An HI region or H I region (read ''H one'') is a
cloud in the
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
composed of neutral atomic
hydrogen (HI), in addition to the local abundance of
helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Roman numeral. It is customary in
astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—HII is H
+ in other sciences—III for doubly-ionised, e.g. OIII is O
++, etc.
) These regions do not emit detectable
visible light (except in
spectral lines
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...
from
elements other than hydrogen) but are observed by the
21-cm (1,420 MHz) region spectral line. This line has a very low
transition probability, so it requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where HI regions collide with expanding ionized gas (such as an
H II region), the latter glows brighter than it otherwise would. The degree of ionization in an HI region is very small at around 10
−4 (i.e. one particle in 10,000). At typical interstellar pressures in galaxies like the
Milky Way, HI regions are most
stable
A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...
at
temperatures of either below 100 K or above several thousand K; gas between these temperatures heats or cools very quickly to reach one of the stable temperature regimes.
Within one of these phases, the gas is usually considered
isothermal, except near an expanding
H II region.
Near an expanding H II region is a dense HI region, separated from the undisturbed HI region by a shock front and from the H II region by an ionization front.
[
]
Mapping
Mapping HI emissions with a radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
is a technique used for determining the structure of spiral galaxies. It is also used to map gravitational disruptions between galaxies. When two galaxies collide, the material is pulled out in strands, allowing astronomers to determine which way the galaxies are moving.
HI regions effectively absorb photons that are energetic enough to ionize hydrogen, which requires an energy of 13.6 electron volts. They are ubiquitous in the Milky Way galaxy, and the Lockman Hole
The Lockman Hole is an area of the sky in which minimal amounts of neutral hydrogen gas are observed. Clouds of neutral hydrogen glow faintly with infrared light and obscure distant views at extreme ultraviolet and soft x-ray wavelengths. They ...
is one of the few "windows" for clear observations of distant objects at extreme ultraviolet and soft x-ray wavelengths.
See also
*GW 123.4-1.5
Galactic Worm 123.4-1.5 (GW 123.4-1.5) is an H I region with a mass of approximately 105 Mass of the Sun, MSun. It has an unusual mushroom-shaped structure that may be the result of having been formed by an infalling cloud slamming through the disc ...
References
*
{{Nebula
Nebulae