Monday, Nov. 3Syllabus, class notes, and homeworks are at:www.as.utexas.edu courses AST 301, LacyReading for this week: chapter 12The Wednesday help session is in GRG 424 at 5:00 (forthe entire semester).Topics for this weekDescribe the Milky Way GalaxyDescribe the Standard Candle method of determiningdistances and how Cepheid variable stars are used asstandard candles.Describe how astronomers measure the distribution of massin the Milky Way and what they find.Explain why we might expect the spiral arms in the MilkyWay to become more tightly wrapped and how densitywave theory solves this problem.Describe the various types of galaxies, both normal andactive, and how they differ from the Milky WayThe Milky WayA hazy band of light across the skyLatin: Via Lactea, or Road of MilkAstronomy: GalaxyGalileo saw that it was made of many stars.The stars form a band across the sky because they are in aflat distribution, like people in this room.Could you determine the distribution of people in this room,and your position in that distribution from observationswithout leaving your seat?Mapping out the Milky WayWilliam Herschel (early 1800s)Used two methods to map out the stars in the MilkyWaystar counting: more stars should be seen in thedirections where the distribution extends fartherstar brightnesses: more distant stars should appearfainter (assuming all stars have the same luminosity)Concluded that we are near the center.Why didn’t Herschel use parallax to measure the distancesto stars?Doing the MathThe flux (or apparent brightness) of a star is the light powercollected by a telescope divided by the collecting area ofthe telescope.Flux depends on distance from the star because as thelight goes away from the star it spreads out over a largerarea.If you can measure the flux, F, and can somehow figure outthe luminosity, L, you can solve for the distance.2d 4L F!=F 4L d!=Mapping out the Milky WayIf we know how much light a star puts out (its luminosity)and we measure how bright it appears (its flux),we can calculate its distance.This is referred to as the standard candle method ofmeasuring the distance to a star.Henrietta Leavitt (early 1900s)Studied variable stars in the Small Magellanic Cloud.Found that flux and period are proportional.Since all of her stars were about equally distant, thismeans that luminosity α period of variation.Our place in the Milky WayHarlow Shapley (about 1920)Determined the constant of proportionality betweenperiod of variation and luminosity, so the luminosity ofa Cepheid variable star could be determined from itsperiod.Then he could use Cepheid variables as standardcandles.He mapped out the distribution of globular clusters.He found that they formed a distribution centered somedistance from us in the direction of Sagittarius.He concluded that we are not at the center of the MilkyWay and that the Milky Way is more than 10,000
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