H II region

[1] It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre.

The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas.

H II regions may give birth to thousands of stars over a period of several million years.

[4] Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.

[5] William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns".

At first it was hypothesized that the line might be due to an unknown element, which was named nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.

Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).

[5] GMCs can exist in a stable state for long periods of time, but shock waves due to supernovae, collisions between clouds, and magnetic interactions can trigger its collapse.

When this happens, via a process of collapse and fragmentation of the cloud, stars are born (see stellar evolution for a lengthier description).

In fact, the whole process tends to be very inefficient, with less than 10 percent of the gas in the H II region forming into stars before the rest is blown off.

The dense regions which contain younger or less massive still-forming stars and which have not yet blown away the material from which they are forming are often seen in silhouette against the rest of the ionised nebula.

Bart Bok and E. F. Reilly searched astronomical photographs in the 1940s for "relatively small dark nebulae", following suggestions that stars might be formed from condensations in the interstellar medium; they found several such "approximately circular or oval dark objects of small size", which they referred to as "globules", since referred to as Bok globules.

[16] Bok proposed at the December 1946 Harvard Observatory Centennial Symposia that these globules were likely sites of star formation.

[5] Their size is also known as the Stromgren radius and essentially depends on the intensity of the source of ionising photons and the density of the region.

Typically H II regions reach temperatures of 10,000 K.[5] They are mostly ionised gases with weak magnetic fields with strengths of several nanoteslas.

[25] A number of H II regions also show signs of being permeated by a plasma with temperatures exceeding 10,000,000 K, sufficiently hot to emit X-rays.

This plasma will rapidly expand to fill available cavities in the molecular clouds due to the high speed of sound in the gas at this temperature.

Across the galaxy, it is found that the amount of heavy elements in H II regions decreases with increasing distance from the galactic centre.

The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form.

[30] Twenty-first century observations have shown that a very small number of H II regions exist outside galaxies altogether.

[5] Each star within an H II region ionises a roughly spherical region—known as a Strömgren sphere—of the surrounding gas, but the combination of ionisation spheres of multiple stars within a H II region and the expansion of the heated nebula into surrounding gases creates sharp density gradients that result in complex shapes.

[33] For a H II region which cannot be resolved, some information on the spatial structure (the electron density as a function of the distance from the center, and an estimate of the clumpiness) can be inferred by performing an inverse Laplace transform on the frequency spectrum.

[12] The Large Magellanic Cloud, a satellite galaxy of the Milky Way at about 50 kpc (160 thousand light years), contains a giant H II region called the Tarantula Nebula.

Measuring at about 200 pc (650 light years) across, this nebula is the most massive and the second-largest H II region in the Local Group.

Measuring at approximately 240 × 250 pc (800 × 830 light years) across, NGC 604 is the second-most-massive H II region in the Local Group after the Tarantula Nebula, although it is slightly larger in size than the latter.

It contains around 200 hot OB and Wolf-Rayet stars, which heat the gas inside it to millions of degrees, producing bright X-ray emissions.