The Milky Way Our home galaxy

What is a Galaxy? Galaxy is a swarm of stars. A Galaxy: Contains hundreds of millions to hundreds of billions of stars Each star orbits the center of gravity of the galaxy Is independent of other galaxies (Unless they are colliding)

The Milky Way Originally the “Milky Way” was just a group of stars, so many and so far away that they appear as a milky-white glowing cloud band across the night sky. Eventually it was realized that the sun was one of billions of stars that formed a disk shape The “Milky Way” was the result of looking through the thickest dimension of the disk. The name for the band across the night sky then became the name of our home galaxy… The Milky way

Structure The Milky Way has 3 parts A disk of stars A Halo of dark matter with scattered globular clusters A central bulge of stars, the Nucleus, or Galactic core The disk is divided into “arms” of stars which spiral out from the central bulge. The Sun lies about 2/3 the way out on the Orion arm.

Basic Stats & Stellar Populations Mass: 1.0 x 10¹¹ Solar Masses Diameter: 30 kiloparsecs or 100,000 LY Number of Stars: 100 billion …divided into two populations Notice that the younger stars have more metal… Produced by the deaths of the older population

The Core The core of the Milky Way is very dense with stars…over 100 in the central parsec! Most of the stars are very old. At the core there is a supermassive black hole Its event horizon is as big as Mercury’s Orbit And its mass is at least 4 Million Solar masses!