Our galaxy the Milky Way is approximately 100,000 light years in diameter. At its center is a black hole. In the 1920s, Harlow Shapley thought the Milky Way was part of an isolated galaxy and the Sun was not at its center. The Milky Way will collide with its closest neighbor, the Andromeda Galaxy, in 4 billion years!
In the Southern Hemisphere, the Magellanic Clouds, or the galaxy’s satellite galaxies (revolves around Milky Way), are visible. Because of interstellar dust (rocky planets and other material), we can only see 6,000 stars, but the Milky Way has 100 billion stars total. The farthest are 4,000 light years away. Earth’s atmosphere smears the sky, so stars appear to twinkle. About 10^6 stars— old as the universe— inhabit In globular clusters (~200 in Milky Way’s halo).
Shapley’s Subdivision of the Milky Way
- Nuclear Bulge: (10^6 solar masses) nucleus in the center, old stars (red)
- The Disk: (10^11 solar masses) thin, diffuse layer of material revolving around the bulge; the Sun is half-way on the disk; all young stars
- The Halo: hot gas about 100,000 K
- Galactic Corona: mass exists but unseen; 5-10 times as much mass as the nucleus, disk, and halo together, 95% of galaxy mass unknown matter
- Visible Matter: 96% stars, 4% interstellar gas
- (13.6 billion years ago) A gas cloud of 75% hydrogen and 25% helium with mass ~ 1 trillion solar masses
- Contraction and rotation form spherical shape
- Inner part flattens to form disk of younger stars
- Galactic rotation forms spiral arms
- Supernovae gives off more heavy elements that eventually become the Sun
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
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 outside 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