The History and Philosophyof Astronomy (Lecture 17: Birth of Astrophysics II)Instructor: Volker BrommTA: Jarrett JohnsonThe University of Texas at AustinAstronomy 350L (Fall 2006)• Which questions would an astronomer haveasked about the stars in the early 1800s? • How far away are they (stellar distance scale)?• What are the stars made of (stellar composition)?• How long do they live (stellar lifetimes)?• By what mechanism do they shine?• Is the Sun just a (nearby) star?A: Yes, already widely believed (Descartes, Newton)• How massive are they?• How far away are they (stellar distance scale)?• What are the stars made of (stellar composition)?The Great Age Controversy • one of the biggest riddles of the 19thcentury!• Claim 1: Earth must be very old (billions of years):- geological time- biological (evolutionary) time• Claim 2: World cannot be so old:- Earth would have cooled too much by now!- Sun cannot shine for so long!Geological Timescale • Big Q: How was Grand Canyon created?A: in a single, catastrophic event (catastrophism)?B: slowly, over many eons (uniformitarianism)?Catastrophism: A Young Earth • James Ussher (Archbishopof Armagh, 1581-1656)• calculated (using the Bible)when God created theuniverse:Oct. 23rd, 4004 BC(Sunday, 8pm)Age of Earth = few 1,000 yearsGeological Timescale • Charles Lyell (1797-1875)• 1830: Principles of Geology- convincingly makes casefor uniformitarianism- age of the Earth =few billion yearsGeological TimescaleBiological Timescale • Charles Darwin (1809-82)• 1859: Origin of Species- theory of evolution:- random mutation- natural selection- age of the Earth =few billion yearsBiological Timescale1850s: Conservation of EnergyJames Joule (1818-89)• In a closed system (e.g., universe):ENERGY=constantHow much Energy does the Sun contain? • Sun’s lifetime=total energy/luminosity• Chemical energy- Sun made up of coal- age = few 1,000 years• What powers the Sun(and the stars)???Powering the Sun: Gravitational Energy???Lord Kelvin (1824-1907)Hermann von Helmholtz(1821-94)Powering the Sun: Gravitational Energy???• Kelvin-Helmholtz contraction:- for the Sun: age = few million years(compared to billions of years required)- need even more efficient energy source!Stellar Distances: From Parallax!Stellar Luminosities: From Inverse-square Law • what we measure: flux = energy/area(`apparent brightness’)• if distance (d) to staris known, can figureout true (intrinsic)brightness = Luminosity (L)• L = 4 x pi x d2x flux (“inverse-square law”)Stellar Surface Temperatures: From SpectraStellar Masses: From Orbits of Binary Stars • Kepler’s Law:How do the Stars work?Ejnar Hertzsprung (1873-1967)Henry Norris Russell (1877-1957)The Hertzsprung-Russell (HR) Diagram Main sequenceGiant BranchThe Hertzsprung-Russell (HR) DiagramThe Mystery of the Main-Sequence • Arthur Stanley Eddington(1882-1944)• proved Einstein’s GR theory(1919 eclipse expedition)• The Internal Constitution of the Stars (1926)- the Laws of Stellar StructureThe Mystery of the Main-Sequence • Mass-luminosity relation (high M high luminosity)The Mystery of the Main-Sequence • Mass-luminosity relation (high M high luminosity)Eddington explains the M-L Relation: • Law of Stellar Structure:- gravity = pressure- pressure = stellar heat- stellar heat = stellarluminosity• gravity (M) ~ stellar luminosity (L)The Mystery of the Main-Sequence • central temperature in star = almost constant!• a stellar thermostat:- Central T~ ten million K- almost independentof mass!!!Eddington’sConjecture for Stellar Energy Source: • nuclear fusion:4 protons (H) 1 helium (He) nucleus• He nucleus has a bitless mass than sum of4 protons (mass defect)• missing mass = energy(Einstein’s E=m c2) • But how does this really work???Chemical Composition of the Stars • Cecilia Payne-Gaposhkin(1900-79)• Harvard PhD 1925• hydrogen and helium aremost abundant elements inthe universe!What makes the Stars Shine?Hans Bethe (1906-2005)Carl Friedrich von Weizsaecker(born 1912)What makes the Stars Shine?• Nuclear Fusion via the CNO Cycle (Bethe and Weizsaecker 1938)Astrophysics and the Bomb• Both worked on bomb during WWII:Bethe (Manhattan Project, USA); Weizsaecker (Nazi bombproject thwarted!?)How much Energy does the Sun contain? • Sun’s lifetime=total energy/luminosity• Nuclear energy- Sun made up of H/He- age = few billion years• Age crisis resolved!How long do the Stars live? • a huge range of stellar lifetimes !All stars (and the Sun) evolve! • Sun’s Life: from molecular cloud to white dwarf(~10 billion years)Birth of Astrophysics (part 2)• Figuring out the energy source of the stars: - “Age crisis” of 19thcentury: How to reconcile thelong timescales of geology and biology (billions of years)with estimated lifetime of the Sun, then estimatedto be only few million years)???- gravitational energy NOT sufficient!- Need nuclear (fusion) energy!• Pioneers of astrophysics:- Kelvin-Helmholtz (gravitational energy)- Hertzsprung-Russell diagram - Arthur Eddington explains the main-sequence- Bethe and Weizsaecker figure out stellar fusion
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