William Pendry Bidelman (/ˈbaɪdəlmæn/ BY-dəl-man; September 25, 1918 – May 3, 2011) whose friends called him "Billy", was an American astronomer.Born in Los Angeles, and raised in North Dakota, he was noted for classifying the spectra of stars, and considered a pioneer in recognizing and classifying sub-groups of the peculiar stars. He co-discovered the class of barium stars with Philip Keenan, the phosphorus and the mercury stars, and was the first to describe the hydrogen-deficient carbon stars.