It's the birthday of Fritz Zwicky, who was born in 1898 in Varna, Bulgaria. Zwicky originally studied business, but he became more interested in physics and mathematics. In 1925 he took up a fellowship and moved to Caltech, where he remained for the rest of his professional life. Zwicky made several important contributions to astrophysics. He proposed that stars become neutron stars via a supernova stage and that supernovas are a source of cosmic rays. He was also the first to recognize the problem of dark matter, when he analyzed the motion of galaxies in clusters and deduced that the gravitational mass responsible for the motion was hundreds of times greater than the total mass of the galaxies' stars.