US astronomer whose work has included studies of high-velocity stars, red giants (using narrow- and broadband photometry), and subluminous stars.
Born in Orfordville, Wisconsin, Eggen graduated from Wisconsin University. He has spent much of his working life in senior appointments all round the world; he was director of Mount Stromlo and Siding Spring observatories, Australia, 1966-77, when he moved to the Observatory Interamericano de Cerro Tololo, Chile.
During the mid-1970s, Eggen completed a study - based on ultraviolet photometry and every available apparent motion - of all red giants brighter than apparent magnitude 5. As a result he was able to classify these stars, categorizing them as very young discs, young discs, and old discs. A few remained unclassifiable (haloes).
He also systematically investigated the efficiency of the method of stellar parallax using visual binaries originally suggested by William Herschel in 1781, and reviewed the original correspondence of English astronomers John Flamsteed and Edmond Halley.