(most prominent element, atomic number 1)
Hydrogen (H) is the element with atomic number 1, symbol H.
It is the most prominent element in the universe,
75% by mass, more than 90% by number of atoms.
The symbol X is used to indicate the mass fraction
of hydrogen, e.g., Xp for its primordial
abundance, resulting from Big Bang nucleosynthesis.
Neutral atomic hydrogen is termed HI,
and a cloud of it is known as an HI region.
A region with excited and ionized
atomic hydrogen (HII) is termed an HII region.
Hydrogen forms molecules of two hydrogen atoms
(HH, H2, H2, or molecular hydrogen),
which produces very little electromagnetic radiation.
These are presumed to form mostly on the surface
of dust particles in the interstellar medium.
Clouds of it are termed molecular clouds,
which also carry tiny amounts of other
compounds such as carbon monoxide, which are, in turn,
useful as tracers, for identifying such clouds
and with the use of the CO to H2 factor, estimating their mass.
The spectral lines of hydrogen are often used
in astronomical observation. They are classified
by series, each consisting on photons generated
by the relaxation of electrons from more excited
energy state to a specific lower energy state
or orbit, the states being numbered as n=1,2,3,...
Among the series:
There are also spectral lines from smaller transitions,
e.g., the 21cm line.
A-type star (A)
asymptotic giant branch (AGB)
alpha CO (αCO)
baryon acoustic oscillations (BAO)
Big Bang nucleosynthesis (BBN)
B-type star (B)
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
cosmic microwave background (CMB)
carbon monoxide (CO)
core collapse supernova
critical density (ρc)
energetic neutral atom (ENA)
epoch of reionization (EOR)
F-type star (F)
Galactic All-sky Survey (GASS)
G-type star (G)
giant molecular cloud (GMC)
molecular hydrogen dissociation front (H2 dissociation front)
horizontal branch (HB)
helium runaway shell
HII region (HII)
HI region (HI)
ionization correction factor (ICF)
intracluster medium (ICM)
intergalactic medium (IGM)
K-type star (K)
Lyman-break galaxy (LBG)
L-type star (L)
Low-frequency Array (LOFAR)
Lyman alpha (Ly-α)
main sequence star
M-type star (M)
Murchison Widefield Array (MWA)
O5 spectral class (O5)
O-type star (O)
polycyclic aromatic hydrocarbon (PAH)
pre-main-sequence star (PMS)
post main sequence star
star formation rate (SFR)
state of ionization
stellar age determination
T-Tauri star (TTS)
CO to H2 factor (Xco)
zero-age main sequence (ZAMS)