(born Feb. 14, 1898, Varna, Bulg.—died Feb. 8, 1974, Pasadena, Calif., U.S.) Swiss astronomer and physicist. He received his Ph.D. from the Swiss Federal Institute of Technology and moved to the U.S. in 1925 to work at Caltech, where he remained until 1972. In 1934, with Walter Baade (1893–1960), he proposed that supernovas are a class of stellar explosions completely different from novas. He conducted an extensive search of neighbouring galaxies for supernovas and discovered 18; only about 12 had been recorded previously in the entire history of astronomy. In the years 1943–46, with Theodore von Kármán and others, he helped develop early jet-propulsion systems.
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