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How does diffuse ionized gas differ from the denser gas in H II regions?
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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.
H II regions and diffuse ionized gas differ primarily in their density, distribution, and ionization states:
- Density: H II regions are relatively dense, with particle counts typically ranging from 10 to 100,000 atoms per cubic centimeter. In contrast, diffuse ionized gas is much more dilute, with average electron densities of approximately 0.05 per cubic centimeter.12
- Distribution: H II regions are generally concentrated near the galactic plane, often associated with molecular clouds and massive, hot stars. Diffuse ionized gas is more widespread, extending thousands of light-years above and below the galactic plane, far beyond the typical thickness of H II regions.12
- Ionization States: Because of its extremely low density, diffuse ionized gas exhibits lower stages of ionization for elements like oxygen, nitrogen, and sulfur compared to the higher stages found in typical H II regions. While H II regions are often powered by a central star or cluster within a cavity, the diffuse ionized gas is thought to be ionized by radiation escaping from O and B stars located in the galactic plane.21