H II region

An H II region (pronounced ha two) is an interstellar cloud of glowing gas with a diameter of sometimes several hundred light years, is the venue for the star formation. Young, hot, blue stars that are caused by local inhomogeneities of this gas cloud, sending large amounts of ultraviolet light, which ionizes the nebula around it.

In H II regions thousands of new stars are formed in a period of a few million years. However, stellar winds of the most massive stars or isolated supernova explosions at the end means that the gas of the H II region is scattered. What remains is an open cluster like the Pleiades visible in the winter sky.

H II regions have include its name from the large amount of ionized atomic hydrogen ( a plasma state of individual protons), they contain, whereas HI- areas atomic non ionized hydrogen and molecular hydrogen (H2). H II regions can be perceived in the universe at very large distances. Therefore, the study of extragalactic H II regions is helpful to determine the distance and chemical composition of other galaxies.

Observation

Some H II regions are so bright that they can still be seen with the naked eye. However, no attention has been paid before the invention of the telescope in the early 17th century. Even Galileo took no notice of the Orion Nebula, as he watched the star clusters in it. Before that, the fog by Johann Bayer as a single star, θ Orionis was cataloged. The French observer Nicolas- Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula 1610. After that, many more H II regions have been discovered in and outside our galaxy.

William Herschel observed the Orion Nebula in 1774 and described him as a " shapeless glowing mist, the chaotic material of future suns". The confirmation of his hypothesis had to wait one hundred years in coming. William Huggins ( with the help of his wife, Margaret Lindsay Huggins ) turned his spectroscope to various fog. Some, such as the Andromeda nebula had star-like spectra and appeared to consist of several hundreds of millions of individual stars. In other nebulae but the was not true. Instead of pointing continuously superimposed absorption lines, objects like the Orion Nebula showed some emission lines. The brightest had a wavelength of 500.7 nm, this was not related with any known chemical element. First, it was assumed that it was an unknown element that nebulium was called. A similar idea led in 1868 to the discovery of helium, as the solar spectrum was analyzed.

Although one can not detect the helium shortly after its discovery in the solar spectrum on Earth, nebulium not found one. In the 20th century Henry Norris Russell suggested that it was not a question, a new element that the wavelength of 500.7 nm caused, but rather a known element in unknown conditions.

In the 1920s, physicists showed that the gas cloud has an extremely low density. Electrons can reach in the atoms and ions metastable energy levels that may exist at higher densities due to the continuous collision barely long otherwise. Electron transitions in hydrogen lead to a 500.7 -nm wavelength. Such spectral lines that are observed only in gases with very low densities are called forbidden lines. Spectroscopic observations have shown that the fog made ​​of extremely rarefied gas.

During the 20th century, it was observed that the H II regions contain mostly hot bright stars. These stars have many times our Sun's mass and are the shortest-lived stars with only a few million years service life (compared to our Sun lives of several billion years). It has been speculated that H II regions are regions where new stars are forming. Over a period of several million years, a star cluster from an H II region is formed before the stellar wind of hot young stars scattered the mist. The Pleiades are an example of such a cluster that has blown away the H II region from which he originated. Only a small remnant remained as a reflection nebula.

Origin and CV

Harbingers of an H II region are dark nebulae in the form of giant molecular clouds (german giant molecular clouds, GMC). They are very cold ( 10-20 K) and consist mainly of molecular hydrogen. Such giant molecular clouds can remain stable for a longer time. However, shock waves through supernovae, collisions between the mists and magnetic interactions lead to the collapse of a cloud part. If that happens, it occurs during the Kollabierungsprozesses and the fragmentation of the cloud to star formation.

When stars form in a giant molecular cloud that will achieve the most massive among them temperatures sufficient to ionize surrounding gas. Shortly after the ionizing radiation field is created that produce high-energy photon an ionization front, which propagates through the surrounding gas at supersonic speed. The further this front moves away from its star, the more it is slowed down. By the pressure of the ionized gas even occurs to spread the ionized volume. Finally, the ionization front reaches subsonic speed and is overtaken by the shock front of the ionized nebula. This is the birth of an H II region

An H II region persists for a few million years. The stellar wind of hot young stars pushes away most of the gas of the nebula. Overall, the process seems to be very inefficient. Less than 10 % of the gas of an H - II region is used to form new star, while the remainder is blown off. A further contribution to the loss of gas control during the supernova explosions of the most massive stars, which already occur after 1 to 2 million years ago.

Stellar birthplaces

The birth of a star in an H II region is obscured by dense clouds and dust around forming stars. Only if the stellar wind his " cocoon " blows away, the star becomes visible. The dense fog regions containing the stars are often seen as a shadow over the rest of the ionized nebula. These dark spots are called globule (English Bok globules ), astronomer Bart Bok after which suggested in the 1940s that they are the birthplaces of stars.

Boks hypothesis was confirmed in 1990, penetrated as infrared observations the thick dust and young stars disclosed. Today it is believed that a Bok globule has about ten times the mass of the sun, which is spread over a diameter of about one light- year. Most of it comes from a formation of a double or multiple star system.

H II regions are both a birthplace for young stars, but also show evidence of planetary systems. The Hubble Space Telescope has discovered hundreds of protoplanetary disks in the Orion Nebula. At least half of the stars in the Orion Nebula have disks of gas and dust, and indeed much more than they would need for the emergence of a planetary system like ours.

Properties

Physical Properties

H II regions vary greatly in their physical properties. They range in size from so -called ultra - compact areas of about one light- year or less to gigantic H II regions that are several hundred light -years across. Their density ranges from one million particles per cubic centimeter. In the ultra - compact H II regions to only a few particles per cm ³ in the most extensive regions

Depending on the size of the H- II domain can comprise up to several thousand stars. This makes it complicated to understand H II regions as, for example, planetary nebulae, which have only one central ionization source. Most of the H II regions have a temperature of about 10,000 K.

With the ever successful recombination to neutral hydrogen ( and re- ionization) a characteristic line emission is generated. Therefore, such areas are among the emission nebulae. Hydrogen has a relatively low ionization energy. But because there is the interstellar matter to 90% of hydrogen, therefore many fog lights on the brightest in the characteristic red hydrogen at a wavelength of 656.3 nm, the so-called Hα line of the Balmer series.

Other lines in the visible range at 486 nm are Hβ, hv at 434 nm and at 410 nm Hδ Depending on the pressure and temperature in the fog vary the proportions of these usually weaker lines. The color of the total light of an emission nebula can thus move into the Pink, such as in the comparatively very dense prominences of the sun. Conversely, one can determine the pressure and temperature of this so-called Balmerdekrement.

The remaining portion of a H- II - domain consists of 10% of helium. The heavier elements only make up a very small fraction. It was found that in our galaxy decreases the amount of heavy elements more and more the farther the distance of the H II region from the galaxy center. This is because that form more star formation in centers of greater density, and so the Interstellar matter is more enriched with the reaction products of nuclear fusion.

Number and distribution

H II regions can be found only in spiral galaxies and irregular galaxies. They were never seen in elliptical galaxies. For the irregular galaxies can find them everywhere, but they are found in spiral galaxies mostly in the arms. A large spiral galaxy could contain thousands of H II regions.

The reason for the failure in elliptical galaxies is that these are caused by galaxy merger. In galaxy clusters, such mergers are common. When galaxies collide, individual stars rarely collide with each other. However, giant molecular clouds and H II regions constantly get in the way. The results under these conditions to a strong star formation, the vonstattengeht so fast that most of the gas stars are formed. Galaxies that undergo such a rapid star formation process are called starburst galaxies.

H II regions, there are also outside of galaxies. This intergalactic H II regions appear to be remnants of the destruction of smaller galaxies.

Morphology

H II regions is available in different sizes. Each star ionizes a roughly spherical region. However, the combination of ionized lead ball rooms of various stars and the heating of the nebula into complex shapes. In addition, supernova explosions affect an H II region. Sometimes the formations of a large star cluster lead to the erosion of the H II region from the inside. This is the case for NGC 604, a giant H II region in the Triangulum Galaxy.

Well-known H II regions

The best-known H II region in our galaxy is the Orion Nebula. It measures about 30 light-years in diameter and stands at a distance of 1,400 light years. The fog is part of a giant molecular cloud, whose central part is already freiäugig to detect. Had he seen as a whole, he would fill most of the Orions. The smaller Horsehead Nebula and Barnard 's Loop are two more glowing parts of this vast cloud of gas.

The Large Magellanic Cloud is a satellite galaxy of the Milky Way. It contains a giant H II region with the name of the Tarantula Nebula (30 Dor ). This nebula is much larger than the Orion nebula, forming thousands of stars. Some of them have 100 times the sun's mass. If the Tarantula so close to the Earth as the Orion Nebula, then rail as bright as the full moon in the night sky. The supernova SN 1987A occurred in the outskirts of the Tarantula Nebula.

NGC 604 is still greater than the Tarantula Nebula and has a diameter of around about 1300 light years, although it is almost devoid stars. She is one of the largest H II regions of the Local Group.

Current research topic of H II regions

Just as in planetary nebulae prepares for H II regions, the determination of some elements difficulties. There are here two paths that emanate from various types of spectral lines. However, there is between the results, which come both methods, sometimes disagreements. It is believed that the reason lies in the temperature variations of the H II regions, or that some cold areas with very little hydrogen are responsible.

Many details of massive star formation in H II regions are still unknown, because - apart from the large distances ( the nearest H II region is 1,000 light- years away from Earth ) - the star formations are largely obscured by dust. It is thus impossible to observe the stars in visible light. Although the radio or infra-red radiation can pass through the dust formation, but young stars emit no light in these wavelengths.