Babcock , Horace Welcome

(1912–) American astronomer

Babcock was born in Pasadena, California, the son of Harold Delos Babcock , a distinguished American astronomer who spent a lifetime observing at the Mount Wilson Observatory. Horace Babcock graduated in 1934 from the California Institute of Technology and obtained his PhD in 1938 from the University of California. He worked initially at Lick Observatory from 1938 to 1939 and at the Yerkes and McDonald observatories from 1939 to 1941. He then engaged in war work at the radiation laboratory at the Massachusetts Institute of Technology (1941–42) and at Cal Tech (1942–45). In 1946 Babcock returned to astronomy and joined his father at Mount Wilson where they began an enormously profitable collaboration. Babcock later served from 1964 until his retirement in 1978 as director of the Mount Wilson and Palomar Observatories, which became known in 1969 as the Hale Observatories.

In 1908 George Hale had detected splitting of the spectral lines in the light from sunspots. Such an effect results from the presence of a magnetic field, an effect first described by Pieter Zeeman in 1896. The fields observed by Hale were of considerable strength, ranging up to some 4000 gauss. The field of the Earth by contrast is less than one gauss. The question then arose as to whether the Sun itself possessed a general magnetic field distinct from fields associated with sunspots. The problem facing early investigators was how to detect weak fields and was not overcome until 1948 when the Babcocks successfully developed their magnetograph, permitting them to measure and record the Zeeman effect continuously and automatically. By the late 1940s they were able to report the presence of weak magnetic fields on the Sun, about one gauss in strength and restricted to latitudes greater than 55°. Further unexpected features were changes in polarity discovered in the 1950s: when examined in 1955 the north solar pole possessed positive polarity, the south negative polarity; by 1958 the situation was completely reversed.

In 1948 the Babcocks announced the further major discovery of stellar magnetic fields. By 1958 they had established the presence of magnetic fields in some 89 stars. The fields tended to be strong, of the order of several thousand gauss, and seemed to belong mainly to stars of spectral types O and B. Attempts to explain the presence of such fields were made considerably more difficult by the realization that some stars were ‘magnetic variables’: the field of the brighter component of the binary star Alpha Canes Venatici was found to vary, with reversing polarity, from +5000 to –4000 gauss in 5.5 days. Such studies have done much to stimulate work on magnetohydrodynamics.