A red giant star is a star that is in the last stages of its stellar evolution. That is, a star that is slowly fading. In a few billion years, our own Sun will turn into a red giant star. It will expand and engulf numerous planets, possibly even Earth.
How are they formed?
Most of the stars in the universe are in their main sequence, converting hydrogen to helium through nuclear fusion. A main sequence star can have a mass between one third and eight times that of the Sun, occasionally burning hydrogen in its core. During its life, the external pressure of fusion is balanced by the internal pressure of gravity. However, once the merger stops, gravity compresses the star and it begins to contract, triggering the transformation of the star into a red giant.
This has as a first effect an increase in the volume of the star and a cooling of the surface, which causes the color of the star to become more reddish. Red giant stars reach sizes of one hundred million to one billion kilometers in diameter. That is, one hundred to one thousand times the size of the Sun. Also, because energy is distributed over a larger space, surface temperatures are cooler. This difference in temperature causes stars to glow in the reddest part of the spectrum, although in reality they tend to appear more orange.
The future of the Sun
In approximately five billion years, the Sun will begin the process of burning helium, thus becoming a red giant star. When it expands, its outer layers will swallow up Mercury and Venus first, eventually reaching Earth. Scientists still debate whether or not our planet will be eaten, although what is certain is that life as we know it will cease to exist.
However, the Sun could give life to other planets. When stars transform into red giants, the habitable zones of your system change. A habitable zone is one where liquid water can exist, which is essential for life to evolve. Because a star remains a red giant for at least a billion years, it is possible for life to emerge on other bodies in the outer solar system.
