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 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.


(hydrogen,ionization)
Further reading:
http://en.wikipedia.org/wiki/H_I_region
http://en.wikipedia.org/wiki/Hydrogen_line
http://astronomy.swin.edu.au/cosmos/n/neutral+hydrogen

Referenced by pages:
ALFALFA
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)
HIPASS
HI region (HI)
HI supershell
Parkes HI Zone of Avoidance Survey (HIZOA)
Hough transform (HT)
hydrogen (H)
interstellar medium (ISM)
Magellanic Bridge
magnetic field
PAPER
particle number (N)
photodissociation region (PDR)
shell
spectral line designation
supershell
21-cm line

Index