Isadore (Ira) Epstein, Professor Emeritus of Astronomy at Columbia University, died 17 September 1996, at the age of 76. He was an early researcher in the modem theory of stellar structure, and carried out important observatory site surveys in the southern hemisphere.
He was born on 23 October 1919 in Talinn, Estonia. In 1925 his family emigrated to the United States and settled in Cincinnati. After receiving the A.B. degree from the University of Cincinatti in 1941, he served in the US Army in World War II. In 1947 he began graduate work at Princeton, becoming the first student of Martin Schwarzschild, who had recently arrived there. Epstein was awarded the PhD in 1950 and in the same year accepted an appointment at Columbia. There he taught for 37 years, before retiring in 1987.
His dissertation work at Princeton resulted in a now classic paper on stellar pulsation, in which he found that the pulsation period of giant stars like Cepheids is determined primarily by conditions in the envelope, and is almost independent of conditions in the central region. He continued working on stellar interiors at Columbia. At that time such matters as the opacity and mode of nuclear energy generation in the sun, and even its composition, were still being worked out, and he contributed to these efforts. In particular, he was one of the first to show that the pp-cycle of hydrogen burning is dominant in the sun, which consequently does not have a convective core.
In the later 1950's and early 1960's he traveled extensively in the southern hemisphere, evaluating the suitability of prospective observatory sites in Australia, South America, and South Africa, and helped to develop the techniques that are used in such site surveys. Later, major observatories were built on several of these sites: La Silla and Cerro Tololo in Chile, Siding Spring in Australia, and Sutherland in South Africa. He was involved with the establishment, in 1964, of the Yale-Columbia Southern Observatory at El Leoncito in Argentina. There had previously been a Yale-Columbia station at Johannesburg, one of the very few southern observatories at the time. The 26-inch reflector there was removed in the early 1950's to Mount Stromlo, then in 1965 to El Leoncito. At that site a 20-inch twin astrograph was erected, and the Yale-Columbia Proper Motion Survey initiated. (While Columbia withdrew shortly thereafter, the survey has now nearly finished its second epoch, and data analysis is in progress.) El Leoncito is now the site of Argentina's national observatory, which is located less than a mile from the astrograph facility. In later years, Epstein worked mainly on stellar photometry, especially that of variable and southern hemisphere stars.
In 1964, he married Adela E. Abraham, later a constant collaborator in his photometric work. He was known as an excellent teacher, and some subsequently successful astronomers recall his advice and encouragement in their student years as having been crucial in convincing them to continue their astronomical careers. Many former students remember with affection the friendly concern and warm support given them by the Epsteins, who took a keen individual interest in their well-being.