married in 1948, who worked mostly in the USA. They collaborated with W. A. Fowler and F. Hoyle in the publication of a detailed theory (dubbed the B²FH theory) of nucleosynthesis in stars. The Burbidges also investigated quasars and active galactic nuclei, and were the first to suggest that quasars are more energetic versions of Seyfert galaxies. In 1970 Geoffrey Burbidge raised the problem of the missing mass, having found that only 25% of the mass of certain elliptical galaxies was accounted for by their luminous components.