Here's why:
* Hydrogen Fusion: Stars primarily produce energy through nuclear fusion, where hydrogen atoms are converted into helium. This process occurs in the star's core.
* Hydrogen Depletion: As a star ages, the hydrogen in its core is gradually depleted.
* Core Contraction: Without hydrogen fuel, the core contracts under its own gravity, leading to a rise in temperature and pressure.
* Shell Burning: This increased pressure and temperature ignites hydrogen fusion in a shell surrounding the core, causing the star's outer layers to expand significantly.
* Red Giant Phase: The star's expansion cools its surface, giving it a reddish hue. This marks the beginning of the red giant phase.
While the term "red giant" is most common, these stars can also be called "subgiants" depending on their specific stage of evolution.
