The Shapley – Curtis Debate 26 Apr 1920Center of the Galaxy How big is the universeNebula vs. Galaxies Are “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 Astronomer @ Napa College

• Ph.D. in Astro from UVa in 1902

• Director of Allegheny Observatory @ 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. 1913 @ 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