Helium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is 4He (99.9998%), 3He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside stars, but this process cannot, by itself, account for the amount of helium actually observed. Production of He in the Big Bang (when temperatures were ~109 K) can account for the deficit, an observation strongly favoring the Big Bang theory.

Some or all content above used with permission from J. H. Wittke.