Region H II

NGC 604, a giant H II region in the Triangle galaxy

An H II region is a cloud of glowing gas and plasma that can reach a size of several hundred light-years and in which massive stars form. These stars emit copious amounts of extreme ultraviolet light (with wavelengths less than 912 angstroms) that ionize the nebula around them.

These regions can give birth to a large number of stars over a period of several million years. Ultimately, intense stellar winds and supernova explosions in the resulting star cluster disperse gases from the region, leaving behind a Pleiades-like cluster.

H II regions are named for the large amount of ionized atomic hydrogen they contain. In astronomy, molecular hydrogen is called H₂, neutral hydrogen is called HI, and ionized hydrogen is called HII. They can be seen at a great distance in the universe and their study is important to determine the distance and chemical composition of other galaxies.

Observations

Dark star formation zones in the eagle nebula, popularly known as the Pillars of Creation

Some of the brighter H II regions are visible to the naked eye. Despite this, there has been no record of observations of them prior to the invention of the telescope, at the beginning of the 17th century. Not even Galileo Galilei appreciated the existence of the Orion Nebula when he first observed the star cluster within it with his telescope (the cluster had previously been cataloged, by Johann Bayer, as a single star: θ Orionis). The discovery of the Orion Nebula is attributed to the French observer Nicolas-Claude Fabri de Peiresc in 1610. Since this first observation, large numbers of H II regions have been discovered in our galaxy and in others.

Orion Nebula

In 1774, William Herschel observed the Orion Nebula and later described it as "a fiery formless mist, the chaotic material of future suns". One hundred years later the hypothesis was confirmed when William Huggins, helped by his wife Margaret Huggins, studied various nebulae with his spectroscope. Some had spectra very similar to that of stars, turning out to be galaxies, which consist of billions of individual stars. However other nebulae were very different. Instead of a strong continuous spectrum with overlapping absorption lines, the Orion Nebula and other similar objects displayed only a small number of emission lines. The brightest of these lines had a wavelength of 500.7 nanometers, which which did not correspond to any known chemical element. The first hypothesis was that this unknown line corresponded to a chemical element not yet discovered, which was called Nebulium. A similar idea led to the discovery of helium from the analysis of the solar spectrum in 1868. However, while helium was isolated on Earth shortly after its discovery in the solar spectrum, nebulium did not suffer the same fate. In the early 20th century, Henry Norris Russell proposed that rather than a new element, the line at 500.7 nm was caused by the presence of a known element under unknown conditions.

In the 1920s some physicists showed that, in a gas that is under extremely low density conditions, excited electrons can occupy metastable energy levels in ions and atoms that at higher densities would be quickly de-excited by collisions between them. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines that can only be seen in very low-density gases are called forbidden lines. This theory was later confirmed by spectroscopic observations showing that nebulae are made of extremely rarefied gas.

During the XX century observations showed that H II regions often contain OB (blue) stars. These stars are many times more massive than the Sun, and have the shortest lifetimes, totaling a few million years (compared to stars like the Sun, which live for several billion years). Therefore, it was inferred that the H II regions must be the places where new stars are formed. Over a period of several million years a star cluster can form an H II region before radiation pressure from the young stars results in scattering of the nebula. An example of these dispersions are the Pleiades where only a trace of nebular reflection remains.

Origin and lifetime

A small portion of the tarantula nebula, a giant region H II in the Great Magellan Cloud

The precursor of an H II region is a giant molecular cloud (GMC). GMCs are very cold (10–20 K) and dense clouds, composed mainly of molecular hydrogen. These clouds can remain stable for long periods, but shock waves from a supernova, collisions between clouds, or magnetic interactions can trigger a part of them to collapse. When this happens, new stars are born through a process of fragmentation and collapse of the cloud.

Because stars are born inside a GMC, the most massive stars will reach temperatures high enough to ionize the gas around them. Shortly after the formation of a field of ionizing radiation, energetic photons create an ionization front that sweeps through the gas at supersonic speeds. As the distance from the ionizing star increases, the ionization front slows down and the pressure of the newly ionized gas causes its volume to expand. Finally, the ionization front descends at subsonic speeds, and is overcome by the shock front caused by the expansion of the nebula. Thus concludes the creation of an H II region.

The lifetime of an H II region is on the order of a few million years. Radiation pressure from young stars will eventually carry away all the gas in the area. In fact, the process tends to be very inefficient, since less than 10% of the gas in the H II region is converted into new stars. The rest of the gas is expelled from the region, hastening its demise, since by the time it no longer contains any more gas, it will cease to exist. Also contributing to gas loss are supernova explosions from the most massive stars, which occur just 1-2 million years later.

Star Nurseries

Bok lobes (Thackeray lobes) in region H II IC 2944

The birthplace of stars in the H II regions is hidden by a dense cloud of gas and dust that surrounds the nascent stars. The star becomes visible only when the radiation pressure from another star drives its 'cocoon' away. Of gas. Before that happens, the dense regions containing the new stars are often seen as silhouetted against the rest of the ionized nebula. It should be noted that these black patches are known as Bok's globules, discovered in 1940, by astronomer Bart Bok, who proposed that they could be stellar birthplaces.

Bok's hypothesis was confirmed in 1990, when infrared observations revealed young stars within the dense dust of Bok's globules. Now, a typical Bok globule is thought to contain matter equivalent to about 10 solar masses in a region about a light-year in size or larger, inducing the formation of double or multiple star systems.

As well as a birthplace of stars, H II regions also present evidence of containing planetary systems. The Hubble Space Telescope has revealed hundreds of protoplanetary disks in the Orion Nebula. At least half of the young stars in this nebula appear to be surrounded by disks of gas and dust, which contain enough matter to create a planetary system like ours.

Features

Physical characteristics

H II regions vary greatly in their physical characteristics. They range from the ultra-compact, with a size of only one light-year or less, to giant H II regions, which can reach sizes of hundreds of light-years. Its size is also known as the Strömgren radius and depends essentially on the intensity of the ionizing photon source and the density of the region. Their density ranges from millions of particles per cm³, in the ultra-compact H II regions, to others that only have a few particles per cm³. This implies that the total masses range from 10² to 10⁵ solar masses.

Depending on their size, H II regions can contain hundreds of stars inside. This makes H II regions more complex than planetary nebulae, which have a single central point of ionization. Typically these regions can reach temperatures of up to 10,000 K. They are normally (mostly) ionized, so plasma (ionized gas) can contain magnetic fields with the strength of several nanoteslas. Magnetic fields are produced by the motion of electric charges within the plasma, suggesting that these regions also contain electric fields.

Chemically, H II is made up of 90% hydrogen. The strongest emission line from hydrogen reaches 656.3 nm, giving these regions a characteristic reddish color. The remainder of the H II region consists of helium, with small traces of heavier elements. The percentage of heavy elements in the regions decreases with distance from the center of the galaxy. This is because throughout the lifetime of the galaxy star formation has been greatest in its denser central regions. This has made the interstellar medium in these areas richer in elements produced by nucleosynthesis.

Number and distribution

The spirals of a pink H II region line the arms of the Remolino galaxy (NGC 5194 or Messier 51)

H IIs can be found not only in spiral galaxies like ours, but also in irregular galaxies. On rare occasions they have been found in elliptical galaxies. When they are in irregular galaxies, they can be in any position within it. However, in spirals the H II are always arranged in the spiral arms. A large spiral galaxy can contain hundreds of H II regions.

H II regions are not found in elliptical galaxies due to their creation process. Ellipticals are created from mergers between galaxies. In galaxy clusters such mergers are frequent. When galaxies collide, individual stars almost never collide, but giant molecular clouds (GMCs) and the H II regions within them are severely affected. Under these conditions the creation of huge numbers of new young stars is triggered so quickly that most of the gas is converted to stellar fuel, much higher than the usual 10% or less.

Galaxies affected by this rapid creation of new stars are known as starburst galaxies. As a result of the merger and rapid star formation, elliptical galaxies with very low gas content remain, which prevents the formation of new H II regions.

Recent observations have shown that a small number of H II regions exist entirely outside galaxies. These intergalactic H II regions are a direct result of disturbances in small galaxies.

Morphology

H II regions come in a wide variety of sizes. Each H II star ionizes a spherical region of gas—known as a Strömgren sphere—around it. The combination of ionizing spheres from multiple stars within the H II region and the expansion of the nebula (which is at a high temperature), cause gases to form density gradients, resulting in complex shapes. Supernova explosions can also sculpt H II regions. In some cases, the formation of large star clusters within the H II region results in the appearance of "holes" inside. This is the case of NGC 604, a giant H II region in the Triangulum galaxy.

Zone of stellar ionization

Within an H II region, not only are photoionized zones found surrounding young stars; it also contains other types of areas known as photodissociated regions (PDRs). These two types of regions have different structures and sizes which depend on the temperature and luminosity of the star they surround and on the density of the medium in which they are found. Higher magnitude stars produce a large amount of ultraviolet radiation (UV) causing large photoionized and photodissociated areas, in contrast to lower magnitude stars which, by not producing a considerable amount of UV, create very small photoionized areas; however, they have dissociative photon fluxes that create a sizable photodissociated zone.

Calculation of Strömgren's sphere in H II regions

Two methods are used to calculate the Strömgren radius in H II regions:

• The radiation limit: The gas around the H II regions is dense and large, causing the number of recombinations to finally balance with the number of ionizations. This defines the difference between H II and H I regions, and the state of a H I region when an ionization process begins that will turn it into a H II region is known as a transition zone. The radius of the Strömgren sphere (ionized zone) depends on two factors: the stellar temperature and the hydrogen density of the area, both ionized and neutral. The radius of the sphere and star temperature are directly proportional, but its density (hydrogen) is inversely proportional.
• The limit of matter: The gas contained within the entire nebula extension limits the shape and size of the H II regions, causing them to acquire extremely complex and asymmetrical forms. This concept applies to nebulae such as Nebula de la Laguna (M8 - NGC 6523).

Notable H II Regions

Optical images reveal clouds of gas and dust in Orion's nebula; an infrared image (right) reveals the new stars.

Notable H II regions include the Orion Nebula, the Carina Nebula (NGC 3372), and the Berkley 59 / Cepheus OB4 complex. The Orion Nebula lies at a distance of approximately 1,500 light-years and forms part of a molecular cloud (GMC), so if it were visible it would fill most of the constellation Orion. The Horsehead Nebula and Barnard's Ring are two other illuminated parts of this gas cloud.

The Large Magellanic Cloud, satellite of the Milky Way, contains a giant H II region called the Tarantula Nebula. This nebula is much larger than the Orion Nebula, and is made up of thousands of stars, some 100 times as massive as the Sun. If the Tarantula Nebula were as close to Earth as the Orion Nebula, it would be as bright as the full moon in the night sky. Supernova SN 1987A was born on the outskirts of the Tarantula Nebula.

Another giant H II region is NGC 604, which occupies an area of about 800x830 light-years, although it contains slightly fewer stars than the Tarantula Nebula. It is one of the largest H II regions of the Local Group.

Nebulae in the region

Comparison of the nebula Trifida view under different wavelengths

Topics currently studied regarding H II regions

As in a planetary nebula, the determination of the abundance of chemical elements in the H II regions is subject to some uncertainty. There are two different ways to determine the abundance of metals in nebulae, that is, of elements other than hydrogen and helium. These two methods are based on different types of spectral lines, so the results are sometimes very different. Some astronomers believe that small temperature fluctuations cause these discrepancies in the H II regions; others claim that the discrepancies are too large to be caused by temperature effects, and assume the existence of "knots" cold ones that contain small amounts of hydrogen that would explain the fluctuations.

Many of the details about the formation of massive stars in H II regions are still poorly understood. There are two major problems that hinder research in this area. First, the distances from Earth to the large H II regions are considerable, with the nearest H II region being approximately 1,000 light-years away; the other H II regions are at a much greater distance. Second, the formation of these stars is largely hidden by stellar dust, so observations using visible light are impossible. Other sections of the spectrum are used to traverse interstellar dust: radio and infrared, but with the drawback that younger stars do not emit much light at these wavelengths.