Here's a breakdown of why:
* Main Sequence: Stars spend the majority of their lives fusing hydrogen into helium in their core, a phase known as the main sequence.
* Red Giant: When hydrogen runs out, stars like our Sun enter a red giant phase, expanding and cooling as they fuse helium into heavier elements.
* Planetary Nebula: Eventually, the outer layers of a red giant are shed, forming a planetary nebula.
* White Dwarf: The remaining core, consisting mostly of carbon and oxygen, becomes a white dwarf. These are extremely dense and hot, but they have no internal fuel source. They gradually radiate their heat into space, slowly cooling over billions of years.
* Black Dwarf: The final stage, a theoretical stage called a black dwarf, is a white dwarf that has cooled down completely and emits no light. The universe is not old enough for any black dwarfs to have formed yet.
So, while the process of a star cooling and fading away starts with the white dwarf phase, the eventual endpoint of this process is a black dwarf.
