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ASTR 1120General Astronomy:Stars & Galaxies!NNOUNCEMENTS• Alternate final date: Monday, Dec 7, 5:30-8pm.• Next observing night: Mon, Nov. 16, 7pm • Homework #7 due on Tuesday 11/10, by 5pm 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 lyREVIEWMain-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 yearsREVIEWCepheid variable starsbrighter Cepheidshave longer periodsPeriod -Luminosity relationDISTANCEESTIMATE 2 REVIEWTully-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 lyREVIEW Even brighter:White dwarf supernovae• Nearly the sameamount of energyreleased everytime.why?• “Standardexplosion” =fusion of 1.4 solarmasses ofmaterialDISTANCE ESTIMATE 4REVIEWSummary“Distance Ladder” to measure universeDifferent standard candles are usefulfor different distancesREVIEWDistance measurements allowed tomake a MAJOR discovery about ourUniverse• Before 1924, “spiralnebulae” werethought to be smalland located insidethe Milky WayAndromeda found to be far outside Milky Way!• Edwin Hubble in 1924identified Cepheids inAndromeda (M33) !showed they were faroutside of Milky Way!– “Island Universes”• His first big discovery!• But then he turned hisattention to OTHERgalaxiesHubble using new 100” Hooker telescopeat Mt. Wilson (above LA)Hubble’s observations showed a verystartling result about the universe.• Vesto Slipher (1912) reported that mostgalaxies showed Doppler redshifts• Edwin Hubble, using new 100” telescope,started busily measuring galaxy redshifts• Hubble (1929) announced that redshifts ofgalaxies appear to increase with distancefrom us• This was startling: Suggested anEXPANDING UNIVERSE !v = Ho ! dVelocity ofRecession(Doppler Shift)Hubble’sConstantDistance(km/sec) (km/sec/Mpc) (Mpc)“Hubble’s Law”velocitydistanceBest currentvalues forexpansionHo = 71+/- 4 km/s/MpcHubble Space Telescope wasdesigned to accurately measurethe Hubble constant• High resolutionimages to find faintCepheid variablestars in very distantgalaxiesWhat is Hubble’s Law?A. An idea stating that more distant galaxies moveaway from us faster than closer onesB. An equation giving the maximum luminosity for awhite dwarf supernovaC. The relationship between the period andluminosity of a Cepheid variable star.D. The law giving the maximum speed that agalaxy can move at.E. The idea that there are more galaxies outsideour own and that the universe contains immensenumbers of these “island universes.”Clicker QuestionWhat is Hubble’s Law?A. An idea stating that more distant galaxies moveaway from us faster than closer onesB. An equation giving the maximum luminosity for awhite dwarf supernovaC. The relationship between the period andluminosity of a Cepheid variable star.D. The law giving the maximum speed that agalaxy can move at.E. The idea that there are more galaxies outsideour own and that the universe contains immensenumbers of these “island universes.”Clicker QuestionBalloon analogy forexpanding universe• Each dot on the balloon can bethought of as agalaxy. As the balloonexpands, galaxiesmove farther awayfrom each otherWhat are the velocities of B, C and Das measured by A?A. B: 1 cm/s; C: 2 cm/s; D: 3 cm/sB. B: 3 cm/s; C: 1cm/s; D: 2 cm/sC. B: 2 cm/s; C: 4 cm/s; D: 6 cm/sD. B: 1cm/s; C: 3 cm/s; D: 6 cm/sClicker QuestionWhat are the velocities of B, C and Das measured by A?A. B: 1 cm/s; C: 2 cm/s; D: 3 cm/sB. B: 3 cm/s; C: 1cm/s; D: 2 cm/sC. B: 2 cm/s; C: 4 cm/s; D: 6 cm/sD. B: 1cm/s; C: 3 cm/s; D: 6 cm/sClicker QuestionBalloon analogy forexpanding universe• On an expandingballoon, no galaxy isat the “center” ofexpansion; no edge• Expansion happensinto a higherdimension (2-Dsurface into a 3-Dspace)• Is our 3-D spaceexpanding through a4th dimension?A Better Way To Image theExpanding Universe• NOT like an explosion of galaxiesTHROUGH space from a center place• The space BETWEEN galaxies isexpanding, carrying the galaxies awayfrom each other– Why don’t galaxies themselves expand?Gravity!No matter which direction we look, we seegalaxies moving away from us. Therefore, wemust be at the center of the expansion.A. TrueB. FalseClicker QuestionNo matter which direction we look, we seegalaxies moving away from us. Therefore, wemust be at the center of the expansion.A. TrueB. FalseClicker QuestionThe Cosmological Principle• Matter is evenly distributed on very largescales in the universe• No center & no edges• Not proven but consistent with all observationsto dateThe universe looks about the same nomatter where you are within itSince the universeis expanding, light traveling through theuniverse “feels” the stretch as it travelsCosmological Redshiftv = Ho ! dVelocity ofRecession(Doppler Shift)Hubble’sConstantDistance(km/sec)(km/sec/Mpc)(Mpc)“Hubble’s Law”velocitydistanceImplies the Expansion of the Universe!What does the “expansion ofthe universe” most accuratelymean?A. Galaxies are moving apart through spaceB. Space itself is expandingC. Everything is expanding, including the earth,our bodies, etcD. The Milky Way is at the center of theuniverse and all other galaxies areexpanding away from us.Clicker QuestionWhat does the “expansion ofthe universe” most accuratelymean?A. Galaxies are moving apart through spaceB. Space itself is expandingC. Everything is expanding, including the earth,our bodies, etcD. The Milky Way is at the center of theuniverse and all other galaxies areexpanding away from us.Clicker QuestionYour friend leaves your house. She later calls you on hercell phone, saying that she’s been driving at 60 mph(miles per hour) directly away from you the whole timeand is now 60 miles away. Without looking at yourwatch, can you tell how long has she been gone?A. Yes, 1 minuteB. Yes, 30 minutesC. Yes, 60 minutesD. Yes, 120 minutesE. No, not enough information to tellClicker QuestionYour friend leaves your house. She later calls you on hercell phone, saying that she’s been driving at 60 mph(miles per hour) directly away from you the


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CU-Boulder ASTR 1120 - Lecture Notes

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