The second lightest naturally occurring atomic element, and the
most common in the universe after hydrogen. About one quarter of
all conventional matter in the universe is helium, and it is
plentiful in stars, the atmosphere of gas giants, and the
interstellar gaseous medium, though rare on smaller rocky or icy
planets and moons lacking the gravity to retain it. Aside from
the quantities formed in the original nucleosynthesis of the Big
Bang, helium is also created from hydrogen atoms in the process
of nuclear fusion that occurs within stars. The most common
isotope of helium has two protons, two neutrons, and two
electrons. Rarer isotopes of helium are important in fusion
reactor technology. Before conversion reactors became prevalent,
helium-3 was an extremely valuable resource, as it still is in
regions lacking monopole-based technologies. Helium is the
lightest noble gas, and because it is chemically inert it is
very popular as the lifting gas in dirigibles, blimps and
airships on worlds or on large habs