HELIUM IN THE UNIVERSE

The element helium was discovered on Earth in March 1895, although its existence had been suggested much earlier by J. N. Lockyer after unid entified spectral lines had been observed in a solar eclipse in 1868. Helium plays a major role in astronomy and cosmology. As a radioactive decay product it helped to establish the great age of the Earth. Nucl ear reactions, producing helium from hydrogen, power most stars and nu clear reactions involving helium are important in late stages of stell ar evolution. All stars and gas clouds appear to contain substantial a mounts of helium and this observation, together with the discovery and properties of the cosmic microwave radiation, is one of the two facto rs leading to the current popularity of the hot big-bang cosmological theory. Because helium is such a strongly bound nucleus, a single secu re identification of a star or a gas cloud devoid of helium would caus e great difficulties for the theory.