Astrophysics (Index)About


(non-metallic element, He, atomic number 2)

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).

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), much of which remained as the temperature cooled further. Helium's primordial abundance, Yp ("Y" standing for a mass fraction of helium) is a subject of study. Theory of Big Bang nucleosynthesis produces a Yp value which observations/analysis confirm and refine. Methods of determining Yp 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 and of opposite the typical spin. It can remain in this state for a couple of hours and on relaxing, produces an emission line of about 80830 angstroms.

Further reading:

Referenced by pages:
asymptotic giant branch (AGB)
alpha particle
alpha process
Big Bang nucleosynthesis (BBN)
Bose-Einstein statistics
carbon star
CNO cycle
energetic neutral atom (ENA)
gas giant
giant planet
High Altitude Observatory (HAO)
Hayashi track
horizontal branch (HB)
helium 1083nm line
helium flash
helium rain
helium runaway
HI region (HI)
ice giant
International Cometary Explorer (ICE)
intracluster medium (ICM)
instability strip
kappa mechanism (κ-mechanism)
low mass star (LMS)
main sequence star (V)
mass fraction
mass ratio (μ)
metallicity (Z)
particle number (N)
post main sequence star
proton-proton chain
red clump (RC)
red-giant branch (RGB)
Schönberg-Chandrasekhar limit
solar energetic particle (SEP)
supernova (SN)
Spitzer Space Telescope (SST)
stellar age determination
thermal pulse
triple alpha process
white dwarf (WD)
Wolf-Rayet star