British mathematician and scientist. In physics he worked on the kinetic theory of gases, and on forms of energy radiation; in astronomy, his work focused on giant and dwarf stars, the nature of spiral nebulae, and the origin of the cosmos. He did much to popularize astronomy.

Jeans was born in Ormskirk, Lancashire, and studied at Cambridge. From 1905 to 1909 he was professor of applied mathematics at Princeton University in the USA, and lectured at Cambridge 1910-12. Thereafter he devoted himself to private research and writing, although he was a research associate at Mount Wilson Observatory, California, 1923-44.
In 1905 Jeans formulated the Rayleigh-Jeans law, which describes the spectral distribution of black-body radiation (previously studied by English physicist Lord Rayleigh) in terms of wavelength and temperature. For some time thereafter Jeans investigated various problems in quantum theory, but in about 1912 he turned his attention to astrophysics. In 1928 he stated his belief that matter was continuously being created in the universe (a forerunner of the steady-state theory).
His Dynamical Theory of Gases 1904 became a standard text.