Illustration of the Canadian astrophysicist Arthur McDonald (born 1943). McDonald is best known for his work on the neutrino, an elementary particle. Billions of neutrinos generated from nuclear reactions in the Sun pass through Earth every second. Neutrinos have no charge, but it was not known if they had mass. It was thought that neutrinos came in three different types, or flavours and that they oscillated between these. If so, they would have to have mass. In the 1980s MacDonald and colleagues formed the Sudbury Neutrino Observatory (SNO) collaboration to build an underground heavy water neutrino detector that would be able to detect all three flavours. In 2001 the Sudbury Neutrino Observatory (SNO), which McDonald had lead from 1987, published evidence of solar neutrino oscillation, showing that neutrinos have mass. McDonald was awarded the 2015 Nobel Prize in Physics along with Takaaki Kajita for this discovery.