Transcript Great Astronomers of the 20th Century

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
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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
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Henrietta Leavitt
Annie Cannon/Cecilia Payne-Gaposchkin
Jocelyn Bell Burnell
Vera Rubin
Sandra Faber
Jill Tarter
Distances between the Stars
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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)
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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 d 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, 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
Spectral Type (Temperature) and
Luminosity
• The Hertzsprung-Russell Diagram
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”
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
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Active Galaxies
Pulsed Radio Frequency Objects
• Rotating fields
• Pulsed emission
• Lighthouse
Rotating Spiral Galaxies
Keplerian Rotation
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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
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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?