Stellar type

When Astronomers refer to a star's stellar type they refer to its color. Letter codes were applied to those colors before the colors were well understood, so they are not in order. The types, in order by color, are "O B A F G K M".

The color of Sol, our sun, makes it a G type star. O B A F stars are more massive than Sol, much brighter than Sol, and run out of Hydrogen much faster than Sol. These stars are less common than stars like Sol, but most of the stars visible to the naked eye are O B A F stars.

Ninety percent of stars are less massive than Sol, less bright than Sol, and are expected to have much longer lifespans than Sol.

The reason why a star's lifespan and brightness depend on its mass is that the energy consumed keeping the star's gravity from forcing it to collapse goes up as the cube of the mass. Stars form from loose clouds of interstellar gas. If tbe cloud is massive enough its gravity forces it to form a Denser core. As the core grows more dense this gravitational collapse heats it up. If it is massive enough the centre of the core will become hot enough, and dense enough, for Hydrogen fusion. When Hydrogen, the most common element in the Universe, fuses to Helium, a huge amount of energy is released. During their Hydrogen fusion period stars are said to be on the main sequence.

Main sequence stars in the K and M classes are dimmer than Sol. K stars are considered to be orange dwarf stars, that fuse their Hydrogen over a lifespan of up to 20 billion years. Only a few very nearby orange dwarf stars can be seen with the naked eye. M stars are considered to be red dwarf stars and they can take up to 3 trillion years to fuse all their Hydrogen.