The Size of the Milky Way

Early models for the Milky Way:
  • early 1900s: Jacobus Kapteyn uses quantitative star counts to measure the size of the Galaxy.
    • ellipsoidal
    • ~ 10 kpc in size
    • Sun near center
  • ~1920: Harlow Shapley uses RR Lyrae variable stars to get distances to globular clusters.
  • Galaxy is ~ 100 kpc in size
  • Sun is ~ 15 kpc away from the center of the GC distribution.

Kapteyn's model
Globular cluster distribution
(Note: this figure uses the modern day distance estimates;
Shapley thought everything was ~ twice as big)


Who's right? Actually, both were wrong. Their observations were compromised by the effects of interstellar dust, the presence of which was unknown at the time.

Of the two, Shapley was much closer to the truth. When the effects of dust were realized and corrected for, studies of galactic structure entered the modern era.