About 90 percent of all stars are main sequence stars. Main sequence stars are the fourth stage of a star's evolution. Nuclear fusion occurs during this stage of a star's life cycle; this is where a star burns hydrogen to make helium.
Stars in the main sequence stage can be very bright. Their brightness depends upon their temperatures. The sun is a main sequence star. It is categorized as a yellow dwarf because of its relatively small size. As small stars burn up all of their hydrogen, they produce cooler temperatures and are categorized as red dwarfs. Red dwarfs are the most common type of stars. Red giants are cooling stars that are massive in size.