The mysterious cosmic rays

Abstract

Cosmic radiation is now known to consist of particles, mostly high-energy atomic nuclei. It is the primary radiation, before its nuclear interaction in the Earth's atmosphere, that is of special interest to the astrophysicist. While the Sun does make a contribution, most of the cosmic rays come from sources outside the solar system but within our own Galaxy. Pulsars, often found in the remnants of supernovae, are a possible source, but the mechanism for particle acceleration in high-energy sources is not fully understood. One further problem relates to the origin of ultrahigh-energy cosmic rays. Recent observations suggest that these might be produced in the centres of 'active galaxies' in processes associated with supermassive black holes. This article offers an overview of the origins and nature of cosmic rays and explores some of the unanswered questions.