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 with a standard Terragen atmosphere.