Here's a breakdown of what happens:
1. Hydrogen Fusion: A star like our Sun spends most of its life fusing hydrogen into helium in its core. This process generates energy, causing the star to shine.
2. Hydrogen Depletion: Eventually, the hydrogen fuel in the core runs out. The core contracts under its own gravity, becoming hotter and denser.
3. Helium Fusion Ignition: When the core temperature reaches about 100 million Kelvin, helium nuclei begin to fuse together to form carbon and energy. This is the helium flash. The sudden energy release from the helium flash expands the outer layers of the star, causing it to become a red giant.
4. Helium Burning: The star continues to burn helium in its core, fusing it into heavier elements like carbon, oxygen, and neon.
5. Evolution: As the core continues to fuse heavier elements, the star expands even further. It may undergo further fusion stages, forming even heavier elements, depending on its mass.
6. Stellar Remnant: Eventually, the star will run out of fuel and its core collapses under its own gravity. The outcome depends on the star's mass:
* Low-mass stars: They become white dwarfs, slowly cooling and fading away.
* Intermediate-mass stars: They experience a planetary nebula phase, shedding their outer layers and leaving behind a white dwarf.
* High-mass stars: They experience a supernova explosion, leaving behind either a neutron star or a black hole.
So, the process of helium fusion is the "second life" of a star like our Sun, allowing it to burn for billions of years after it exhausts its hydrogen fuel.
Important Note: The Sun's mass is about 1 solar mass. The "second life" of the Sun is relatively short compared to its hydrogen burning phase. It's expected that the Sun will become a red giant in about 5 billion years and eventually become a white dwarf.
