A red giant star ranges from 62 million to 621 million miles in diameter, or 100 to 1,000 times the size of the sun. However, red giants have cooler temperatures than the sun because the energy travels over a larger surface area.
A red giant star is in the last phase of its life. It forms when nuclear fusion stops and gravity forces the star to contract. Temperatures increase, and helium fuses into carbon. The resulting explosive energy makes the star expand in size and turns it into a red giant. A red giant is so named because its temperature, which reaches 4,000 to 5,800 F, makes the star shine in the red part of the light spectrum.