The History and Philosophyof Astronomy (Lecture 21: Hubble I)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)Edwin P. Hubble: Mariner of the Nebulae • 1889 (Missouri) – 1953 (Pasadena)• leading observationalastronomer of 20thcentury:- discovers galaxies (1924): Milky Way but one ofinnumerable “island universe”- expansion of the universe (1929): Hubble’s Law:v = H0 x dHubble’s Early Life • 1889: born in Marshfield,Missouri • born into uppermiddle-class family• 1899: family moves toIllinois (Evanston/Wheaton)•1906: Graduation fromHigh-SchoolHubble memorial in MarshfieldUndergrad at University of Chicago• 1906-10: physics, mathematics, and (at insistence offather) law• great athletic aptitudeRhodes Scholar at Oxford• 1910-12: Queen’s College• continued athletic prowessCecil Rhodes: Archimperialist• 1903: Rhodes Scholarship program established!Cecil Rhodes (1853-1902)PhD Thesis at Yerkes, Univ. of Chicago• PhD 1917: “Photographic Investigations of Faint Nebulae”Hubble joins Army in WW I• 1917-19: Major Hubble, 86thDivisionMount Wilson Observatory• 1919 onwards: world’s largest telescopeMount Wilson ObservatoryGeorge Hale (1868-1938)100-inch Hooker TelescopeMount Wilson Observatory“The Monastery”: Mt Wilson’s libraryThe Riddle of the Nebulae• Big Q: What are the (Spiral) Nebulae? part of the Milky Way? “Island Universes” in their own right?The Earl of Rossediscovers Spiral Nebulae • His sketch of M51 (1845): First Spiral Nebula• Speculate: -- Galaxy in rotation? -- A galaxy (system of stars), or gaseousnebula?Riddle of the (Spiral) Nebulae One-island universe (Kapteyn):- model accepted ~1900- basis for Einstein’s static GR model of the universe• Two basic models:Many-island universe (Kant)MWApril 1920: The Great Debate • National Academy of Sciences, Washington, D.C.• Resolved: “What is the nature of the Spiral Nebulae?”vsHarlow Shapley(Mt. Wilson Obs.)Heber Curtis(Lick Observatory)Shapley demotes Sun from Galactic Center (1918) SunGlobular Clusters• Sun’s true location is ~30,000 LY away from Centerof the Milky Way!April 1920: The Great Debate • National Academy of Sciences, Washington, D.C.• Resolved: “What is the nature of the Spiral Nebulae?”vsHarlow Shapley(Mt. Wilson Obs.)Heber Curtis(Lick Observatory)Great Debate: Shapley’s Argument • Spiral nebulae are part of MW (=Universe)!Great Debate: Shapley’s Evidence I • Q: If nebulae were truly extragalactic, they shouldbe seen all over the sky!• all-sky map of nebulae (NGC): “Zone of Avoidance”Great Debate: Shapley’s Evidence II • If Andromeda were truly extragalactic (i.e., very distant), nova would have had luminosityof 100 million Suns!  preposterous!?• 1885 Nova in Andromeda Nebula (S Andromedae)Great Debate: Curtis’Argument • Spiral nebulae are “island universes” (=galaxies)in their own right, consisting of billions of stars!Great Debate: Curtis’Rebuttal I • “Zone of Avoidance” is due to interstellar dustobscuration!  Nebulae can be all over the sky!• all-sky map of nebulae (NGC): “Zone of Avoidance”Interstellar DustGreat Debate: Curtis’Rebuttal II • Curtis claims: S Andromeda likely abnormal!(We now know this is correct: S And was a supernova!)• 1885 Nova (?) in Andromeda Nebula (S Andromedae)Great Debate: Who won? • With hindsight, we know that Curtis was right• But: Scientific controversies cannot be decided bydebate, only by hard evidence!• To empirically decide the issue: Need to measuredistance to the spiral nebulae!• Big Q: How to achieve this???- parallax method only works for nearest starsHubble and the Distance to Andromeda (M31) • Hubble discovers a Cepheid variable in Andromeda!• October 1923: He obtains photograph of M31Brightness vs timeDiscovery of Cepheid Variables • 1784: John Goodricke discovers delta CepheiLightcurveJohn Goodricke (1764-86)- also dicovered Algol binarysystemWhat are the Cepheids? • A: Pulsating stars (periods of ~ few days)Cepheids: Period-Luminosity Relation (1908) • for Cepheids: longer period higher luminosity!Henrietta Leavitt (1868-1921)- part of the famous team ofwomen “computers” at HarvardObservatory• Cepheids in Small Magellanic Cloud (SMC):- can assume that theyare all at same distanceCepheidsas Standard CandlesCepheidsas Standard Candles 1) Measure Period3) Calculate distance2) Calculate luminosity • inverse-square law:flux=luminosity/distance2measureHubble and the Distance to Andromeda (M31) • Cepheid distance = ~ 900,000 Lightyears !• Compare to size of MW (~100,000 LY)!• October 1923: He detects Cepheid in M31Brightness vs timeGreat Debate: Hubble’s Empirical Answer • Spiral nebulae are “island universes” (=galaxies)in their own right, consisting of billions of stars!• Andromeda is typicalexample of spiral nebulae• Andromeda lies outsideof Milky Way, and byextension all spiral nebulaedo!Hubble (part 1)• Early Life:- 1889: born in Marshfield, Missouri- 1906-10: Underraduate at University of Chicago- 1910-13: Rhodes Scholar in Oxford- 1917: PhD from Yerkes, Univ. of Chicago- 1917-19: Army officer during WWI- 1919 onwards: joins Mount Wilson Observatory• True Nature of Spiral Nebulae (1923-24):- resolves Cepheid variables in Andromeda (M31)- estimates distance: ~ 900,000 LY- this safely places M31 outside of the MW (size: ~100,000 LY)- Kant (and Curtis in “Great Debate”) was correct:- Spiral Nebulae are “Island Universes”