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Why are planetary nebulae brighter at their surfaces than 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 are significantly brighter at their surfaces than H II regions primarily because they are much denser. While H II regions typically contain about 10 atoms per cubic centimeter, planetary nebulae are considerably more compact, containing between 1,000 and 10,000 atoms per cubic centimeter in their dense regions. This higher density results in a surface brightness that is approximately 1,000 times greater than that of typical H II regions.12