An H II region or HII region is a region of interstellar atomic hydrogen that is ionized.[1] It is typically a cloud 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 cm. 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.
They may be of any shape, because the distribution of the stars and gas inside them is irregular. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds. They often appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
H II regions can be observed at considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregulargalaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy.
The term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ in other sciences—III for doubly-ionised, e.g. O III is O++, etc.[3] H II, or H+, consists of free protons. An H I region is neutral atomic hydrogen, and a molecular cloud is molecular hydrogen, H2. In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H2".
Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse.[13] The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it was formed. Only a trace of reflection nebulosity remains.
The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20 K) and dense cloud consisting mostly of molecular hydrogen.[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).[13]
As stars are born within a GMC, the most massive will reach temperatures hot enough to ionise the surrounding gas.[5] Soon after the formation of an ionising radiation field, energetic photons create an ionisation front, which sweeps through the surrounding gas at supersonic speeds. At greater and greater distances from the ionising star, the ionisation front slows, while the pressure of the newly ionised gas causes the ionised volume to expand. Eventually, the ionisation front slows to subsonic speeds, and is overtaken by the shock front caused by the expansion of the material ejected from the nebula. The H II region has been born.[14]
The lifetime of an H II region is of the order of a few million years.[15] Radiation pressure
Stars form in clumps of cool molecular gas that hide the nascent stars. It is only when the radiation pressure from a star drives away its 'cocoon' that it becomes visible. The hot, blue stars that are powerful enough to ionize significant amounts of hydrogen and form H II regions will do this quickly, and light up the region in which they just formed.
Physical properties[edit]
H II regions vary greatly in their physical properties. They range in size from so-called ultra-compact (UCHII) regions perhaps only a light-year or less across, to giant H II regions several hundred light-years across.[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. Their densities range from over a million particles per cm³ in the ultra-compact H II regions to only a few particles per cm³ in the largest and most extended regions. This implies total masses between perhaps 100 and 105 solar masses.[22]
There are also "ultra-dense H II" regions (UDHII).[23]
Depending on the size of an H II region there may be several thousand stars within it. This makes H II regions more complicated than planetary nebulae, which have only one central ionising source. 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.[24] Nevertheless, H II regions are almost always associated with a cold molecular gas, which originated from the same parent GMC.[5] Magnetic fields are produced by these weak moving electric charges in the ionised gas, suggesting that H II regions might contain electric fields.[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. X-ray observatories such as Einstein and Chandra have noted diffuse X-ray emissions in a number of star-forming regions, notably the Orion Nebula, Messier 17, and the Carina Nebula.[27] The hot gas is likely supplied by the strong stellar winds from O-type stars, which may be heated by supersonic shock waves in the winds, through collisions between winds from different stars, or through colliding winds channeled by magnetic fields. 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. It will also leak out through holes in the periphery of the H II region, which appears to be happening in Messier 17.[28]
Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line, the H-alpha line at 656.3 nm, gives H II regions their characteristic red colour. (This emission line comes from excited un-ionized hydrogen.) Most of the rest of an H II region consists of helium, with trace amounts of heavier elements. Across the galaxy, it is found that the amount of heavy elements in H II regions decreases with increasing distance from the galactic centre.[29] This is because over the lifetime of the galaxy, star formation rates have been greater in the denser central regions, resulting in greater enrichment of those regions of the interstellar medium with the products of nucleosynthesis.
Morphology[edit]
H II regions come in an enormous variety of sizes. They are usually clumpy and inhomogeneous on all scales from the smallest to largest.[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.[32] Supernova explosions may also sculpt H II regions. In some cases, the formation of a large star cluster within an H II region results in the region being hollowed out from within. This is the case for NGC 604, a giant H II region in the Triangulum Galaxy.[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.
Secondly, the formation of these stars is deeply obscured by dust, and visible light observations are impossible. Radio and infrared light can penetrate the dust, but the youngest stars may not emit much light at these wavelengths.[35]
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