How does a star turn into a red giant?

Answer 1

A star turns into a red giant when it lose fuel and die. If a star isn't a massive star then it expands into a red giant. After that it becomes a white dwarf. If its massive then it collapses on itself and turns into a black hole.

Answer 2

As a star ages, it runs out of hydrogen fuel in its core and starts burning helium. This process causes the star's core to contract and its outer layers to expand, creating a red giant. The star's increased size and decreased temperature cause it to appear red in color.