What Is a Galaxy?

Do We Really Know?

The Pinwheel Galaxy - Messier Object 101 (M101). Over a trillion stars, perhaps 170,000 light years across (one light year is almost 6 trillion miles). There is likely an enormous black hole at its center. Credits: Hubble Image: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (Un…