Great Astronomers of the 20th Century: A Brief Review of Astronomy C. G. De Pree RARE CATS June 2002

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” pc 0.01” pc

Variable stars John Goodricke ( ) –Delta Cepheus –Period of 5.4 days –Change of 0.7 magnitudes –(about 2x) Pulsating variables –Cepheid –RR Lyrae

Henrietta Leavitt (Radcliffe, ) 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

Stars in the Magellanic Cloud

Hubble Expansion Law

Spectra of Selected Elements

The Sun’s Spectrum

Annie Cannon (Wellesley, ) 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

Spectral Type (Temperature) and Luminosity The Hertzsprung-Russell Diagram

Cecilia Payne-Gaposchkin (Cambridge, ) 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”

When Stars Collapse Massive stars Supernovae Rotating remnant (neutron star)

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)

Active Galaxies

Pulsed Radio Frequency Objects Rotating fields Pulsed emission Lighthouse

Rotating Spiral Galaxies

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…

Galaxy Rotation Curves

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

Galaxy Classification

“Tuning Fork” Diagram Not an evolutionary scheme Classification scheme How does one become another? –Possibly through mergers

Galaxy Evolution Galaxy clusters –Mergers Time evolution –need to see distant galaxies

Large Scale Structure Density variations Top-down vs. bottom-up

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

Does Life Always Arise?

Can Life Survive?

Will Civilizations Communicate?

Our Interstellar Postcard

Recent Contrary Proposal e.g. Rare Earth, Peter D. Ward & Donald Brownlee “the alien search is likely to fail…”

Milestone Questions of 20th Century Astronomy How distant are the stars and galaxies? What other types of stars exist? What happens when stars die? What is dark matter and where is it located? How are galaxies organized and why? Are we alone in the universe?