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What changes occur before a red giant ejects the gas that becomes a planetary nebula?
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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 a red giant ejects its outer layers to form a planetary nebula, it undergoes several critical physical and evolutionary changes:
- Mass Loss: The star experiences a rapid loss of mass, sometimes reaching up to 0.01 Earth mass per day, through a slowly expanding stellar wind. This wind may contain dust formed from heavy elements, which can heavily obscure the star.1
- Nuclear Instability: The ejection is triggered by instabilities in the helium-burning shell, which cause rapid variations in the star's nuclear luminosity.2
- Convective Mixing: Internal convection currents transport products of nuclear fusion—such as nitrogen from the carbon-nitrogen cycle and carbon from helium burning—to the star's surface.2
- Radiation Pressure: The outward force of radiation acting on the star's outer layers eventually overcomes gravity, facilitating the ejection of the material that becomes the nebula.2
These processes occur as the star evolves along the asymptotic giant branch, eventually exposing its hot, dense core, which becomes a white dwarf.213