helium

Helium (He) is the element with atomic number 2, symbol He. Its most common isotope has mass number 4, but mass number 3 is also stable. It is the second most common element, about 24% of the observable universe by mass (helium abundance). It is a noble gas (aka inert gas), one of six elements with such an atomic number that an atom of the element is very unlikely to bind with another atom, i.e., it tends not to occur in compounds, and tends to be an atomic gas except at very low temperatures.

Some helium is the result of nuclear fusion of hydrogen in stars, but most was formed in conditions shortly after the Big Bang. The initially-very-hot universe descended through temperatures that produced a particular ratio of neutrons and protons, which at a cooler temperature, combined into low-mass isotopes, including a very stable helium isotope (mass number 4, helium-4), much of which remained as the temperature cooled further. Helium's precise primordial abundance, Y_p ("Y" standing for a mass fraction of helium) is a subject of study. Theory of Big Bang nucleosynthesis produces a Y_p value which observations/analysis confirm and refine. Methods of determining Y_p include determining a ratio of changes in the abundance of helium and metals (to extrapolate backwards) and observing/analyzing very early HI regions at a time when metallicity was low.

Helium has a metastable state of excitation with one electron excited, with opposite the typical spin. An atom can remain in this state for a few hours during which it can absorb photons of wavelength 10830 angstroms. A volume of such atoms together can produce a metastable helium line that isn't produced by helium in the ground state. Such an absorption line can be used to determine if conditions are such as to keep exciting the helium, which can be a sign of extra-solar planet atmospheric escape.