence Fitting• Start with cluster A(upper) whosedistance known viaparallax• Compare with othercluster B (lower)• Get distance to Bfrom brightnessdifferenceDISTANCEESTIMATE 1ABDistances up to ~1 million light yearsWhich cluster is closer?ABA. HyadesB. PleiadesC. Not enoughinformation totellClicker QuestionWhich cluster is closer?ABA. HyadesB. PleiadesC. Not enoughinformation totellClicker QuestionMain Sequence Fitting “pinnedto” nearby Hyades ClusterOnly 151ly awayCepheid variable stars• “Instability strip” --region in H-R diagramwith large, bright stars• Outer regions of star areunstable and tend topulsate• Star expands andcontracts, getting brighterand fainterReminder (Fig 15.14)DISTANCEESTIMATE 2 Cepheid variable starsbrighter Cepheidshave longer periodsPeriod -Luminosity relationDISTANCEESTIMATE 2 Two Cepheid stars, Fred and Barney, havethe same apparent brightness. Fred has aperiod of 5 days, and Barney of 10 days.Which is closer ?A. FredB. BarneyClicker QuestionWhy A. Fred ?• Fred has a shorterperiod and so mustbe less luminous• Less luminous butthe same apparentbrightness meansthat Fred is closerto usPeriod-Luminosity RelationTully-Fisher Relation• Fast rotation speeds inspiral galaxies! more mass in galaxy! higher luminosityMeasure rotation speedsto infer luminosityNeed bright “edge-on”spirals, estimate tiltDISTANCEESTIMATE 3Distances up to ~1 billion ly Even brighter:White dwarf supernovae• Nearly the sameamount of energyreleased everytime.why?• “Standardexplosion” =fusion of 1.4 solarmasses ofmaterialDISTANCE ESTIMATE 4Bright enough to be seen halfwayacross observable universeUseful for mapping theuniverse to the largestdistancesSummary“Distance Ladder” to measure universeDifferent standard candles are usefulfor different