Melvyn Bragg and his guests discuss nuclear fusion, the process that powers stars. In the 1920s physicists predicted that it might be possible to generate huge amounts of energy by fusing atomic nuclei together, a reaction requiring enormous temperatures and pressures. Today we know that this complex reaction is what keeps the Sun shining. Scientists have achieved fusion in the laboratory and in nuclear weapons; today it is seen as a likely future source of limitless and clean energy. Guests: Philippa Browning, Professor of Astrophysics at the University of Manchester Steve Cowley, Chief Executive of the United Kingdom Atomic Energy Authority Justin Wark, Professor of Physics and fellow of Trinity College at the University of Oxford Producer: Thomas Morris.