H II region

An H II region is a large area of interstellar atomic hydrogen that has become ionized. In astronomy, the Roman numeral II indicates that the hydrogen is singly-ionized. This means the atoms have lost an electron, leaving behind free protons. These regions are vital to our understanding of the universe. They serve as massive star nurseries where new stars are born. They are typically found within giant molecular clouds, which are cold and dense. By studying these regions in distant galaxies, scientists can determine the chemical composition and distance of those galaxies.

The birth of an H II region follows a specific sequence of physical events. It begins with a giant molecular cloud, or GMC, which is mostly molecular hydrogen. These clouds are very cold, often between 10 and 20 K. A collapse is triggered by shock waves, collisions between clouds, or magnetic interactions. As the cloud fragments and collapses, new stars begin to form. The most massive stars created are very hot and blue. These stars emit intense ultraviolet light, which creates an ionization front. This front sweeps through the surrounding gas at supersonic speeds. As the ionized gas expands, it eventually creates the distinct H II region.

H II regions vary significantly in their physical characteristics. They can be categorized by their size and density. Some are ultra-compact H II regions, or UCHII, which may be one light-year across or less. These small regions are very dense, containing over a million particles per cubic centimeter. Other regions are giant H II regions that span hundreds of light-years. The density in these larger regions can drop to just a few particles per cubic centimeter. The temperature of an H II region typically reaches about 10,000 K. They also possess weak magnetic fields, often measured in several nanoteslas.

Humans have studied these glowing clouds for centuries. The French observer Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula in 1610. In 1774, William Herschel observed the same nebula and called it a "fiery mist." Later, William and Mary Huggins used spectroscopy to study nebulae. They noticed unique emission lines in the light from the Orion Nebula. Scientists once thought these lines belonged to a new element called nebulium. However, Henry Norris Russell later proved the lines were actually from familiar elements in unusual conditions. This discovery helped astronomers understand the true nature of ionized gas.

The scale of these regions is truly immense. A giant H II region can contain tens of thousands of stars. For example, 30 Doradus in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy are massive examples. The mass of these regions can range between 100 and 10^5 solar masses. While spiral galaxies like the Milky Way have H II regions concentrated in their spiral arms, irregular galaxies have them scattered chaotically. In contrast, elliptical galaxies almost never contain these ionized regions.

While they are called nurseries, H II regions are also places of destruction. The process of star formation is quite inefficient. Less than 10 percent of the gas in the region actually turns into stars. The remaining gas is pushed away by radiation pressure from the hot, young stars. The most massive stars also produce strong stellar winds. After only one to two million years, these stars may explode as supernovae. These explosions, along with stellar winds, eventually disperse the remaining gas. This process effectively destroys the nursery that created the stars.

Within these regions, we find fascinating structures like Bok globules. These are small, dark, circular or oval objects seen in silhouette. Bart Bok and E. F. Reilly identified them in the 1940s as potential sites for star formation. It was later confirmed in 1990 that these globules are indeed where stars are born. As the radiation from the H II region drives material away, it can actually squeeze these globules. This pressure might trigger one final burst of star formation. Additionally, the Hubble Space Telescope has found protoplanetary disks, or proplyds, in the Orion Nebula. These disks contain much of the matter needed to build planetary systems.