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H II region

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   NGC 604, a giant H II region in the Triangulum Galaxy.
   Enlarge
   NGC 604, a giant H II region in the Triangulum Galaxy.

   An H II region is a cloud of glowing gas and plasma, sometimes several
   hundred light years across, in which star formation is taking place.
   Young, hot, blue stars which have formed from the gas emit copious
   amounts of ultraviolet light, ionising the nebula surrounding them.

   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
   such as the Pleiades.

   H II regions are named for the large amount of ionised atomic hydrogen
   they contain, referred to as H II (pronounced 'aitch two') by
   astronomers ( H I region ('aitch one') being neutral atomic hydrogen,
   and H[2] (also 'aitch two') being molecular hydrogen). H II regions can
   be seen out to considerable distances in the universe, and the study of
   extragalactic H II regions is important in determining the distance and
   chemical composition of other galaxies.

Observations

   Dark star-forming regions within the Eagle Nebula.
   Enlarge
   Dark star-forming regions within the Eagle Nebula.

   A few of the brightest H II regions are visible to the naked eye.
   However, none seem to have been noticed before the advent of the
   telescope in the early 17th century. Even Galileo did not notice the
   Orion Nebula when he first observed the star cluster within it
   (previously catalogued as a single star, θ Orionis, by Johann Bayer).
   French observer Nicolas-Claude Fabri de Peiresc is credited with the
   discovery of the Orion Nebula in 1610. Since that early observation
   large numbers of H II regions have been discovered in our galaxy and
   others.

   William Herschel observed the Orion Nebula in 1774, and described it as
   "an unformed fiery mist, the chaotic material of future suns".
   Confirmation of this hypothesis had to wait another hundred years, when
   William Huggins (assisted by his wife Mary Huggins) turned his
   spectroscope on various nebulae. Some, such as the Andromeda Nebula,
   had spectra quite similar to those of stars, and turned out to be
   galaxies consisting of hundreds of millions of individual stars. Others
   looked very different. Rather than a strong continuum with absorption
   lines superimposed, the Orion Nebula and other similar objects showed
   only a small number of emission lines . The brightest of these was at a
   wavelength of 500.7  nanometres, which did not correspond with a line
   of any known chemical element. At first it was hypothesised 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.

   However, while helium was isolated on earth soon after its discovery in
   the spectrum of the sun, nebulium was not. In the early 20th century,
   Henry Norris Russell proposed that rather than being a new element, the
   line at 500.7 nm was due to a familiar element in unfamiliar
   conditions.

   Physicists showed in the 1920s that in gas at extremely low densities,
   electrons can populate excited metastable energy levels in atoms and
   ions which at higher densities are rapidly de-excited by collisions .
   Electron transitions from these levels in oxygen give rise to the
   500.7 nm line. These spectral lines, which can only be seen in very low
   density gases, are called forbidden lines. Spectroscopic observations
   thus showed that nebulae were made of extremely rarefied gas.

   During the 20th century, observations showed that H II regions often
   contained hot, bright stars. These stars are many times more massive
   than the Sun, and are the shortest-lived stars, with total lifetimes of
   only a few million years (compared to stars like the Sun, which live
   for several billion years). Therefore it was surmised that H II regions
   must be regions in which new stars were forming. Over a period of
   several million years, a cluster of stars will form out of an H II
   region, before radiation pressure from the hot young stars resulting
   causes the nebula to disperse. The Pleiades are an example of a cluster
   which has 'boiled away' the H II region from which it formed – just a
   trace of reflection nebulosity remains.

Origin and lifetime

   A small portion of the Tarantula Nebula, a giant H II region in the
   Large Magellanic Cloud.
   Enlarge
   A small portion of the Tarantula Nebula, a giant H II region in the
   Large Magellanic Cloud.

   The precursor to an H II region is a giant molecular cloud (GMC). A GMC
   is a very cool (10–20  K) and dense cloud consisting mostly of
   molecular hydrogen. 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 all trigger the collapse of part of the
   cloud. When this happens, via a process of collapse and fragmentation
   of the cloud, stars are born (see stellar evolution for a lengthier
   description).

   As stars are born within a GMC, the most massive will reach
   temperatures hot enough to ionise the surrounding gas. 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 nebula. The H II region has
   been born .

   The lifetime of an H II region is of the order of a few million years.
   Radiation pressure from the hot young stars will eventually drive most
   of the gas away. In fact, the whole process tends to be very
   inefficient, with less than 10 per cent of the gas in the H II region
   forming into stars before the rest is blown away. Also contributing to
   the loss of gas are the supernova explosions of the most massive stars,
   which will occur after only 1–2 million years.

Stellar nurseries

   Bok globules in H II region IC 2944.
   Enlarge
   Bok globules in H II region IC 2944.

   The actual birth of stars within H II regions is hidden from us by the
   dense clouds of gas and dust which surround the nascent stars. It is
   only when the radiation pressure from a star drives away its 'cocoon'
   that it becomes visible. Before then, the dense regions which contain
   the new stars are often seen in silhouette against the rest of the
   ionised nebula — these dark patches are known as Bok globules, after
   astronomer Bart Bok, who proposed in the 1940s that they might be
   stellar birthplaces.

   Confirmation of Bok's hypothesis had to wait until 1990, when infrared
   observations finally penetrated the thick dust of Bok globules to
   reveal young stellar objects within. It is now thought that a typical
   Bok globule contains about 10 solar masses of material in a region
   about a light year or so across, and that Bok globules most commonly
   result in the formation of double or multiple star systems ^,,.

   As well as being the birth place of stars, H II regions also show
   evidence for containing planetary systems. The Hubble Space Telescope
   has revealed hundreds of protoplanetary disks ( proplyds) in the Orion
   Nebula. At least half the young stars in the Orion Nebula appear to by
   surrounded by disks of gas and dust, thought to contain many times as
   much matter as would be needed to create a planetary system like our
   own.

Characteristics

Physical characteristics

   H II regions vary greatly in their physical properties. They range in
   size from so-called ultra-compact regions perhaps only a light year or
   less across, to giant H II regions several hundred light years across.
   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 10^2 and 10^5 solar masses.

   Depending on the size of an H II region there may be anything up to
   several thousand stars within it. This makes H II regions much more
   complicated to understand than planetary nebulae, which have only one
   central ionising source. Typically, though, H II regions are at
   temperatures of the order of 10,000 K. They are mostly ionised, and the
   ionised gas (plasma) can contain magnetic fields with strengths of
   several tens of microgauss (several nanoteslas). Magnetic fields are
   produced by moving electric charges in the plasma, and some
   observations have suggested that H II regions also contain electric
   fields .

   Chemically, H II regions consist of about 90% hydrogen. The strongest
   hydrogen emission line at 656.3 nm gives H II regions their
   characteristic red colour. 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. 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 the
   interstellar medium with the products of nucleosynthesis.

Numbers and distribution

   Strings of red H II regions delineate the arms of the Whirlpool Galaxy.
   Enlarge
   Strings of red H II regions delineate the arms of the Whirlpool Galaxy.

   H II regions are found only in spiral galaxies like our own and
   irregular galaxies. They are never seen in elliptical galaxies. In
   irregular galaxies, they may be found throughout the galaxy, but in
   spirals they are almost invariably found with the spiral arms. A large
   spiral galaxy may contain thousands of H II regions.

   The reason H II regions are not seen in elliptical galaxies is that
   ellipticals are believed to form through galaxy mergers. In galaxy
   clusters, such mergers are frequent. When galaxies collide, individual
   stars almost never collide, but the GMCs and H II regions in the
   colliding galaxies are severely agitated. Under these conditions,
   enormous bursts of star formation are triggered, so rapid that most of
   the gas is converted into stars rather than the normal 10 per cent or
   less. Galaxies undergoing such rapid star formation are known as
   starburst galaxies. The post-merger elliptical galaxy has a very low
   gas content, and so H II regions can no longer form.

   Recent observations have shown that a very small number of H II regions
   exist outside galaxies altogether. These intergalactic H II regions are
   likely to be the remnants of tidal disruptions of small galaxies .

Morphology

   H II regions come in an enormous variety of sizes. Each star within an
   H II region ionises a roughly spherical region - known as a Strömgren
   sphere - of the gas surrounding it, but the combination of ionisation
   spheres of multiple stars within an H II region and the expansion of
   the heated nebula into surrounding gases with sharp density gradients
   results in complex shapes. 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.

Notable H II regions

   Within our galaxy, the best known H II region is the Orion Nebula,
   which lies at a distance of about 1,500 light years. The Orion Nebula
   is part of a GMC which, if it were visible, would fill most of the
   constellation of Orion. The Horsehead Nebula and Barnard's Loop are two
   other illuminated parts of this cloud of gas.

   The Large Magellanic Cloud, a satellite galaxy of the Milky Way,
   contains a giant H II region called the Tarantula Nebula. This nebula
   is much bigger than the Orion Nebula, and is forming thousands of
   stars, some with masses of over 100 times that of the sun. If the
   Tarantula Nebula was as close to Earth as the Orion Nebula, it would
   shine about as brightly as the full moon in the night sky. The
   supernova SN 1987A occurred in the outskirts of the Tarantula Nebula.

   NGC 604 is even larger than the Tarantula nebula at about 1,300 light
   years across, although it contains slightly fewer stars. It is one of
   the largest H II regions in the Local Group.

Current issues in studies of H II regions

   Optical images reveal clouds of gas and dust in the Orion Nebula; an
   infrared image (right) reveals the new stars shining within.
   Enlarge
   Optical images reveal clouds of gas and dust in the Orion Nebula; an
   infrared image (right) reveals the new stars shining within.

   In common with planetary nebulae, determinations of the abundance of
   elements in H II regions are subject to some uncertainty. There are two
   different ways of determining the abundance of metals (that is,
   elements other than hydrogen and helium) in nebulae, which rely on
   different types of spectral lines, and large discrepancies are
   sometimes seen between the results derived from the two methods. Some
   astronomers put this down to the presence of small temperature
   fluctuations within H II regions; others claim that the discrepancies
   are too large to be explained by temperature effects, and hypothesise
   the existence of cold knots containing very little hydrogen to explain
   the observations .

   The full details of massive star formation within H II regions are not
   yet well known. Two major problems hamper research in this area. First,
   the distance from Earth to large H II regions is considerable, with the
   nearest H II region being over 1,000 light years away; other H II
   regions are several times that distance away from Earth. 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.
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