Darkness at Night (Olbers ’ Paradox)

1. Darkness at Night (Olbers’ Paradox) Imagine you are a pre-20th century astronomer. How many stars would you expect to see (under some simple assumptions about the universe)?

2. How much flux does each thin spherical shell contribute to the total brightness of the sky? # density of stars = n0 radius of shell = r thickness of shell = dr You’ll work this out on HW#1 (already posted on AS413 website)

3. A “forest” of finite-sized stars

4. Resolution of Olbers’ Paradox in Static Universe Can’t see the most distant stars because the light has not had time to reach us (universe is not infinitely old). The edge of the observable universe is the “horizon”. The distance to the horizon changes constantly.

5. William Herschel’s 1785 map of the Galaxy from star counts (The Amoeba Universe) Fig. 4 from “On the Construction of the Heavens” by William Herschel, published in Philosophical Transactions of the Royal Society of London, vol. 75 (1785)

6. The “Kapteyn Universe” (1922), based on star counts and no accounting for dust obscuration

7. Globular Star Cluster M80

8. Shapley’s distribution of globular star clusters (1918) Plane of Milky Way Sun is about 15 kpc from the center, and the whole distribution is about 90 kpc in diameter

9. The Great Debate / The Shapley-Curtis Debate • April 26, 1920 at National Academy of Sciences • What are the distances to the spirals? • Are the spirals composed of gas or stars? • Why do the spirals avoid the plane of the Milky Way? • Harlow Shapley (Mt. Wilson Observatory) and Heber Curtis (Lick Observatory) • Curtis argued for the spirals being “island universes” • He who had seemingly the best arguments turned out to be wrong

10. Distances to Spirals? • Shapley argued that they must be close for two reasons: (1) measurements of proper motions in M101 lead to ludicrous rotation speeds if M101 were as big as the Milky Way; (2) comparison of brightness of SAndromedae in M31 with Nova Persei in the Milky Way • Curtis argued for large distances because: (1) proper motion measurements might be in error, (2) comparison of novae in M31 with novae in the Milky Way • Difference between novae and supernovae not known observationally or theoretically at the time M101, the Pinwheel Galaxy

11. Stars or Gas? (Shapley) • If they are galaxies, they ought to have photometric and spectral properties like those of the Milky Way • Based on star counts, “surface brightness” (= flux/unit area) of the MW in the solar neighborhood is much less than in the spiral nebulae • Spirals are bluer at larger radii and absorption lines characteristic of stars hard to detect in the central regions • Spirals were apparently quite different from what was known about the photometric properties of the MW at the time; Curtis didn’t have an adequate response

12. Avoidance of the Milky Way? • Slipher had shown that the spirals were receding from the MW, and combined with the “Zone of Avoidance” led Shapley to propose that the MW exerted a peculiar (and hitherto unknown!) repulsive force on the spirals • Curtis noted that many of the spirals had dark, thick bands of obscuring material and gave 3 big“Ifs”: (1) if the MW has such a band, (2) if we are located in the mid-plane of the band, and (3) if the spirals are located outside the MW, THEN the Zone of Avoidance is caused by the obscuring material simply blocking them from our view NGC 4565