When does an average star turn into a red giant?

An average star, like our Sun, typically becomes a red giant after it exhausts the hydrogen fuel in its core, which usually occurs after about 10 billion years of fusion. As hydrogen depletes, the core contracts under gravity, raising the temperature until helium fusion begins. This process causes the outer layers to expand and cool, transforming the star into a red giant. The red giant phase can last for several hundred million years before the star eventually sheds its outer layers, leaving behind a white dwarf.