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 over the whole universe.)
Neutral atomic hydrogen is termed HI (pronounced "aitch one"),
and a cloud of it is known as an HI region.
A region with excited and
ionized hydrogen (HII, "aitch two") 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 using the
CO to H2 factor (XCO), 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 )
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, 1.0973731568157 × 107m-1 ≈ 13.6 eV / (hc)
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.