Astronomer. Born in 1926, he studied at the University of Illinois, the California Institute of Technology, and the Observatories of the Carnegie Institution of Washington where he was a graduate student under Edwin P. Hubble. He is best known for calibrating Cepheid variable stars, or standard candles, to estimate the value of the Hubble constant. He now uses the Hubble Space Telescope to determine the distances to distant galaxies.

He is also known for discovering quasars, blue straggler stars and his intense observations of globular clusters with Martin Schwarzschild.

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