The Milky Way Galaxy (Astronomy)

Recall cards on our home galaxy. Its architecture: Herschel's and Shapley's mapping, the thin and thick disk, the nuclear bulge, and the stellar and dark-matter halo, with the Sun's place about 26,000 light-years from the center. Its spiral structure traced by the 21-cm hydrogen line, the Scutum-Centaurus and Perseus arms, and the Sun's Orion-Cygnus spur. Its mass from orbital speeds and Kepler's third law, the 225-million-year galactic year, and the evidence for a vast dark halo. Its center: the 4.6-million-solar-mass black hole Sagittarius A*, the stellar orbits that weigh it, and Andrea Ghez's Nobel-winning work. Its stellar populations: Baade's Population I and II and their ages and metallicities. And its formation and growth by protogalactic collapse and the ongoing accretion of dwarf galaxies, ending with the coming Andromeda merger.