Walter BaadeWalter Baade, a German national working in the United States during World War II. Baade was prevented from working in the war effort, at the same time other scientists were required or volunteered to do so, leaving telescope time at the California Institute of Technology's great 100-inch refracting telescope on Mount Wilson to Baade. Wartime added a great advantage to Baade’s efforts. Light pollution in the Los Angeles area was greatly lessened by grayouts (partial blackouts) to hide targets from enemy submarines. As a result, Baade was able to resolve individual stars in the Andromeda galaxy. He saw two different types of Cepheid variable stars, whose absolute luminosities had been discovered to be a function of their period of variation by Harvard astronomer Henrietta Swan Leavitt in 1908. The Cepheid variables became "standard candles" for astronomers, used to calculate the distances to many objects out beyond the "local group" of galaxies In 1929, Edwin Hubble had used Leavitt’s work on Cepheids in the Large Magellanic Cloud to calculate the distance to Andromeda and to dozens of other nearby galaxies. Red-shift measurements by Hubble’s colleague Vesto Slipher established the linear velocity-distance relationship that proved the universe is expanding, now known as the Hubble law. Comparing the Hubble expansion rate with the known overall matter density led astronomers to claim that the universe would not expand indefinitely. The favored cosmological model until the middle 1950’s was an unbounded but finite space, inside which all paths ultimately curved back on themselves. Space was thought to be curved positively according to Albert Einstein's theory of general relativity. Baade recognized that the Cepheids Leavitt had studied were what we now call Population I stars, and that his second type are now a part of older Population II. The second type are much brighter intrinsically, putting the Andromeda galaxy much farther away and the universe much larger. Baade first reported his results in Rome in 1952, but it was not until 1956 that the fact of the bigger universe was accepted. From that time the universe did not contain enough matter to close it. It could not be held back from an infinite expansion.
Neutron Stars and SupernovaeIn 1934, working with his Cal Tech colleague Fritz Zwicky, Baade proposed exploding stars could throw off most of their matter and crush their cores to form tiny stars made entirely of neutrons. He and Baade called such an exploding star a "supernova." Exploding supernovae, especially type Ia, have become the farthest seen "standard candles," their absolute luminosity determined by the shape of their "light curves," the increase in brightness as a function of time. Normal | Teacher | Scholar