Area HII

In Astronomy, one calls areas HII nebulas in emission consisted of clouds mainly made up of Hydrogène and whose majority of the Atome S are ionized, and extending sometimes on several light-years. Ionization is produced by the proximity of one or more star S very hot, of spectral Type O or B, which strongly radiate in the extreme Ultraviolet, thus ionizing the gas around, from which these stars in the beginning were formed.

Later, the explosions in supernovæ and the forts winds stellar caused by the most massive stars of the stellar Amas will end up dispersing the gas particles remaining, leaving behind them a star cluster such as that of the Pléiades.

Areas HII draw their name from the presence in great quantity of Hydrogène ionized, noted HII (to pronounce “ axe two ”), not to confuse with the molecular hydrogen (H₂), and atomic neutral hydrogen (HI).

These ionized gas clouds are visible with very long distances, and the study of extragalactic areas HII is fundamental to determine the distances and the Chemical composition of the others Galaxie S.

History

Some of the most luminous areas HII are visible with the naked eye, however it seems that none of enter they was not discovered before the invention of the Télescope at the beginning of the 17th century. Even Galileo seems not to have noticed the Nébuleuse of Orion whereas it observed the stellar Amas which it contains. It is the Astronome French Nicolas-Claude Fabri de Peiresc which will discover this nebula in 1610, and since a great number of these areas HII were discovered, inside and apart from our Galaxy.

William Herschel, observing the nebula of Orion in 1774, will describe it like “ a formless fog burning, chaotic material of the future suns ”. A century will have however to be still waited so that this theory is confirmed, when the British astronomer William Huggins pointed his Spectromètre in direction of several nebulas. Among those which were observed, certain, like the nebulous of Andromède, had a spectrum similar to that of stars, and one deduced from it that it was acted in fact of Galaxie S made up of hundreds of million of stars. The others were very different: in the place of a continuous spectrum intersected with absorption lines, that of the nebula of Orion and other similar objects were made up only of some emission lines, very few . One of enter they was located at a Wavelength of 500,7 nanometer S, which, at the time, did not correspond to any chemical element known. The scientists put forth the assumption then that it was about a new chemical element, which was named Nebulium (a similar idea had led to discovered Hélium in 1868 by analysis of the spectrum of the Sun).

However, whereas helium was isolated on Ground shortly after its discovery in the spectrum from the Sun, it was not the case of nebulium. At the beginning of the 20th century, Henry Norris Russell proposed that rather than to be related to a new element, the emission line to 500,7 Nm could the being with an element already known but placed under unusual conditions.

The physicist S showed in the Années 1920 that in an extremely weak gas of Densité, the excited electrons can occupy of the metastable energy levels which would very quickly be de-energized by the collisions in a gas of higher density. However the transition from the electrons between these energy levels in the atom of Oxygène lead precisely to an emission line of 500,7 Nm wavelength. These spectral lines, which can be observed only for very weak gases of density, are called prohibited lines of transition . The spectrometric observations of nebulas thus showed that those consisted of extremely rarefied gas.

During the 20th century, the observations revealed that areas HII often contained hot and very luminous stars. These stars are much more massive than the Sun, and which is those have the shortest lifespan, estimated to a few million years only (compared to stars as the Sun which can live several billion years). One conjectured whereas areas HII were to be one of the places where the stars are born. Thus, over one period of several million years, a star cluster is formed starting from the gas cloud, before the Pression of radiation generated by stars already created does not disperse what remains nebula. The Pléiades are an example of cluster which completely “blew” gas of area HII from which it was formed (only some traces of nebulosity by reflection are still visible).

Formation and evolution

The precursor of an area HII is a giant molecular Nuage. This giant cloud is very cold (from 10 to 20 K) and dense, mainly made up of hydrogen molecular. The giant clouds molecular can remain in a stable condition during very a long time, but the shock waves caused by the close supernovæ, the collisions between galaxies or the interactions gravitational and magnetic can involve the collapse of part of the cloud, which leads to the star formation via a process of collapse and fragmentation of the cloud (→ to see the detailed article: Birth of the stars ).

Following the creation of stars inside the giant molecular cloud, most massive of them reach a very high temperature quickly (several tens of thousands of Kelvins), and the very energy Photon S emitted by star start with to ionize the surrounding gas - this one being mainly made up of hydrogen, one then obtains a plasma of Proton S and free electron S. It is then formed a face of ionization, which extends at very high speed. The internal pressure of gas lately ionized increases with its temperature, involving in fact an increase in its volume. Displacements of matter and generated shock waves support in their turn the stellar formation in the close areas.

The lifespan of an area HII is estimated between 10 and 100 million years following its dimensions, the pressure of radiation and the stellar Vent generated by hot stars completing to evacuate gas still present (see star Wolf-Rayet). In fact, the process has a rather weak output, with only approximately 10 pourcent gas of nebula being used for the formation of stars before being ejected with far. The explosions in supernovæ also contribute for a great part to this gas loss, those being able to occur after only 1 to 2 million years for the most massive stars.

Star Pouponnières

The real process of formation of stars inside areas HII is to us in fact hidden by the dense cold and opaque gas cloud which surrounds incipient star. It is only when the pressure of radiation caused by the radiation of star expels its “cocoon” which it becomes visible. Before that, the dense gas areas which contain new stars in formation are often seen in silhouette in front of the other ionized parts of the nebula (these dark zones are known under the name of globules of Bok, of the name of the astronomer Bart Bok, which put forth the assumption in the Années 1940 that those can be the place of the formation of stars).

The confirmation of the assumption of Bok due to await the Years 1990, so that the improvement of the instruments and the observations Infrarouge end up “boring” this layer of dust and showing young stars in the course of formation . One generally thinks that a globule of typical Bok has a mass of approximately 10 solar masses, concentrated in an area from approximately 1 light-year , and that the globules of Bok lead most of the time to the formation of double stars or multiple .

In addition to being the place of the formation of stars, areas HII also seem to contain planetary systems. The Space telescope Hubble revealed the presence of hundreds of discs protoplanétaires in the nebula of Orion. At least the half of young stars of the nebula of Orion seem surrounded by a gas disc and dust, which one thinks that they contain each one of matter enough to form planetary systems similar to our.

Characteristics

Physical characteristics

The physical properties of areas HII vary one with the other enormously. Their size is between one light-year only for the ultra-compact areas, until several hundred light-years for the giants. The density of the ultra-compact areas is about the million particles per cm ³, and only some particles with cm ³ for the widest areas.

According to the size of the area HII, this one can contain of a star to several thousands. What makes areas HII much more complicated to include/understand and analyze that the planetary nebulas, which they contains only one central source of ionization. Areas HII have however in common to have a temperature about 10 000 K. They are mainly ionized, and this ionized gas can generate a magnetic field of a force of several tens of microphone Gauss . Certain observations also suggest that this gas can contain electric fields.

Chemically, areas HII are made up to hydrogen 90%. The strongest emission line of hydrogen located at 656,3 Nm gets for these areas their characteristic red color. The remainder is mainly made up of helium, plus some traces of heavier elements. Through the Galaxy, it was shown that the proportion of heavy elements in an area HII decrease when one moves away from the galactic Center. This is probably due to the fact that during the life of the Galaxy, the rate of formation of stars was faster in the central areas (denser), implying a faster enrichment of the interstellar Milieu in heavy elements, by the stellar processes of Nucléosynthèse.

Number and distribution

Areas HII were detected only in the spiral galaxies like ours or the irregular galaxies. One on the other hand never found of it in the elliptic galaxies. One can observe some about anywhere inside an irregular galaxy, while they are almost always located in the spiral arms of the spiral galaxies. A spiral galaxy of big size can contain several thousands of areas HII.

The reason which makes that no area HII is observed in the elliptic galaxies is due to the way whose these galaxies are created, by fusion of several galaxies between them. When two galaxies enter in collision, the individual stars which compose them almost never does not come into contact (the density of stars inside a galaxy is altogether relatively weak), but the giant molecular clouds and areas HII are them seriously agitated, in particular because of the gravitational forces. Under these conditions, a very great number of stars are formed, so quickly that the largest part of gas is transformed into stars ( instead of the 10% evoked in the chapter #Formation and evolution ). The elliptic galaxy resulting from this fusion does not contain any more that very little of gas, and areas HII cannot thus be formed any more.

Recent observations showed that there exists a small number of areas HII located apart from the galaxies themselves. It is supposed that these gas clouds were torn off by effect of tide to the outlying areas of galaxies at the time of collisions or even only at the time of passages brought closer between two massive galaxies .

Morphology

Areas HII have a very large variety of forms and sizes. Each star inside an area HII ionizes an overall spherical gas area around it, but the combination of ionized spheres of multiple stars inside the same area HII as well as the expansion of the nebula overheated inside the surrounding gas cloud, which contains itself of weak variations of density, conduit to the formation of complex forms. The supernovæ also contribute “to carve” the shape of the cloud.

In certain cases, the formation of large a stellar Amas inside area HII leads this one to be “enlightened” of the interior by the many stars which composes it. It is the case for example of NGC 604, an area HII giant located in the Galaxie of the Triangle.

Some notable areas HII

most known of areas HII inside our Galaxy is the Nébuleuse of Orion, located at a distance from approximately 1500 light-years of the Solar system. This nebula belongs to a giant molecular Nuage, called cloud of Orion , which if it were entirely visible would fill to it quasi totality of the Constellation of Orion. The Nebulous of the Head of Horse and the Boucle of Barnard are two other enlightened parts of this cold gas cloud.

the Large Cloud of Magellan, a small satellite galaxy of our, contains a very great area HII called Nébuleuse of the Tarantula. This nebula is much wider than that of Orion, and of the thousands of stars are created there, certain having more than 100 solar masses. If the nebula of Tarantule were as close to the Earth as is the nebula of Orion, it would shine as much as full the the Moon in the night sky. The supernova SN 1987A occurred in the surroundings of this nebula.

NGC 604 is an area HII even larger than the nebula of the Tarantula, but it contains slightly less stars. It is one of areas HII more the extents known in the local Groupe.

See too

Internal bonds

Nebulous
Nebulous planet gear
Birth of the stars
Globule of interstellar Bok
Medium

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