1. Expansion into a Red Giant:
* The Sun's core, now mostly helium, will contract under its own gravity, increasing its temperature and density.
* This heat will cause the outer layers of the Sun to expand dramatically, making it a red giant.
* The Sun will engulf Mercury and Venus, and possibly even Earth.
2. Helium Fusion:
* The core will eventually become hot enough (around 100 million degrees Celsius) to start fusing helium into carbon and oxygen.
* This helium fusion will be much more energetic than hydrogen fusion, causing the Sun to briefly become even brighter.
3. Instability and Ejection of Outer Layers:
* Helium fusion is unstable and will not last as long as hydrogen fusion.
* The Sun will pulsate, shedding its outer layers in a series of giant eruptions.
* These eruptions will form a planetary nebula, a beautiful cloud of gas and dust illuminated by the hot core of the Sun.
4. White Dwarf:
* The remaining core, now mostly carbon and oxygen, will cool and shrink into a white dwarf.
* A white dwarf is a dense, Earth-sized object that emits very little light. It will eventually fade into darkness over billions of years.
Timeline:
This process will take a very long time. The Sun is about halfway through its hydrogen-burning phase, and it has about 5 billion years left. After that, it will take another few billion years for the Sun to go through the red giant and white dwarf phases.
Important Note: While the Sun's evolution is a well-understood process, it is impossible to predict exactly what will happen to the Earth during the red giant phase. The Earth might be engulfed, it might be pushed further out into the solar system, or it might be completely destroyed.
Overall, the death of the Sun will be a spectacular and dramatic event, but it is not something we need to worry about for billions of years.
