Red Giant Stars

On the other end of the spectrum are the red giant stars. While blue is the hottest color of stars, red is the coolest color they can have. A red giant is born when a star like our Sun reaches the end of its life and runs out of hydrogen fuel in its core. This forces the star to begin nuclear fusion with helium, increase in luminosity and bloat up many times its original size. When our Sun becomes a red giant, it will expand to consume the orbits of the inner planets, including Mercury, Venus and Earth.
When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star's life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf. Prominent bright red giants in the night sky include Aldebaran, Arcturus, and Gamma Crucis.
So, regular stars become regular red giants. But there are even larger red giants out there; the red supergiants. These are massive stars with more than 20 times the mass of the Sun. They enter the red giant phase of stellar evolution, but instead of merely expanding to the orbit of the Earth, they can expand to more than 1,500 times the radius of the Sun. Imagine a star that extended out past the orbit of Saturn.