1The Expanding UniverseChapter 10 – Hawley & HolcombChapter Overview• Doppler Effect• Scale of Galaxy• Cosmic Distance Ladder• Hubble’s Law• Expansion of the UniverseRedshifting• Objects moving toward us appear more blue, while objects moving away from us appear more red!Image Credit: www.cnrt.scsu.eduRedshifting of SpectraImage Credit: http://www.sdsu-physics.orgRedshifting of GalaxiesLight from a galaxy moving slowly away from EarthLight from a galaxy moving faster away from EarthImage Credit: herschel.jpl.nasa.gov www.cnrt.scsu.eduRedshifted SpectraWavelength (nm)Image Credit: www.cnrt.scsu.eduVery Distant GalaxyNearby GalaxyDistant GalaxyNearby Star2Redshifted SpectraThree of the most distant quasars discovered by the Sloan Digital Sky SurveyREDBLUEInterstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic diskWilliam Herschel maps out the distribution of stars and gets:The sunThe “universe”of HerschelWilliam Parsons, 3rd Earl of Rosse, builds the “Leviathan of Parsonstown” - draws “spiral nebulae”What were they?Stars & planetary systems forming in our own “universe”?Separate “island universes”?Henrietta Leavitt & the Cepheid P-L RelationshipLight curve of a Cepheid variableLarge & Small Magellaic Clouds Period versus magnitude of Cepheids in SMCH. Shapley maps distribution of Globular Star Clusters using “Cepheids”(“where’s the mass centered?”)We are NOT at the center.What happened?Globular cluster with variable stars3“Dust Happened”At visible wavelengths, the center of our galaxy suffers ~ 30 mag of extinction by dust!! Even with big modern telescopes, we cannot see very far in the plane of our galaxy at visible wavelengthsThe Shapley-Curtis Debate (1920)MWGMWGShapleyCurtisThe debate solved nothing!Questions in science are not resolved by debates, but by observations & experimentsNature of the Spiral Nebulae and the Great DebateShapley Novae brightnessesincompatible with M31 being as big as MWG Rotation of M101Curtis Novae indicate a smaller MWG than Shapley’s Galaxy proper motions undetected Zones of avoidance in other systems1923 - Hubble Measures Distance to M 31 using Cepheid Variables100-inch Hooker Telescope, Mt. WilsonEdwin HubbleDebate OVER!Discovery of Cepheids in M 31Star cluster is really HEREBut the extinction makes it fainter, so we would incorrectly think that it is HERE based on brightness measurementsRegion with dust absorption: A magsIgnoring the extinction due to dust will result in deriving a photometric distance that is too large by a factor of 10A/5!Trumpler - 1929Shapley’s MWG was too big for a couple of reasons:Other problems:S Andromedae, a “nova” in the Andromeda Galaxy, was actually a supernova -with much higher L and hence distanceProper motions in galaxies “measured” would require speeds greater than light if they were distant - these measurements turned out to be wrong!Summary: Shapley’s MWG was too big, and his distances to the spiral nebulae too small4Stellar Photometric DistancesFor an apparent (observed) magnitude m, absolute magnitude M, and distance d in parsecs:Without dust: m = M+5logd-5 and so d = 10(m-M+5)/5 pc(reminder: m = Md=10pc)With dust: m = M+5logd-5+A and d = 10(m-M-A+5)/5 pcwhere A is the extinction by dust in magnitudes(Note: sometimes astronomers use the “distance modulus”m-M = 5logd-5 to express the distance to some objects)How do we know the distances to all these galaxies???“Extragalactic Distance Ladder”• Starts with distances to nearest stars…• Ends with distances to furthest galaxies…We use many different methods to measure distance:Some only work nearby.Some only work far away.Parallax works nearbyCan get distances only to the very nearest star clustersin the Milky Way (the Hyades & the Pleiades)Fails when the angular shift is smaller than we can resolve.Main Sequence Fittingaka Spectroscopic Parallax• Make a color-magnitude diagram of individual star clusters• Groups of stars at different distancesappear shiftedup or down on the HR diagramMain Sequence Fitting• Can use to measure distances to anywhere you can resolve large numbers of individual stars.– Star Clusters in the Milky Way– The Large & Small Magellanic Clouds.5which stars are truly luminous, and which stars just look bright?A Trick:Some fraction of very luminous stars “pulsatepulsate”They become brighterbrighterand dimmer in a regular patternWhat about stars that are too far for parallax, but not in clusters?Pulsating or “Variable” Stars• There is an “instability strip” in the HR diagram, where stars sizes aren’t stable.•“Cepheids” are extremely bright!The “period” (duration) of the pulsation is related to the Cepheid’s luminosity1.1.Measure Measure periodperiod2.2.Derive Derive luminosityluminosity3.3.Measure Measure apparent apparent brightnessbrightness4.4.Derive Derive distancedistance!!The luminosity of the observed star is ~1500L~!What about galaxies that are too far to detect Cepheids?• Use the “Tully-Fisher” Relation:Luminosity# of starsrGMVcentralrotation=Rotation speedMassMeasure rotation speed, figure out luminosityWhat about galaxies that are too far to measure rotation speeds?Wait for a supernova to go off!Extremely LuminousLuminousCan be seen to huge distancesIt’s hard to measure a rotation curve if the galaxy just looks like a fuzzy blob…Works for White Dwarf Supernova (Type 1a)• All have about the same luminosity, since detonation of 1.4M~white dwarf!• “standard candles”• Measure the apparent brightness, derive distanceThey have distinctive spectra, so they’re easy to recognize6Lastly, the Hubble Law can be used to estimate distances.Too hard to use Tully-Fisher for all of them, or to wait for SN Ia’s to explode… v = H0dInstead, we can use this observed relationship between speed and distance.Hubble found that more distant galaxies appeared to be moving away from us faster• Measure the redshift “z”, derive the “recessional velocity”zcv×= z ≡ΔλλA “Hubble Diagram”• Recessional velocity is proportional to distance.•H0is the “Hubble Constant”, units are km/s/MpcDHv0=A galaxy’s recessional velocity lets you estimate its distance. v = H0dzcv×=1.Measure the redshift z.2.Derive the velocity v.3.Use Hubble Law to get the distance Measure this……to derive this.Example: A galaxy appears to be