11Look-back timeLook-back timeDistances (ly): 4.3 6.0 8.6 9.5 10.8 11.4•Students sent out pictures of themselves when they were 12.•We get their pictures at the same time (a student doesn’t send oneuntil they get one from a farther-away-student).•Ron’s picture had to travel the farthest and thus took the most time.•Ron is actually 23.4 years old when we get his picture, but he lookslike he is only 12.Apparent age (yrs): 19.1 17.4 14.8 13.9 12.6 12Actual age (yrs): 23.4 23.4 23.4 23.4 23.4 23.42Learning goals: HubbleLearning goals: Hubble’’ss Law Law• Summarize the considerations astronomers must makein interpreting the observations of galaxies far, faraway• What kinds of “standard candles” are used to finddistances to galaxies?• What observations are needed in order to find thedistance to a galaxy that is too far away to have anobservable standard candle?• What is Hubble’s Law and what is Hubble’s constant?3• Universe is expanding:– distant galaxies receding faster than nearer ones.• Cosmological redshift:– light gets redshifted to longer and longer wavelengths• We are observing UV radiation at IR wavelengths.• Galaxies were closer together in the past than they are nowSummarize the considerations astronomers must make in interpreting the observations ofgalaxies far, far away.4 What kinds of “standard candles” are used to find distances to galaxies?Standard CandlesStandard Candles25Need:•spectra of galaxies•independent way ofdetermining distances•the slopeThen all we need areredshifts of thosegalaxies havingunknown distances. What is Hubble’s Law and what is Hubble’s constant?HubbleHubble’’s Law: v = s Law: v = HHood y = mx + bd y = mx + b6VideoVideo What observations are needed in order to find the distance to a galaxy that is too far away to have anobservable standard candle?7 What observations are needed in order to find the distance to a galaxy that is too far away to have anobservable standard candle?8Cosmological Redshift“Nearby” -- ultraviolet visible visible infraredFarther -- ultraviolet infraredzcvtruetruemeasured!="### What observations are needed in order to find the distance to a galaxy that is too far away to have anobservable standard candle?39REVIEW:Once we have theslope, then all weneed are redshifts.Have the redshifts,and velocities, weuse this graph tofind the distances. What observations are needed in order to find the distance to a galaxy that is too far away to have anobservable standard candle?HubbleHubble’’s Law: v = s Law: v = HHoodd10Hubble Deep FieldHubble Deep Fieldhttp://www.aip.de/groups/galaxies/sw/udf/swudfV1.0.htmlHubble Ultra-Deep Field11•What kinds of “standard candles” are used to find distances to galaxies?• What observations are needed in order to find the distance to a galaxy that istoo far away to have an observable standard candle?12The ExpansionThe Expansionof the Universeof the UniverseTutorialTutorial04. If this represents what actually happens to galaxies during the expansion of theUniverse, will the stars within a galaxy move away from each other? Why or whynot?05. Using the dashed lines “connecting” the galaxies A – E, compare the amount thatthe distance between galaxies D and C changed in comparison to the amount that thedistances between galaxies D and E changed. Did galaxy C or galaxy E appear tohave moved farther from D?06. If you were in galaxy D, how would the galaxies A, B, C, and E appear to moverelative to your location?41307. If you were in galaxy D, would the galaxies A, B, C, and E all appear to move bythe same amount in the time interval from “Early” to “Some Time Later”? _____08. Imagine that you are still in galaxy D. Rank galaxies A, B, C, and E in terms oftheir relative speeds away from you, from fastest to slowest:fastest _____ _____ _____ _____ slowest09. Now imagine that you in galaxy E. Rank galaxies A, B, C, and D in terms of theirrelative speeds away from you, from fastest to slowest:fastest _____ _____ _____ _____ slowest1410. What is the relationship between an object’s distance away from you in theuniverse and its apparent speed?11. Would your answer to question 10 be true in general for all locations in theuniverse sketched above?12. If this represents the true motion of real galaxies, would your answer toquestion 10 be true in general for all locations in the real
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