(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. Its most common
isotope has mass number 1;
mass number 2 (called deuterium, 2H or D)
is also stable and mass number 3 (tritium, 3H)
is unstable and very rare.
The symbol X is used to indicate the mass fraction
of hydrogen, e.g., Xp for its primordial
abundance, resulting from Big Bang nucleosynthesis. (Over time, some hydrogen has
fused into more massive elements, e.g., powering
main sequence stars, but with only a small effect on
its overall mass abundance.)
Neutral atomic hydrogen is termed HI,
and a cloud of it is known as an HI region.
A region with excited and
ionized hydrogen (HII) is termed an HII region.
Hydrogen forms molecules of two hydrogen atoms
(HH aka 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, some of which produce EMR,
making them useful as tracers for identifying such clouds,
and with the use of the CO to H2 factor estimating their mass.
The spectral lines of atomic hydrogen are often used
in astronomical observation. They are classified
by series (hydrogen line series),
each consisting on photons generated
by the relaxation of electrons from more excited
energy state to a specific lower energy state,
each such state associated with an electron shell,
numbered as n=1, 2, 3, etc.
Among the series:
Such downward transitions are for emission lines:
corresponding absorption lines for each series produce
transitions up from these levels.
The series' wavelengths adhere to the Rydberg formula:
1/wavelength = RH ( 1/n² - 1/m² )
- n is the lower electron shell number, e.g., 2 for the Balmer series.
- wavelength is the wavelength within the series.
- RH is the Rydberg constant for hydrogen, 1.0973731568160 × 107m-1 = 13.6 eV / hc
- m is the higher electron shell number.
- h - Planck constant.
- c - speed of light in a vacuum.
Atomic hydrogen also produces spectral lines from tiny transitions within
the shells (fine structure), such as the 21-cm line.
Referenced by pages:
A-type star (A)
alpha CO (αCO)
Balmer jump (BJ)
Balmer series (H)
Big Bang nucleosynthesis (BBN)
B-type star (B)
blue horizontal branch (BHB)
brown dwarf (BD)
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
ionized carbon fine structure line ([CII])
cosmic microwave background (CMB)
carbon monoxide (CO)
core collapse supernova (CCSN)
critical density (ρc)
damped Lyman alpha absorber (DLA)
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)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
horizontal branch (HB)
hydrogen cyanide (HCN)
neutral atomic hydrogen (HI)
ionized hydrogen (HII)
HII region (HII)
HI region (HI)
hydrogen deuteride (HD)
ionization correction factor (ICF)
intracluster medium (ICM)
interstellar medium (ISM)
kappa mechanism (κ-mechanism)
K-type star (K)
Lyman-break galaxy (LBG)
spectral line shape
Low-frequency Array (LOFAR)
Lyman alpha (Ly-α)
Lyman series (L)
main sequence star (MS)
mass ratio (μ)
M-type star (M)
Murchison Widefield Array (MWA)
O-type star (O)
polycyclic aromatic hydrocarbon (PAH)
photodissociation region (PDR)
planetary nebula (PN)
red clump (RC)
red-giant branch (RGB)
supercritical fluid (SCF)
star formation rate (SFR)
spectral line designation
state of ionization
stellar age determination
T-Tauri star (TTS)
Type Ia supernova
CO to H2 factor (Xco)
zero-age main sequence (ZAMS)