A Great Debate: Shapley vs. Curtis and the Scale of the Universe (1920)

Main Questions:

Some ``Nebulae''

Andromeda Spiral ``Nebula'' (Messier 31)

Just fuzzy blobs to astronomers at turn of century...

Who are these Guys?

Harlow Shapley

Heber Doust Curtis

To read their obituaries, click Shapley or Curtis.

Shapley believed:

Curtis believed:

To understand why, first need to understand Cepheid variable stars...

The period over which a Cepheid variable star fluctates in observed brightness is related to its intrinsic brightness (or luminosity). Therefore, by measuring the period, you can determine the intrinsic brightness. By comparing the intrinsic brightness to the observed brightness, you can derive the distance to the star.

Shapley used Cepheids/RR Lyraes to determine distances to clusters.

Curtis did not believe that Cepheid's had a strong period-luminosity relation.

Plot of Time vs. Brightness

To look at a real movie of a Cepheid varying, click here.

To learn more about Cepheid variables, click here.

Shapley's arguments:

Sun far from center of Galaxy because Milky Way large because Spiral nebulae within our Galaxy because

Curtis's arguments:

Sun at center of relatively small Milky Way because Spiral nebulae are galaxies beyond our own because

Edwin Hubble to the rescue...

at the 100in on Mount Wilson

identification of Cepheids in Andromeda

So, who won the debate?

Correct Both were incorrect that interstellar absorption of starlight by dust was unimportant.

The picture of our Galaxy today...

  • bulge, thin disk, and spherical halo

  • globular clusters reside mostly in halo

  • disk consists of mostly young stars, gas, and dust

  • spiral arms of disk are where most new stars form

  • bulge is mostly older stars

  • Milky Way is over 100,000 light-years across

  • Sun is located in disk, one-half to two-thirds of distance from center


Our galaxy itself contains a hundred billion stars;
it's a hundred thousand light years side to side.
It bulges in the middle - sixteen thousand light years thick -,
but out by us it's just three thousand light years wide.
With thirty thousand light years from galactic centrepoint
we go round every two hundred million years -
and our galaxy is only one of millions, of billions,
in this amazing and expanding universe!
                   Eric Idle, The Galaxy Song ("The Meaning of Life")

Our Galaxy in visible light (an edge-on disk as seen from Earth)

Our Galaxy at other wavelengths...

To read some more about the Shapley-Curtis Debate, click on the following: