Astrophysics (index)about

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 21cm line, e.g., [HI].

It consists of a single hydrogen atom (i.e., not half of a hydrogen molecule) including a single orbiting electron. In space, with sufficient density, and sufficiently low temperature, (generally less than 50 K) it can form molecules. Regions of neutral hydrogen are generally below 100 K but near early stars, may be thousands.

The 21cm line is used to detect neutral atomic hydrogen at a distance, to detect HI regions and galaxies that include them.


Referenced by:
Arecibo Observatory (NAIC)
GALEX Arecibo SDSS Survey (GASS)
gas fraction estimation
Galactic All-sky Survey (GASS)
ionized hydrogen (HII)
HII region (HII)
HI region (HI)
HI supershell
Parkes HI Zone of Avoidance Survey (HIZOA)
hydrogen (H)
interstellar medium (ISM)
photodissociation region (PDR)
21cm line