After the red giant phase, the Sun will go through the following stages as it ends its life cycle:
1. Helium Flash and the Horizontal Branch (or Asymptotic Giant Branch):
• Helium Fusion: As the Sun's core contracts and heats up, it will begin fusing helium into carbon and oxygen. This starts with a "helium flash"—a rapid onset of helium fusion in the core.
• Asymptotic Giant Branch (AGB): After the helium flash, the Sun will enter a phase where it alternates between contracting and expanding, leading to significant mass loss. It will expand further, becoming a more unstable giant.
2. Planetary Nebula:
• Outer Layers Shed: The Sun will shed its outer layers due to intense solar winds. These layers will be expelled into space, creating a beautiful glowing shell of gas and dust called a planetary nebula. The core that remains behind is extremely hot and dense.
3. White Dwarf:
• Core Remains: The leftover core, which is mainly composed of carbon and oxygen, will become a white dwarf. It will no longer undergo fusion but will remain very hot (around 100,000 K initially) and will slowly cool and fade over billions of years.
• Stability: The white dwarf will gradually radiate away its heat, shrinking and cooling over time. Eventually, it will become a cold, dark "black dwarf," though this will take longer than the current age of the universe (more than 10 billion years).
Summary of the Process:
• Red giant phase (fusion of hydrogen and helium)
• Helium flash and transition to AGB
• Planetary nebula (expulsion of outer layers)
• White dwarf (remnant core)
• Black dwarf (after billions of years)
So, the next phase after the red giant stage is the shedding of the outer layers to form a planetary nebula, leaving behind the white dwarf core. This marks the Sun's transformation from an active star to a faint, cooling remnant.
