Copyright © 2009 Pearson Education, Inc.Chapter 15Galaxies and the Foundationof Modern CosmologyCopyright © 2009 Pearson Education, Inc.Should the Bonfire return toA&M?A. YesB. NoC. What’s a bonfire?Copyright © 2009 Pearson Education, Inc.Was the Midterm too hard?A. YesB. NoC. What?? I missed the midterm??Copyright © 2009 Pearson Education, Inc.Copyright © 2009 Pearson Education, Inc.15.1 Islands of StarsOur goals for learning:• What are the three major types of galaxies?• How are galaxies grouped together?Copyright © 2009 Pearson Education, Inc.Spiral GalaxydiskbulgehaloCopyright © 2009 Pearson Education, Inc.Spheroidal Component:bulge and halo, old stars,few gas cloudsDisk Component:stars of all ages,many gas cloudsCopyright © 2009 Pearson Education, Inc.DiskComponent:stars of all ages,many gascloudsSpheroidalComponent:bulge andhalo, old stars,few gas cloudsCopyright © 2009 Pearson Education, Inc.DiskComponent:stars of all ages,many gascloudsSpheroidalComponent:bulge andhalo, old stars,few gas cloudsBlue-white colorindicates ongoingstar formationRed-yellow colorindicates older starpopulationCopyright © 2009 Pearson Education, Inc.DiskComponent:stars of all ages,many gascloudsSpheroidalComponent:bulge and halo,old stars,few gas cloudsBlue-white colorindicates ongoingstar formationRed-yellow colorindicates older starpopulationCopyright © 2009 Pearson Education, Inc.Thought QuestionWhy does ongoing star formation lead to a blue-white appearance?A. There aren’t any red or yellow stars.B. Short-lived blue stars outshine others.C. Gas in the disk scatters blue light.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhy does ongoing star formation lead to a blue-white appearance?A. There aren’t any red or yellow stars.B. Short-lived blue stars outshine others.C. Gas in the disk scatters blue light.Copyright © 2009 Pearson Education, Inc.Barred Spiral Galaxy: Has a bar of stars across the bulgeCopyright © 2009 Pearson Education, Inc.LenticularGalaxy:Has a disklike a spiralgalaxy butmuch lessdusty gas(intermediatebetween spiraland elliptical)Copyright © 2009 Pearson Education, Inc.EllipticalGalaxy:All spheroidalcomponent,virtually no diskcomponentCopyright © 2009 Pearson Education, Inc.EllipticalGalaxy:All spheroidalcomponent,virtually nodiskcomponentRed-yellowcolor indicatesolder starpopulation.Copyright © 2009 Pearson Education, Inc.Irregular Galaxy: Neither spiralnor ellipticalCopyright © 2009 Pearson Education, Inc.Blue-white colorindicates ongoingstar formation.Irregular Galaxy: Neither spiralnor ellipticalCopyright © 2009 Pearson Education, Inc.Hubble’s galaxy classesSpheroidDominatesDiskDominatesCopyright © 2009 Pearson Education, Inc.How are galaxies groupedtogether?Copyright © 2009 Pearson Education, Inc.Spiralgalaxies areoften foundin groups ofgalaxies (upto a fewdozengalaxies pergroup).Copyright © 2009 Pearson Education, Inc.Ellipticalgalaxies aremuch morecommon inhuge clustersof galaxies(hundreds tothousands ofgalaxies).Copyright © 2009 Pearson Education, Inc.What have we learned?• What are the three major types of galaxies?— Spiral galaxies, elliptical galaxies, andirregular galaxies— Spirals have both disk and spheroidalcomponents; ellipticals have no disk.• How are galaxies grouped together?— Spiral galaxies tend to collect into groups ofup to a few dozen galaxies.— Elliptical galaxies are more common in largeclusters containing hundreds to thousands ofgalaxies.Copyright © 2009 Pearson Education, Inc.15.2 Distances of GalaxiesOur goals for learning:• How do we measure the distances to galaxies?• What is Hubble’s law?• How do distance measurements tell us the age ofthe universe?Copyright © 2009 Pearson Education, Inc.How do we measure the distancesto galaxies?Copyright © 2009 Pearson Education, Inc.Brightnessalone doesnot provideenoughinformationto measuredistance.Are Bright Stars Nearby or Luminous?Copyright © 2009 Pearson Education, Inc.Step 1Determine sizeof solar systemusing radarRadar PulsesCopyright © 2009 Pearson Education, Inc.Step 2Determinedistances ofstars out to afew hundredlight-yearsusing parallaxCopyright © 2009 Pearson Education, Inc.Luminosity passingthrough each sphereis the same.Area of sphere: 4π (radius)2Divide luminosity byarea to get brightness.Copyright © 2009 Pearson Education, Inc. The relationship between apparent brightness andluminosity depends on distance: Luminosity Brightness = 4π (distance)2 We can determine a star’s distance if we know itsluminosity and can measure its apparent brightness: Luminosity Distance = 4π × Brightness A standard candle is an object whose luminosity wecan determine without measuring its distance.Copyright © 2009 Pearson Education, Inc.Step 3Apparentbrightness ofstar cluster’smain sequencetells us itsdistanceCopyright © 2009 Pearson Education, Inc.Knowing a star cluster’s distance, we can determine theluminosity of each type of star within it.Copyright © 2009 Pearson Education, Inc.Thought QuestionWhich kind of stars are best for measuring largedistances?A. High-luminosity starsB. Low-luminosity starsCopyright © 2009 Pearson Education, Inc.Thought QuestionWhich kind of stars are best for measuring largedistances?A. High-luminosity stars B. Low-luminosity starsCopyright © 2009 Pearson Education, Inc.Cepheidvariable starsare veryluminous.Copyright © 2009 Pearson Education, Inc.Cepheid Variable StarsThe light curve of this Cepheid variable star shows that itsbrightness alternately rises and falls over a 50-day period.Copyright © 2009 Pearson Education, Inc.Cepheid variable stars with longer periods have greaterluminosities.Copyright © 2009 Pearson Education, Inc.Step 4Because theperiod of aCepheidvariable startells us itsluminosity, wecan use thesestars asstandardcandles.Using Cepheid Variables as Standard CandlesCopyright © 2009 Pearson Education, Inc.White dwarfsupernovaecan also beused asstandardcandles.Copyright © 2009 Pearson Education, Inc.Step 5Apparentbrightness of awhite dwarfsupernova tellsus the distanceto its galaxy(up to 10billion light-years).Copyright © 2009 Pearson Education, Inc.What is Hubble’s law?Copyright © 2009 Pearson Education, Inc.The Puzzle of “Spiral Nebulae”• Before Hubble, some scientists argued that“spiral nebulae” were
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