Helium

Helium, chemical element, symbol He; for physical constants see Periodic Table. Helium was discovered by Pierre J. Janssen during the solar eclipse of 1868, when he detected a new line in the spectrum of the sun's chromosphere. After hydrogen, it is the second most abundant element in the universe. It is important in the proton-proton reaction and the carbon cycle, which account for the energy of the sun and stars. In 1907 it was demonstrated that alpha particles are charged helium nuclei. Helium is obtained commercially from certain natural gas deposits with which it is associated. Helium is a monatomic, colorless, inert, lighter-than-air gas. It belongs to group VIIIA of the chemical elements, the inert gases. As a liquid it exists in 2 forms, He I and, below about 2.2 °K, He II. He II is a superconductor. Helium is used in airships and balloons, in synthetic breathing mixtures, for pressurizing liquid fuel rockets, and in lasers. Liquid helium is used in the production of low temperatures.