Interstellar ionized atomic hydrogen regions are composed of gas ionized by extreme ultraviolet (EUV) photons with energies greater than the hydrogen ionization energy of 13.6 eV. These objects include HII regions ionized by hot O and B-type stars and associated with regions of recent massive star formation. HII regions range in size from parsec-scale nebulae ionized by a few O or B stars (e.g., the Orion Nebula), up to 100 parsec-scale complexes ionized by clusters containing hundreds of O stars (e.g., 30 Doradus in the Large Magellanic Cloud). They have diverse morphologies, due to the clumpy and inhomogeneous distribution of gas and stars inside them.
The UV, visible and IR spectra of HII regions contain many emission lines, primarily recombination lines of hydrogen and helium and collisionally excited lines of metal ions. HII regions are also observed at radio wavelengths, primarily free-free radio emission from thermalized electrons and radio recombination lines from highly excited states of hydrogen, helium and metals. In the visible spectrum, the dominant emission line is the 656.3 nm H-alpha recombination line. The photographs of HII regions in this gallery were taken using a 3 nm bandwidth filter centered on the H-alpha recombination line to reduce background noise and increase contrast.
The gallery is divided into two sections, Photographs and Descriptions. Each photograph is cross-linked to a description that contains a map of the objects in the photograph, a brief explaination of the objects, and technical information about the photograph.
All of the photographs were acquired with the same 530 mm f/5 telescope and camera and are uncropped with a 1.9° x 1.5° field of view and a 4.2 arcsecond per pixel image scale.