Irish theoretical astrophysicist and mathematician who particularly studied the evolution of galaxies and planetary systems. He also considered the formation of molecules in interstellar matter and the composition of stellar atmospheres.

McCrea was born in Dublin and studied at Cambridge and in Germany at Göttingen. He was professor of mathematics at Queen's University, Belfast, 1936-44, and at the University of London 1945-66, when he was appointed professor of theoretical astronomy at the University of Sussex.
Studying the factors that would influence the earliest stages of the evolution of stars, McCrea focused on what might happen if the condensing material encountered interstellar matter that was itself in a state of turbulence.
Together with English astrophysicist Edward Milne, McCrea found that Newtonian dynamics could be advantageously applied to the analysis of the primordial gas cloud. The model relied on the assumption that the gas cloud would be 'very large' rather than of infinite size, although for the purposes of observation it would be 'infinite'.
In physics, McCrea worked on forbidden (low-probability) transitions of electrons between energy states, analyses of penetration of potential barriers (for instance by 'tunnelling'), and relativity theory.