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How does an H II region differ from a planetary nebula?
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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.
While both H II regions and planetary nebulae are types of nebulae consisting of ionized gas, they differ significantly in their origins, physical characteristics, and evolutionary stages.
Key Differences
- Origin and Evolution: An H II region is a cloud of interstellar matter ionized by the radiation of massive, hot, young stars, often associated with active star formation within molecular clouds. In contrast, a planetary nebula consists of an expanding shell of gas expelled by a dying, low-mass star (a former red giant) late in its evolution.312
- Structure and Appearance: H II regions typically lack regular structure, appearing chaotic or patchy with luminous filaments. Planetary nebulae generally exhibit more compact, regular, and often circular or ring-like forms.314
- Density and Brightness: Planetary nebulae are considerably denser than H II regions, often containing 1,000–10,000 atoms per cubic cm, and possess a surface brightness roughly 1,000 times greater.4
- Ionization Levels: The central stars of planetary nebulae can reach much higher temperatures (up to 200,000 K) than the stars exciting H II regions (less than 60,000 K), leading to higher stages of ionization in planetary nebulae, such as doubly ionized helium.4