Here's why:
* Nuclear Fusion Changes: Stars like our Sun spend most of their lives fusing hydrogen into helium in their core. As this hydrogen runs out, the core contracts and heats up.
* Helium Fusion: Eventually, the core gets hot enough to start fusing helium into heavier elements like carbon and oxygen. This fusion process is less efficient than hydrogen fusion and produces a lot of energy, causing the outer layers of the star to expand dramatically.
* Red Color: The star's surface cools as it expands, giving it a reddish color.
* Giant Size: The star becomes much larger than its original size, hence the term "giant."
Note: The specific details of the red giant phase vary depending on the star's initial mass. Larger stars become even more massive red supergiants.
