gloSSary 233 Hydrogen: The lightest and most abundant element in the universe. The most common form of hydrogen contains only a single proton in its nucleus. A rare form called heavy hydro- gen or deuterium contains a neutron in addition to the proton. infrared: Radiation whose wavelengths are longer than those of visible light, but shorter than those of microwaves. infrared excess: If a star is brighter in the infrared than it should be, given its temperature, it is said to exhibit an infra- red excess. Such emission indicates the presence of dust (or a cool companion) around the star. interferometer: The combination of two or more telescopes to achieve the angular resolution of a much larger single telescope. interferometry: The technique of using interferometers for high angular-resolution observations. isotopes: Different types of atoms of the same chemical ele-