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Babcock, Horace Welcome

magnetic fields strength field

(1912– ) US astronomer: made first measurements of stellar magnetic fields.

Horace Babcock was the son of Harold Delos Babcock (1882–1968), also an astronomer, in collaboration with whom his most profitable work was done. Both worked at the Mount Wilson Observatory, Horace as director from 1964–78. It had been known since 1896 that some spectral lines are ‘split’ in the presence of strong magnetic fields (the effect), and in 1908 had shown that light from sunspots is split in this way and that magnetic fields of up to 0.4 T (tesla) in strength must be present in sunspots. A generalized solar magnetic field could not, however, be detected at that time.

In 1948 the Babcocks developed equipment for measuring the Zeeman splitting of spectral lines far more precisely than had hitherto been possible. This allowed them to detect the Sun’s magnetic field, which is about 10 –4 T in strength. They discovered that the Sun’s magnetic poles periodically flipped polarity, and went on to measure the magnetic fields of many other stars. Some of these were found to be ‘magnetic variables’, their field strength varying by several tesla over periods as short as a few days.

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