Sooner or later, a main sequence star's core starts to run out of hydrogen. This makes the star expand to perhaps 1000 times the volume of the Sun. Indeed, in another five billion years, the Sun itself will expand and consume both Mercury and Venus and maybe even the Earth.
Being bigger but with fewer core reactions means that that star's surface cools to about 3000 kelvin* and therefore turns to a deep red colour. The star also produces silicon, oxygen, carbon and iron.
The main sequence star has become a red giant.