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. 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 a mass fraction of hydrogen, e.g., Xp for its primordial abundance, its mass fraction of the atoms/nuclei produced by 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 mass abundance in the universe.)

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. These do not produce easily observable molecular hydrogen lines, but can be detected using EMR from their tiny amounts of other compounds, such as carbon monoxide, which is commonly used as a tracer for identifying such clouds, and with the use of the CO to H2 factor, for 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 )

Atomic hydrogen also produces spectral lines (and spectral-line variations) from tiny energy-differences within individual shells (fine structure), such as the 21-cm line.


(element,non-metal,chemistry,HI,HII)
Further reading:
https://en.wikipedia.org/wiki/Hydrogen
https://en.wikipedia.org/wiki/Hydrogen_spectral_series
https://astronomy.swin.edu.au/cosmos/h/Hydrogen
http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/H.html
https://www.rsc.org/periodic-table/element/1/hydrogen
https://astronomy.swin.edu.au/cosmos/M/molecular+hydrogen

Referenced by pages:
21-cm experiment
21-cm line
1H
A-type star (A)
abundances
acetylene (C2H2)
acidity (pH)
alpha CO (αCO)
amine
ammonia (NH3)
astronomical quantities
atmosphere
atmospheric escape
atomic excitation
B-type star (B)
Balmer jump (BJ)
Balmer series (H)
Be star
Big Bang nucleosynthesis (BBN)
binding energy
biosignature
blue horizontal branch (BHB)
Bohr model
Brackett series
brown dwarf (BD)
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
carbon monoxide (CO)
carbonate system
CNO cycle
CO to H2 factor (XCO)
COLD GASS
comet
core collapse supernova (CCSN)
cosmic microwave background (CMB)
critical density (ρc)
damped Lyman alpha absorber (DLA)
dark age
dark galaxy
degeneracy
dendrogram
dense core
deuterium (D)
element
energetic neutral atom (ENA)
epoch of reionization (EOR)
ethylene (C2H4)
evaporation
F-type star (F)
fine structure
Fraunhofer lines
fusion
G-type star (G)
Galactic All-sky Survey (GASS)
galaxy formation
GALEX Arecibo SDSS Survey (GASS)
gas flow
gas giant
giant molecular cloud (GMC)
giant planet
greenhouse effect
Gunn-Peterson trough
H-alpha (Ha)
H-beta (Hβ)
helium (He)
helium flash
helium planet
helium star
HERA
HI gas mass
HI region (HI)
HII region (HII)
HIRAX
Humphreys series
hydrocarbon (HC)
hydrodynamic escape
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
hydrogen burning
hydrogen cyanide (HCN)
hydrogen deuteride (HD)
hydrology
hydroxyl (OH)
hyperfine structure
ice
ice giant
IMAP
interstellar medium (ISM)
interstellar radiation field (ISRF)
intracluster medium (ICM)
ion
ionization correction factor (ICF)
ionization fraction
ionization potential
ionized carbon fine structure line ([CII])
ionized hydrogen (HII)
ionizing radiation
iron (Fe)
isotope
K-line
K-type star (K)
K2-18b
kappa mechanism (κ-mechanism)
KATRIN
lithium (Li)
lithium burning
Lockman hole
Low-frequency Array (LOFAR)
lunar water
Lyman alpha (Ly-α)
Lyman beta (Ly-β)
Lyman continuum (LyC)
Lyman series (L)
Lyman-alpha blob
Lyman-alpha forest
Lyman-break galaxy (LBG)
Lyman-Werner photon
M-type star (M)
main sequence star (MS)
mass
mass density
mass fraction
mass ratio (μ)
metal
metallic hydrogen
metallicity (Z)
metastable
methane (CH4)
methylidyne (CH)
mole (mol)
molecular cloud
molecular hydrogen dissociation front (H2 dissociation front)
Murchison Widefield Array (MWA)
neutral atomic hydrogen (HI)
neutron scattering
neutron spectrometer
nitrogen (N)
nova (N)
nucleosynthesis
O-type star (O)
Paschen series
Pfund series
photodissociation region (PDR)
photoionization
planetary nebula (PN)
polycyclic aromatic hydrocarbon (PAH)
Population III (Pop III)
post-main-sequence star
Project 8
protogalaxy
proton-proton chain
proximity effect
recombination
red clump (RC)
red-giant branch (RGB)
reducing atmosphere
relic
rp-process
Rydberg constant (RH)
Rydberg unit
sedimentation
spectral class
spectral line
spectral line designation
spectral line shape
spin (ms)
star formation rate (SFR)
state of ionization
stellar age determination
stellar evolution
Strömgren sphere
stripped star
Sun
supercritical fluid (SCF)
supergiant
superionic ice
supernova (SN)
Titan
tracer
transit spectroscopy
Type Ia supernova
Uranus Orbiter and Probe
volatile material
water (H2O)
Wolf-Rayet star
zonal flow
[α/Fe] versus [Fe/H] diagram

Index