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Where are H II regions located in the Milky Way Galaxy?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

In the Milky Way Galaxy, H II regions are primarily located within the galactic plane, where they are found intermixed with young stars, stellar associations, and the youngest open clusters.1

These regions are concentrated within the spiral arms of the Galaxy, though some are also located in the spaces between them. While they are distributed throughout the Galaxy, they are most numerous at an intermediate distance of approximately 10,000 light-years from the galactic centre. Because these regions are associated with areas of ongoing star formation, they are almost always found in proximity to molecular clouds.12

Astronomers map their distribution across the Galaxy by detecting the characteristic thermal radio radiation emitted by the ionized gas, which allows for observation even in distant parts of the Galaxy that are obscured from visible light.1