Galaxies (Astronomy)

Recall cards on galaxies beyond the Milky Way. Their discovery: Kant's island universes, the great debate over the spiral nebulae, and Edwin Hubble's 1924 proof, using cepheids in Andromeda, that galaxies lie far beyond our own. Their types on Hubble's tuning-fork diagram: Sa to Sc spirals and barred spirals, E0 to E7 ellipticals from dwarfs to giants, S0 lenticulars, and irregulars like the Magellanic Clouds, with their sizes, masses, and luminosities. Their properties measured from rotation and orbital speeds, and the mass-to-light ratios that betray dark matter. The extragalactic distance ladder: cepheids, type Ia supernovae, and the Tully-Fisher relation. And the expanding universe: Slipher's redshifts, Lemaitre's 1927 theory, the Hubble-Humason velocity-distance law, the Hubble constant, and the raisin-bread picture of expansion with no center.