AST101 The Nature of the Nebulae

Tuning Fork Diagram

Nebulae

The Nature of the Nebulae : Vesto Slipher obtains spectra of spiral nebulae. Spectra are continuous, not emission line spectra. Slipher shows spectra are redshifted to 1000 km/s, unlike anything else in Milky Way : Adriaan van Maanen claims to detect proper motions in M101 and 6 other spirals. Conclusion: they must be close. The 1920 debate at the National Academy of Sciences.

Spectra of Nebulae Gaseous nebula (Orion nebula) Spiral galaxy (M 83)

The Great Debate Harlow Shapley Large Galaxy Spirals: Gaseous nebulae Heber D Curtis Small Galaxy Spirals: “Island Universes”

Size of the Galaxy Distance scale Trigonometric Parallax (to 300 ly) Spectroscopic Parallax –Based on brightness of main sequence –Based on brightness of tip of giant branch

Shapley’s Argument Spirals are nearby: Van Maanen’s proper motions in M101 –Shown to be erroneous S Andromedae like Nova Persei 1901 –S And is a supernova, unrecognized in 1920 Spirals are gaseous: Bluer than Milky Way –We see redder bulge of spiral galaxies Spirals avoid plane of Milky Way Suggests influence High Recessional Velocities Suggests influence

Curtis’ Argument Star counts require small Galaxy Radius ~ 30,000 ly Globular cluster stars are dwarfs The brightest are giants S And unlike N Per 1901 Colors of galaxies look like stars

The Effect of Dust Dust Reddens light Absorbs light, and makes things appear further away than they really are The existence of dust was unknown in 1920

Standard Candles In 1908, Henrietta Swan Leavitt noted a correlation between brightness and period in certain variable stars in the LMC. These are the Cepheids

Cepheids Yellow Supergiants Luminous: observable at great distances Pulsationally unstable, Period ~ 1/density Named after δ Cephei

Other Standard Candles Planetary Nebulae –All have similar surface brightnesses Tully Fisher Relation –If all spiral galaxies are similar, L~V 4 Type Ia supernovae –All have same peak luminosity

Hubble Law Hubble, The Realm of the Nebulae, 1935

Hubble Law

V = H 0 d H 0 = 73.8 ± 2.5 km/s/Mpc Recessional velocities of galaxies (redshifts) are the ultimate distance indicator.