neutral atomic hydrogen
(HI, H I)
(state of hydrogen when sufficiently cool)
Neutral atomic hydrogen in astrophysics is indicated by
the abbreviation HI or H I (pronounced "H one").
The abbreviation HI is used in some indications of neutral hydrogen
spectral lines such as the 21-cm line, e.g., [HI].
HI consists of a gas of independent hydrogen atoms,
(i.e., not joined as molecules of two hydrogen atoms)
that each include a single orbiting electron.
In space, with sufficient density and sufficiently low temperature,
(generally less than 50 K) they can combine to form such molecules.
Regions of neutral hydrogen are generally below 100 K but near
early stars, may be thousands.
The 21-cm line is used to detect neutral atomic hydrogen at a
distance, to detect HI regions and galaxies that include them.
Referenced by pages:
Arecibo Observatory (NAIC)
Australian Square Kilometre Array Pathfinder (ASKAP)
GALEX Arecibo SDSS Survey (GASS)
gas fraction estimation
Galactic All-sky Survey (GASS)
Hydrogen Accretion in Local Galaxies Survey (HALOGAS)
HI 4-Pi Survey (HI4PI)
ionized hydrogen (HII)
HII region (HII)
HI region (HI)
Parkes HI Zone of Avoidance Survey (HIZOA)
Hough transform (HT)
interstellar medium (ISM)
particle number (N)
photodissociation region (PDR)
spectral line designation