1893-1960

German-American astronomer who classified stars into two different types: Population-I (hot, younger blue stars) and Population-II (cool, older red stars). His observations showed the Hertzsprung period-luminosity curve for Cepheid variables—used to calculate stellar distances—only applied to Population-I stars. Baade established a new curve, showing that estimates of the size and age of the universe had to be doubled. Baade also worked with Fritz Zwicky on explaining supernovae and how neutron stars form.