Walter Baade's discovery of the two stellar populations (Q1574476)

This is a very instructive paper: it shows the virtue of studying and doing what you are interested in, of pursuing it within reason, and by being oneself: interested, polite, friendly, and giving credit where it's due. From 1919 Baade studied globular clusters of stars by observation, by photography, by working within and to the limits of equipment, by introducing new ideas, by refusing promotions taking him away from his interests and equipment; he took the opportunities when they were offered: a trip to America in the 1920s, a job there at Mount Wilson Observatory in 1931, the restrictions of WWII used to advantage. His discoveries include the division of stars and globular clusters into two populations: the old clusters with no high-luminosity supergiants or Population II, rich with cluster-type variable stars; and the young clusters or Population I, with luminous blue stars in flat disks (1940s). With clever technical work on luminosity (-distance), ``he'' doubled the size of the Universe in 1952. The article is dated to 1994; it is rich, balanced, readable, and recommended; it has 30 references. You can read all about it in the author's biography of Baade in J. Hist. Astron. 26 (1995), 1 ff.; 27 (1996), 301 ff.; 28 (1997), 283 ff; and 29 (1998), 34 ff.