HII REGIONS FRANK P. ISRAEL Early type (OB) stars hotter than about 25,000 K are usually surrounded by ionized regions called HII regions or Stromgren spheres. In reality, these regions contain dust, He *, and other ionized elements in addition to H *, are irregular in shape, and very inhomogeneous in structure. Often an HII region is excited by more than one star, further complicating the picture. Because hot ionizing stars live for a comparatively short time, HII regions are, astronomically speaking, of recent origin (at most 10 million years old). For this reason, HII regions also indicate where hot and massive stars form at the present time. Diffuse nebulae such as the Orion nebula had been observed for centuries, but it was not until 1939 that Bengt Stromgren first explained their basic structure, showing how to interpret the observations quantitatively. Technological progress following World War II, especially in spectroscopy and radio astronomy, made much more detailed observations possible. The advent of aperture synthesis radio telescopes around 1970 allowed high resolution studies of H II regions to be made, unhampered by optical extinction. Since then millimeter-wave observations have shown that all but the most evolved H II regions are intimately associated with dark and massive molecular cloud complexes. Observations of HII regions provide astronomers with information on their physical properties, evolution in time, and occurrence throughout galaxies. This can be used to find where and at what rate stars are being formed in galaxies. HII region observations also provide astronomers with the abundance (relative to hydrogen) of elements, notably He, C, N, and O as a function of galaxy type and of position in a galaxy. This in turn yields important information on galaxy evolution. Also, HII region sizes are sometimes used to estimate distances of galaxies. HII REGION PARAMETERS HII regions are studied by observing line emission resulting from bound-bound transitions in the hydrogen atom (optical Balmer lines, infrared Brackett lines, radio recombination lines) and in other elements, as well as by observing radio continuum emission resulting from free-free transitions. In the Galaxy, distances of HII regions are found from the properties of their ionizing stars, or the Doppler shift of their emission lines together with a galactic rotation model. Sizes are measured directly, whereas optical emission line ratios yield density ([O II] and [S II] doublets) and temperature ([O II/[O III] and [N II]/[N III] ratios). Mean densities also follow from radio continuum observations, and temperatures follow from radio recombination lines. The observed amount of line or continuum radiation emitted by an HII region measures the rate at which the exciting stars produce ionizing ultraviolet photons, and hence yields information on the temperature (spectral type) and number of these stars. Determined with sufficiently high resolution, the observed temperature, velocity, and density distributions of hydrogen and other elements are used to construct elaborate models of the complex physical and excitation structure of HII regions. From these, the time evolution of HII regions can be reconstructed. COMPACT H II REGIONS Hot, massive stars form in molecular clouds, ionize their dense immediate surroundings, and heat nearby dust. The result is an ultracompact HII region less than 0.1 pc in size and of density greater than 10**cm**, surrounded by dense, obscuring cocoon of warm dust. Thus, ultracompact HII regions can only be observed at radio or infrared wavelengths. They are often associated with OH or H ,0 masers. Because of its high temperature and density, the ultracompact HII region expands into the equally dense, but much cooler molecular cloud (see Fjg. 1): it excavates an ionized cavity. After ******* years, the cavity breaks through the nearest molecular cloud edge. With speeds up to 20 km s**, ionized material of high temperature and density flows through the break into the less dense surrounding medium, not unlike champagne coming out of a freshly opened bottle. At this stage, the HII region becomes optically visible as a compact HII region (size typically 0.5 pc; density 5000 cm***). The champagne concept has been successfully used to describe the flows observed in such HII regions. Because of the flow, the density in the cavity decreases, so that more ionizing photons from the exciting star can reach the ionization front that forms the boundary between the HII region and the molecular cloud. The ionization front moves slowly (typically at 1 km ***) into the molecular cloud. Neutral material is ionized and fed through the ionization front into the ionized cavity from which it streams into interstellar space. The cavity shows a steep gradient in density, highest at the ionization front, decreasing outward, and finally merging into the general interstellar medium, without a clear boundary. The Orion nebula is such a compact HII region. It also hides an ultracompact HII region, known as the Becklin-Neugebauer object (BN) at its edge. FURTHER EVOLUTION OF HII REGIONS As the HII region cavity further expands, it develops into a classical HII region such as the Omega nebula with typical densities of a few hundred hydrogen atoms per cubic centimeter or less and sizes of a few parsecs. Its morphology is that of a blister on the skin of a much larger molecular cloud. This (simplified) blister model is useful to describe several observed properties, such as excitation and density structures. The majority of optically visible normal emission nebulae have such a blister structure, albeit with many irregularities introduced by dense clumps, shock fronts, and additional stars. A consequence of the champagne and blister concepts is that HII regions cannot be considered as isolated entities. Their properties and evolution are closely tied to those of the (invisible) neutral atomic and molecular surroundings, and observations of these surroundings are essential to obtain the full picture. As stars tend to form in groups, further expansion often leads to the overlap of different blisters, forming even more complex structures. This is characteristic of large H II regions (size typically 10 pc; density 50 cm***) such as the North America nebula. Finally, ultraviolet photons escaping from the blisters cause the tenuous interstellar medium to become ionized to great distances, leading to common ionized envelopes for groups of HII regions. Such giant HII regions (size 100-1000 pc; density below 10 cm***) are often seen in other gas-rich galaxies (30 Doradus in the Large Magellanic Cloud; NGC 5461 in M101) and signify the presence of as many as several thousand hot, young stars dominating a large section of a spiral arm. After about 10 million years, the exciting stars of evolved HII regions have almost completely eroded their molecular clouds, and start to die themselves; their HII regions expand over further into the interstellar medium and fade away. Elsewhere, new hot stars and new HII regions have been formed, starting the process anew. H II REGIONS IN GALAXIES Otical telescopes can detect HII regions about a hundred times less intense than the weakest seen with radio telescopes. Most of the optical emission from stars and nebulae in the Galaxy is, however, obscured by dust clouds. Thus, radio measurements are the only way to determine the overall content and distribution of galactic HII regions. They show the Galaxy to contain several hundred large and giant HII region complexes, and many more small ones. These are concentrated between distances of 4 and 7 kpc from the galactic center, where giant molecular clouds are also concentrated (the "molecular ring"). From 4500 k at distances of 4 kpc from the galactic center, H II region (electron) temperatures increase to 10,000 K at 14 kpc. This is a consequence of the observed decrease in abundances of elements such as O and N (hence a decrease in cooling) by factors of 5-8 over the same range. Many other spiral galaxies show abundance gradients in their HII region populations. However, absolute abundances vary greatly from galaxy to galaxy, the smallest galaxies usually having the lowest abundances. Temperatures as high as 13,000 K are measured in HII regions in the low-abundance satellite galaxies of the Milky Way, the Magellanic Clouds, as well as in more distant irregular dwarf galaxies. Some of the latter are little more than a single giant HII region, such as the object II Zwicky 40. In most other galaxies, astronomers can observe only a few dozen or less of the brightest HII regions. In spiral galaxies these occur preferentially halfway out from the center in the galactic disk; sometimes, bright H II regions are concentrated in narrow rings in the inner galaxy (e.g., NGC 1097). Sa galaxies generally have the smallest and weakest HII regions; Sc spirals and irregular galaxies have the largest and brightest. In the former, HII regions frequently nicely delineate the major spiral arms (e.g., Walter Baade's "beads on a string" in M31), whereas in the latter the distribution is more chaotic. At the edges of the nearest galaxies, large faint ringlike HII regions have been found, of an appearance not yet seen in the galaxy, and of unclear, origin. There are many more weak than bright HII regions. Radio studies show that in a given galaxy, the number of HII regions above a certain luminosity generally decreases with the square of that luminosity. Among galaxies, the luminosity of the brightest HII region complexes varies widely, irrespective of galaxy mass or luminosity. Giant galaxies such as M101 and dwarfs such as M33 and the Large Magellanic Cloud contain HII region complexes more luminous than any found in our own Galaxy. The resolution and sensitivity of astronomical measurements continues to increase, bringing more HII regions in more distant galaxies within reach. The observational emphasis is shifting from studies of individual HII regions to studies of HII regions as part of their atomic and molecular surroundings and to the use of HII regions as a tool to determine large-scale properties of galaxies. Thus, in only a few decades, two largely unrelated fields, The study of the (galactic) interstellar medium and the study of the evolution of galaxies, are merging into one. Additional Reading Gordon, M.A.(1988). H II regions and radio recombination lines. In Galactic and Extragalactic Astronomy, G.L. Verschuur and K.I. Kellermann, eds. Springer, Berlin, p.37. Habing, H.J. and Israel, F.P.(1979). Compact H II regions and OB star formation. Ann. Rev. Astron. Ap. 17 345. Osterbrock, D.E.(1989). Astrophysics of Gaseous Nebulae and Active Galactic Nuclei. University Science Books, Mill Valley, California. Pagel, B.E.J. and Edmunds, M.G.(1981). Abundances in stellar populations and the interstellar medium in galaxies. Ann. Rev. Astron. Ap. 19 77. Shields, G.A.(1990). Extragalactic H II regions. Ann. Rev. Astron. Ap. 28 525. See also H II Regions, Dynamics and Evolution; Interstellar Medium; Masers, Interstellar and Circumstellar.