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How are planetary nebulae different from H II regions?
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Planetary nebula
Southern Ring Nebula as captured by the James Webb Space Telescope in the near-infrared (left) and the mid-infrared (right). The dimmer star at the center of this scene has been sending out rings of gas and dust for thousands of years in all directions. The Southern Ring Nebula is approximately 2,500 light-years away.
Planetary nebulae and H II regions differ significantly in their physical characteristics, origins, and structures:
- Size and Density: Planetary nebulae are relatively small, typically having a radius of about 1 light-year and containing approximately 0.3 solar masses of gas. They are significantly denser than most H II regions, with densities of 1,000β10,000 atoms per cubic cm, and possess a surface brightness 1,000 times greater. In contrast, H II regions vary widely in size, with typical galactic examples measuring about 30 light-years in diameter and having lower average densities of roughly 10 atoms per cubic cm.12
- Structure: Planetary nebulae generally exhibit regular, compact, and often ring-like shapes with sharp boundaries. H II regions are typically chaotic, patchy, and lack regular structure or sharp boundaries.132
- Evolutionary Origin: A planetary nebula is an expanding shell of gas recently expelled by a dying star in a late stage of its evolution. H II regions are clouds of interstellar matter ionized by massive, hot stars, often associated with star-forming molecular clouds.32
- Ionization: The central stars of planetary nebulae are much hotter (up to 200,000 K) than the exciting stars of H II regions (less than 60,000 K), leading to higher stages of ionization in planetary nebulae.1