1. Main Sequence: A yellow star spends most of its life in the main sequence, fusing hydrogen into helium in its core. This is where the star is in its prime, like our Sun currently is.
2. Red Giant Phase: As the star runs out of hydrogen in its core, it begins to fuse helium into carbon and oxygen. This causes the star to expand significantly, becoming a red giant. The Sun is expected to become a red giant in about 5 billion years.
3. Planetary Nebula: The red giant phase is relatively short-lived. Eventually, the star's outer layers are ejected into space, forming a beautiful, colorful shell known as a planetary nebula.
4. White Dwarf: What remains at the core is a dense, hot object called a white dwarf. It's primarily made up of carbon and oxygen, with a mass similar to the Sun but compressed into the size of the Earth. It gradually cools and dims over billions of years, becoming a black dwarf, a cold and dark object.
Important Note: While we theoretically predict black dwarfs, we haven't observed any yet because the process of cooling down takes incredibly long.
