AST- 105 1st EditionLecture 25Outline of Last LectureI. Galaxy CollisionsII. Dark Matter in ClustersIII. Superclusters Outline of Current LectureI. Extragalactic Distance ScaleII. The Hubble LawCurrent Lecture I. Extragalactic Distance Scale a. Use standard candles and apply inverse square law to find galaxy distances B = L / d2 b. Examples of standard candles:1. Cepheid variables2. Supergiant stars3. Globular star clusters4. Galaxies 5. Supernovac. More distant galaxies require more luminous candlesd. Must calibrate the candles1. i.e. must know L accuratelyII. The Hubble Law (1929)a. Hubble measured Doppler shifts of absorbtion lines in galaxy spectra1. Result: all but the nearest galaxies recede from Milky Way – have red shifts2. Recession velocity v is proportional to distance of galaxy d v = H0 d3. H0 is the Hubble constantb. The more distant the galaxy, the faster it recedesc. Determination of Hubble constant H0v/c = (λobs – λrest) / λrest1. Measure v from Doppler shift2. Measure d using standard candle methods 3. Plot v vs. d and determine slope: H0 = v/dd. Interpretation of the Hubble Law1. The universe is expanding!2. Distances between galaxies (except within the same cluster) all increase uniformly in time3. The expansion started with the Big Bang about M x 10^9 years ago.4. It has continued ever
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