The Milky Way – Timeline

The Milky Way Galaxy

HISTORY

1750: Immanuel Kant: advocated the “lens-shaped” distribution of stars, or an “island universe” with galaxies like the Milky Way

1785: William Herschel + Caroline (wife): made the first attempt to determine the shape of the galaxy; found few stars near the edge  and many stars toward the center; determined the galaxy to be an irregular “grindstone” or hockey puck

1900: If the Sun is at the center of the Universe, why is it not brighter at the center? Gas and gas prevent seeing far toward the center and light absorbed and refracted by Earth’s atmosphere  only allows us to see a small portion of the galaxy

Harlow Shapley and Herbert Curtis

1920: The Curtis- Shapley Debate

Harlow Shapley: rising star and “golden boy” of astronomy

  • Since globular clusters are not uniformly distributed uniformly around the Sun, the center of the Milky Way must be centered 30,000 light years away
  • Concluded that the Milky Way is much larger than previously believed (>100,000 light years in diameter)
  • The “nebulae” seen are not island universes but contained in the Milky Way

Herbert Curtis: established astronomer and respected

  • Spiral nebulae are galaxies out side the Milky Way, with high recessional velocities
  • Predicted that these spiral nebulae are the right size to be galaxies –> “huge” galaxy idea

While Shapley advanced that the Sun is not at the center of the galaxy and the galaxy is much larger than believed, Curtis argued that since spiral galaxies are external, there must be more big galaxies.

Who was right? BOTH. Who was wrong? BOTH.

Shapley was right the Sun is not at the center of the Universe. Curtis was right the Universe is composed of many galaxies. However, the size of the Milky Way was in-between their estimates.

1920s-1930s: Edwin Hubble: With the Hooker Telescope on Mt. Wilson, Hubble observed Cepheid Variable stars in the Andromeda Galaxy (M31); Cepheid Variable stars are 500-10,000 brighter than the Sun (in absolute magnitudes)

  • 1920s: Discovered that M31’s distance is too large to be within the Milky Way; M31 is a galaxy like the Milky Way
  • 1930s: Further understanding of the distances and distribution of globular clusters; the scientific community accepted that they underestimated the size of the Milky Way and the Sun is not at its center
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