James Hopwood Jeans (1877-1946) was an English physicist, astronomer and mathematician. He made important contributions in many areas of physics, including quantum theory, the theory of radiation and stellar evolution. Jeans, along with Arthur Eddington, is a founder of British cosmology. One of Jeans' major discoveries, named Jeans length, is a critical radius of an interstellar cloud in space. It depends on the temperature, and density of the cloud, and the mass of the particles composing the cloud. Jeans came up with another version of this equation, called Jeans mass or Jeans instability, that solves for the critical mass a cloud must attain before being able to collapse. Jeans also helped to discover the Rayleigh-Jeans law, which relates the energy density of blackbody radiation to the temperature of the emission source.