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Announcements • 4th test is finished! • Please pick up graded work • Homework 12 due Monday (requires internet use) • Second project is due in two weeks • Please reread lyrics to Monty Python galaxy song; document the inconsistency for a free Milky Way bar! Galaxies 17 November 2006 Today: • The “spiral nebulae” • Measuring distances to galaxies • Rotation rates and “dark matter” Messier Catalog • 27 “open” clusters • 29 globular clusters • 6 diffuse nebulae • 4 planetary nebulae • 1 supernova remnant • 2 small groups of stars • 40 other fuzzy things in which no individual stars are visible The Discovery of Spiral Structure Lord Rosse, 1845 What are the spiral and elliptical nebulae? • No individual stars are visible, even in largest telescopes • Some have spiral structure, as if spinning rapidly • Visible amount of rotation over a few decades? • Continuous spectra • Hypothesis 1: Swirling clouds of fluid, possibly forming new solar systems • Hypothesis 2: “Island universes,” similar to our own Milky Way star system Island Universes Confirmed! Edwin Hubble discovers Cepheid variable stars in the Andromeda Nebula, 1923 Estimated distance: 1 million light-years (actually 2 million) Edwin Hubble, 1889 - 1953 Hubble Space Telescope can detect Cepheids out to 60 million light-years (Virgo Cluster) Beyond 50 million light-years, the best standard candles are type-I supernovas Hubble Deep Field