An H II region is a volume of interstellar gas where hydrogen is predominantly ionized by Lyman‑continuum photons from nearby O‑ and early B‑type stars, yielding characteristic optical emission (notably Hα) and radio free–free radiation that mark sites of recent massive star formation. According to the Encyclopaedia Britannica, typical gas temperatures are about 8,000 K and particle densities span roughly 10–10^5 cm⁻³, with sizes ranging from tens to hundreds of light‑years. Britannica; Essential Radio Astronomy, NRAO.

Physical conditions and ionization balance

The structure of an H II region is governed by photoionization–recombination equilibrium and thermal balance, classically idealized by the Strömgren sphere concept in which ionizations by a central source are balanced by case‑B recombinations; the sphere’s radius depends on the ionizing photon rate and ambient density. Strömgren’s original analysis appeared in 1939; comprehensive modern treatments are given in standard nebular-astrophysics texts. book://Donald E. Osterbrock|Astrophysics of Gaseous Nebulae and Active Galactic Nuclei|University Science Books|2006; [book://A. G. G. M. Tielens|The Physics and Chemistry of the Interstellar Medium|Cambridge University Press|2005).

Electron temperatures in classical H II regions are typically 7,000–10,000 K, set by the balance of photoelectric heating and line cooling by metals; ultracompact regions reach much higher densities (n_e ≳ 10^4–10^5 cm⁻³) while maintaining similar temperatures. Britannica; Annual Review: Ultra‑Compact H II Regions.

Morphology and environment

H II regions form where massive stars ignite within or adjacent to a Molecular cloud, and the ionized zone typically borders a neutral Photodissociation region (PDR) where far‑UV photons heat and dissociate gas without ionizing hydrogen. PDR physics and diagnostics explain strong C II 158 μm and O I fine‑structure lines from the neutral interface. Annual Review: Dense PDRs; [book://A. G. G. M. Tielens|The Physics and Chemistry of the Interstellar Medium|Cambridge University Press|2005).

Observed morphologies include spherical, blister/cometary “champagne flows” where ionized gas expands down a density gradient, and shell or bubble structures inflated by stellar winds and supernovae. Analytic and numerical models trace an initial R_S phase followed by pressure‑driven D‑type expansion; blister and champagne‑flow scenarios are common at cloud edges. MNRAS overview of classical expansion; MNRAS hydrodynamics of cometary H II regions.

Subclasses and early phases

Ultracompact (UC) H II regions are tiny (≲0.1 pc), dense (n_e ≳ 10^5 cm⁻³) nebulae still embedded in dusty natal clouds and best seen in the infrared and radio; they mark the earliest observable stages of massive stars. Annual Review: Ultra‑Compact H II Regions; Britannica on ultracompact H II.

Radiation and emission diagnostics

At optical wavelengths, Balmer recombination lines (Hα, Hβ) and collisionally excited “forbidden” lines such as O III λ5007, N II λ6584, and S II λλ6716, 6731 diagnose temperature, density, ionization parameter, and abundances; at radio wavelengths, thermal bremsstrahlung (free–free) continuum and radio recombination lines provide extinction‑free probes of the ionized plasma and ionizing photon budget. book://Donald E. Osterbrock|Astrophysics of Gaseous Nebulae and Active Galactic Nuclei|University Science Books|2006; Essential Radio Astronomy, NRAO.

In galaxy‑scale studies, H II region spectra are separated from non‑stellar ionization using the Baldwin–Phillips–Terlevich diagram, which employs O III/Hβ versus N II/Hα (and related ratios) to distinguish H II regions from AGN and shocks. journal://Publications of the Astronomical Society of the Pacific|Classification parameters for the emission‑line spectra of extragalactic objects|1981; A&A WHaD/diagnostics overview.

Gas‑phase metallicity in extragalactic H II regions can be estimated via “strong‑line” methods such as N2 and O3N2 calibrated against direct T_e measurements; usage and limitations are widely discussed in the literature. MNRAS review of calibrations; MNRAS discussion of N2/O3N2 (PP04/M13).

Role in galaxies and surveys

Because ionizing photons come from short‑lived massive stars, H II regions map current star formation and underpin widely used Hα star‑formation rate calibrations in nearby and distant galaxies. journal://Annual Review of Astronomy and Astrophysics|Star Formation in Galaxies Along the Hubble Sequence|1998; A&A SFR calibrations from emission lines.

In the Milky Way, mid‑infrared surveys (WISE) combined with radio continuum and recombination lines have produced the largest catalogs of H II regions and candidates (>8,000 entries), enabling studies of Galactic structure and spiral arms. ApJS WISE catalog summary (WVU repository); IPAC DOI entry for WISE Catalog v2.2.

Notable examples

The Orion Nebula (M42) is the nearest bright H II region, illuminated chiefly by the Trapezium cluster; Hubble imaging revealed hundreds of externally irradiated protoplanetary disks (“proplyds”), directly linking H II environments to early planetary‑system formation. ESA/Hubble proplyds; NASA Hubble press imagery and description.

The Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud is the most massive and largest H II region in the Local Group; its core cluster NGC 2070 contains the extremely dense R136 aggregate hosting several of the most massive known stars, and its hot, young population excavates cavities and drives superbubbles. NASA Science: 30 Doradus; NASA Science: Heart of the Tarantula.

NGC 604 in M33 is a prototypical giant extragalactic H II region whose luminosity and extent are powered by a large OB association rather than a compact cluster, providing a contrast to 30 Doradus. Britannica general context; see nebular diagnostics in [book://Donald E. Osterbrock|Astrophysics of Gaseous Nebulae and Active Galactic Nuclei|University Science Books|2006).

Observational techniques

Optical narrowband imaging and spectroscopy (Hα, [O III], N II, S II) trace ionized gas morphology and kinematics but are affected by dust extinction; radio free–free continuum and recombination lines penetrate dust to yield electron temperatures, emission measures, and ionizing photon rates; mid‑IR imaging delineates PDRs via PAH emission at 8–12 μm surrounding 20–24 μm hot dust. Essential Radio Astronomy, NRAO; IPAC/WISE catalog description; Annual Review: Dense PDRs.

Historical development and nomenclature

The modern physical picture was established using emission‑line spectroscopy and the Strömgren equilibrium framework; “H II” follows spectroscopic notation where the Roman numeral denotes ionization state (II = once ionized). Classic texts synthesize the atomic physics, radiative transfer, and diagnostics that remain foundational to H II region studies. Strömgren 1939; [book://Lyman Spitzer|Physical Processes in the Interstellar Medium|Wiley|1978); [book://Donald E. Osterbrock|Astrophysics of Gaseous Nebulae and Active Galactic Nuclei|University Science Books|2006].

Strömgren sphere: an idealized ionized volume in photoionization equilibrium around a hot star. Photodissociation region: a predominantly neutral layer heated by far‑UV radiation adjoining an H II region. Baldwin–Phillips–Terlevich diagram: an emission‑line diagnostic to classify ionization sources. Orion Nebula: a nearby H II region in Orion with rich young stellar content. Tarantula Nebula: giant H II region 30 Doradus in the LMC. Molecular cloud: cold, dense interstellar cloud where stars form.