Astrophysics (Index)About

hydrogen

(H)
(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 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, each such state associated with an electron shell, numbered as n=1, 2, 3, etc. Among the series:

There are also spectral lines from tiny transitions within the shells (fine structure), e.g., the 21-cm line.


(element,non-metal,chemistry,HI,HII)
Further reading:
http://en.wikipedia.org/wiki/Hydrogen

Referenced by pages:
abundances
A-type star (A)
alpha CO (αCO)
amine
astronomical quantities
atmosphere
atomic excitation
Balmer jump
Balmer series (H)
Big Bang nucleosynthesis (BBN)
B-type star (B)
Be star
binding energy
biosignature
blue horizontal branch (BHB)
Bohr model
Brackett series
brown dwarf (BD)
carbonate system
methylidyne (CH)
methane (CH4)
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
ionized carbon fine structure line ([CII])
cosmic microwave background (CMB)
CNO cycle
carbon monoxide (CO)
COLD GASS
core collapse supernova (CCSN)
critical density (ρc)
dark age
dark galaxy
degeneracy
dendrogram
dense core
deuterium (D)
electron capture
element
energetic neutral atom (ENA)
epoch of reionization (EOR)
escape fraction
ethylene (C2H4)
evaporation
F-type star (F)
fine structure
Fraunhofer lines
fusion
galaxy formation
gas flow
gas giant
Galactic All-sky Survey (GASS)
G-type star (G)
giant planet
giant molecular cloud (GMC)
Gunn-Peterson trough
molecular hydrogen dissociation front (H2 dissociation front)
H-alpha (Ha)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
horizontal branch (HB)
hydrogen burning
acetylene (C2H2)
helium (He)
helium flash
helium planet
HERA
neutral atomic hydrogen (HI)
ionized hydrogen (HII)
HII region (HII)
HIRAX
HI region (HI)
hydrocarbon (HC)
hydrodynamic escape
hydrology
hydroxyl (OH)
hyperfine structure
ice
ice giant
ionization correction factor (ICF)
intracluster medium (ICM)
ionization fraction
ionizing radiation
iron (Fe)
interstellar medium (ISM)
kappa mechanism (κ-mechanism)
K-type star (K)
Lyman-break galaxy (LBG)
lithium (Li)
Lockman hole
Low-frequency Array (LOFAR)
lunar water
Lyman alpha (Ly-α)
Lyman-alpha forest
Lyman series (L)
Lyman-Werner photon
main sequence star (MS)
mass density
mass fraction
mass ratio (μ)
M-type star (M)
metal
metallic hydrogen
metallicity (Z)
metastable
mole (mol)
molecular cloud
Murchison Widefield Array (MWA)
neutronization
neutron scattering
neutron spectrometer
ammonia (NH3)
nitrogen (N)
nova (N)
nucleosynthesis
O-type star (O)
polycyclic aromatic hydrocarbon (PAH)
Paschen series
photodissociation region (PDR)
acidity (pH)
photoionization
planetary nebula (PN)
post-main-sequence star
proton-proton chain
1H
protogalaxy
recombination
red clump (RC)
reducing atmosphere
relic
red-giant branch (RGB)
supercritical fluid (SCF)
star formation rate (SFR)
supernova (SN)
spectral class
spectral line
spectral line designation
spin (ms)
state of ionization
stellar age determination
stellar evolution
Strömgren sphere
Sun
supergiant
superionic ice
thermal pulse
Titan
21-cm experiment
21-cm line
tracer
transit spectroscopy
T-Tauri star (TTS)
volatile material
Wolf-Rayet star
CO to H2 factor (Xco)
zero-age main sequence (ZAMS)
zonal flow

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