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Great Moments in 20th Century Astronomy
• Determining the distances between the stars
• Spectral classification of stars
• Discovery of pulsars
• Evidence for dark matter
• The structure and evolution of galaxies
• Search for life in the universe
The Discoverers
• Henrietta Leavitt
• Annie Cannon/Cecilia Payne-Gaposchkin
• Jocelyn Bell Burnell
• Vera Rubin
• Sandra Faber
• Jill Tarter
Distances between the Stars
• Stellar Parallax (ancient times)
• First telescopes
• Resolution limit from the Earth
• 1” -- 1 pc (206,265 A.U.)
• 0.1” -- 10 pc
• 0.01” -- 100 pc
Variable stars
• John Goodricke (1764-1786)– Delta Cepheus– Period of 5.4 days– Change of 0.7 magnitudes – (about 2x)
• Pulsating variables– Cepheid– RR Lyrae
Henrietta Leavitt(Radcliffe, 1868-1921)
• Worked with Magellanic Cloud data– Discovered 1700 variable stars, 20 Cepheid variable
stars (after prototype Cepheus)– Discovered a magnitude-period relationship--
brighter stars had longer periods– All stars were essentially at the same distance
• Once calibrated--period luminosity relationship
• Hubble’s expanding universe theory
Annie Cannon (Wellesley, 1863-1941)
• Harvard College Observatory– Henry Draper (HD) catalog– Stellar spectral classes (O, B, A, F, …)
• Prolific classifier of stars– Spectral Type and Temp.– Uniform database of stars
Cecilia Payne-Gaposchkin (Cambridge, 1900-1979)
• Assigned specific surface temperatures to Annie Cannon’s spectral/temperature classes
• 1956: Full professor and chair of Harvard Astronomy Department
• Her PhD thesis on the physics of stellar atmospheres was deemed “brilliant”
What’s Left?
• Supernova Remnant– e.g. the Crab Nebula
• Neutron star– high density, strong magnetic fields
Jocelyn Bell Burnell(Cambridge)
• Investigating radio emission from quasars– Distant, energetic objects related to the
formation of galaxies
• Discovered pulsars
• Little Green Men – (LGM)
Keplerian Rotation
• Near the galaxy center--fast rotation
• Far out in the galaxy--slow rotation
• Just like the planets
• Only true when the mass is centrally concentrated– Solar system is a good example– Galaxies apparently are not
Vera Rubin(Vassar)
• Rotation of spiral galaxies
• Outer reaches of galaxies do not rotate more slowly– Mass of visible matter does not explain the rotation
curve
– Presence of “dark matter”
• What is dark matter?– Big Jupiters, massive neutrinos,
– Black holes, brown dwarfs…
Sandra Faber(Swarthmore)
• Structure and evolution of galaxies and galaxy clusters
• Faber-Jackson relation to determine distances to galaxies – Picks up where Period-Luminosity relationship runs out
of steam– Luminosity of galaxy is correlated to the width of its
spectral absorption lines– Velocity dispersion of the inner few kiloparsecs of a
galaxy
“Tuning Fork” Diagram
• Not an evolutionary scheme
• Classification scheme
• How does one become another?– Possibly through mergers
Jill Tarter
• Joint appointment at UC Berkeley and SETI Institute
• Pioneered efforts to look for artificial radio signals
• The Drake Equation and the search for life– Star formation rate– “Habitable Zone”– Probability that life will arise– Probability that intelligence will arise
Recent Contrary Proposal
• e.g. Rare Earth, Peter D. Ward & Donald Brownlee
• “the alien search is likely to fail…”