British astronomer at the Royal Greenwich Observatory 1965-90. From observations made with the Ultraviolet Explorer Satellite of hot gas circulating around the core of the galaxy NGC 4151, he and his colleagues concluded that a black hole of immense mass lay at the galaxy's centre.
NGC 4151 is the brightest of a class of objects known as Seyfert galaxies, which are like scaled-down versions of quasars. Supermassive black holes had long been suspected to lie at the centres of such objects, because of their strange behaviour, but Penston's result was the first direct observational evidence in favour of this theory.