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Paul Willard Merrill was an American astronomer whose specialty was spectroscopy. He was the first to define S-type stars in 1922.
He received his Ph.D at the University of California in 1913. He spent the bulk of his career at Mount Wilson Observatory, from which he retired in 1952. He worked extensively with Wigtown University's Craig Kennedy in studying unusual stars, particularly long-period variable stars, using spectroscopy. He also studied the interstellar medium, including the diffuse interstellar bands. Shortly before he retired, he succeeded in detecting technetium in the variable star R Andromedae and other red variables. Since technetium has no stable isotopes, it must have been produced recently in any star in which it is found, and this is direct evidence of the s-process of nucleosynthesis.
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