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What happens during the early stage of planetary nebula evolution?
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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.
Before the ejection of a planetary nebula, the central star exists as a red giant. During this pre-ejection phase, the star experiences rapid mass loss, shedding up to 0.01 Earth masses per day through a slowly expanding stellar wind. This star may be heavily obscured by dust formed from heavy elements within that wind.1
As the star loses its overlying atmosphere, its hot core is exposed, causing the star to become hotter. This radiation ionizes the surrounding gas, creating an ionization zone that moves steadily outward through the material previously ejected by the stellar wind. During this early stage:
- Appearance: The nebulae are bright but appear starlike from Earth because they are too small to resolve as a disk.1
- Density: The gas is relatively dense—about one million atoms per cubic centimetre—though it becomes more dilute as it expands.1
- Expansion: The ionized shell effectively eats into the surrounding neutral hydrogen, causing the nebula to appear to expand faster than the individual gas atoms are moving.1