The Shapley – Curtis Debate 26 Apr 1920 Center of the GalaxyHow big is the universe Nebula vs. GalaxiesAre “nebula” local or distant

Question of Scale—How big is the Galaxy or the Universe?

Are “Nebula” Local or Distant?

Heber Curtis Degree in Classical Languages Amateur Napa College Ph.D. in Astro from UVa in 1902 Director of Allegheny Pitt

Heber Curtis Sun is at the center of the Milky Way—oops Cepheids are not good for distance measurements—oops Milky Way size about 10K parsecs—oops Nebula are Galaxies beyond the MW--yea

Harlow Shapley Missouri to study journalism Switched to astronomy Ph.D. Princeton under H.N. Russell Director of Harvard Observatory Founder of UNESCO FBI Investigation

Harlow Shapley Galaxy 100K parsecs— oops (too big) Sun is 20K parsecs from the center—yea Nebula minor gaseous populations in the Milky Way--oops

The Distance Ladder Photometric parallax-- using geometry to measure distance to nearby stars Standard Candles— using decreasing brightness from certain stars to measure distance to nearby galaxies

Cepheid Variables Class of stars whose brightness varies with time Period is proportional to intrinsic brightness Using inverse square law and knowing period distance can be calculated

Cepheid Variables Period vs. luminosity discovered by Henrietta Swan Leavitt Worked at Harvard classifying stars Opened the way for Hubble, Hurtzsprung and Russell

The Great Debate--Resolved Edwin Hubble Used Cepheids to show Andromeda was beyond the Milky Way Discovered “Red Shift”—evidence of an expanding universe Erased Einstein’s “Cosmological Constant ”

Structure of Galaxies Hubble’s work led to understanding the structure of galaxies

And to Types of Galaxies