Death of Sun-Like Stars: Red Giants and White Dwarfs

## Death of Sun-Like Stars

Stars with masses up to about 8 solar masses end their lives gently compared to their more massive siblings. The process transforms a stable main-sequence star into a spectacular planetary nebula surrounding a tiny, dense white dwarf.

### Leaving the Main Sequence

When hydrogen is exhausted in the core, fusion ceases and the core contracts. A hydrogen-burning shell ignites around the inert helium core, and the outer layers expand dramatically. The star becomes a **red giant**, growing to 100-200 times its original radius.

### The Helium Flash

As the core contracts and heats to about 100 million K, helium fusion ignites — for stars below ~2.25 solar masses, this happens in a violent thermal runaway called the **helium flash**. In just seconds, the core produces energy at a rate comparable to the entire Milky Way galaxy, though this energy is absorbed internally and not directly visible.

### The Asymptotic Giant Branch

After helium core fusion exhausts, the star enters the asymptotic giant branch (AGB), with alternating hydrogen and helium shell burning. Thermal pulses drive intense mass loss, shedding the star's outer layers into space.

### Planetary Nebulae

The ejected shell of gas, illuminated by the hot exposed core, creates a **planetary nebula** — among the most beautiful objects in the sky. The Ring Nebula, Helix Nebula, and Cat's Eye Nebula are famous examples. Despite the name, they have nothing to do with planets.

### The White Dwarf

The remaining core, typically 0.5-0.8 solar masses, becomes a **white dwarf** — an Earth-sized object supported by electron degeneracy pressure. It slowly cools over billions of years, eventually fading to a hypothetical black dwarf (none yet exist, as the universe is not old enough).