AST101 The Nature of the Nebulae Tuning Fork Diagram Nebulae The Nature of the Nebulae • 1912-1915: 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. • 1916-1923: 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 Heber D Curtis Large Galaxy Small Galaxy Spirals: Gaseous nebulae Spirals: “Island Universes” http://apod.nasa.gov/diamond_jubilee/debate20.html 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~V4 • Type Ia supernovae – All have same peak luminosity Hubble Law Hubble, The Realm of the Nebulae, 1935 Hubble Law Hubble Law Hubble Law V = H0 d H0 = 73.8 ± 2.5 km/s/Mpc Recessional velocities of galaxies (redshifts) are the ultimate distance indicator. .