[00:00]	The Milky Way
[00:01]	The Shapley-Curtis Debate
[01:46]	Distance versus Size
[02:05]	Solar System, or Spiral Galaxy?
[04:18]	Lines of Sight
[06:18]	Globular Clusters
[08:27]	The Shapley-Curtis Debate (reprise)
[10:28]	Variable Stars
[12:25]	Galactic Rotation
[15:46]	The Shapley-Curtis Debate (reprise)
[18:22]	Reading Galaxy Spectra
[23:20]	Expansion of the Universe

Learning Objectives

1. Consider the key points made in the Shapley-Curtis debate about the nature of spiral nebulae, and evaluate the observational evidence.
   1. Shapley thought the Milky Way was 300,000 light-years in diameter, and not centered around the Sun. He concluded that spiral nebulae were local clouds of gas.
   2. Curtis argued that the Milky Way was only 30,000 light years in diameter, was centered around the Sun, and that spiral nebulae were island universes located beyond the Milky Way.
   3. Shapley's estimate of the size of the galaxy was fairly good, and he was absolutely correct that it is not heliocentric. However, he did not realize that the spiral nebulae were external galaxies, similar in nature to the Milky Way itself.

   4. Shapley assumed that all globular clusters were similar in size, mapped their distribution, and used parallax measurements of the distance to one globular cluster to determine the size and extent of the Milky Way. [good data, reasonable analysis, good conclusions]
   5. Curtis' analysis of the globular cluster data produced a significantly smaller size estimate for the Milky Way.
      [good data, difficult analysis, flawed conclusions]
   6. Leavitt's observations and analysis of variable stars in the nearby Magellanic Cloud galaxies showed that stellar period correlated with the intrinsic luminosity for these objects. Shapley deduced a distance to the Magellanic Cloud's which lay within the Milky Way, and concluded that spiral nebulae lay within it, perhaps being late-forming solar systems.
      [good data, risky analysis, biased sample, flawed conclusions]
   7. van Maanen measured a rotational speed for M101 of 0.02 arc-seconds per year, forcing it to either lie within the Milky Way or to rotate faster than the speed of light.
      [inaccurate data, reasonable analysis, flawed conclusions]
   8. Recessional velocities for spiral nebulae implied that they were close to the Milky Way, near enough to be profoundly influenced by it.
      [good data, incomplete analysis, flawed conclusions]
   9. Curtis' study of spiral nebulae spectra showed them to be indistinguishable from the Galactic spectrum, suggesting a common form.
      [good data, good analysis, good conclusions]

2. Connect emission peaks and absorptions troughs in galaxy spectra to atomic transitions within atoms, and to chemical composition.

3. Calculate redshifts and blueshifts for galaxies, based on wavelength offsets in position of features based on rest-wavelengths.

4. Connect the recessional velocities of galaxies with their distance from the Milky Way.

5. Calculate distances to galaxies based upon their redshifts and blueshifts.

6. Visualize how an expanding Universe causes galaxies to appear to be receding, when viewed from the Milky Way or from any other galaxy.