A star is born. The expanding force of the fusion reactions and the compressing force of gravity keep the star in balance. A main sequence star in the H-R diagram, for example, our own Sun . Dwarfs are in the luminosity class V. Stars below dwarfs are divided into red dwarfs, subdwarfs, and white dwarfs . Above are giants .
Luminosity class describes the absolute brightness, or magnitude, of a star.
This is called the Yerkes classification. A supergiant is the largest of all stars. The surface temperature of a red giant is usually under 5,000 degrees, causing its colour, and giving rise to its popular name of red giant. Our Sun will also become a red giant when it stops fusing hydrogen. It will expand so much that our planet will be swallowed up.