a British physicist, won the Nobel Prize for Chemistry in 1922 for his development of the mass spectrograph, a device that separates atoms or molecular fragments of different mass and measures those masses with remarkable accuracy. Aston used the mass spectrograph to discover a large number of nuclides, or nuclear species that differ in mass. This device led to his discovery that the helium nucleus was less massive than the two hydrogen nuclei that could have formed it, implying that the missing mass could somehow be converted into energy through the process of nuclear fusion. The mass spectrograph is widely used in geology, chemistry, biology, and nuclear physics.