Star Classifications
Type O Star (Violet)  -  25,000 K +
Type B Star (Blue)  -  10,000 to 25,000 K
Type A Star (White)  -  7,500 to 10,000 K
Type F Star (White/Yellow or Green)  -  6,000 to 7,500 K
Type G Star (Yellow)  -  5,000 to 6,000 K
Type K Star (Orange)  -  3,500 to 5,000 K
Type M Star (Red Giant)  -  2,000 to 3,500 K
Class O stars are very hot and luminous, being bluish in color, with most of their output being in the ultraviolet range. These are the rarest of all main sequence stars, constituting as few as 1 in 32,000. Class O stars shine with over a million times that of Earth's sun. These stars have dominant lines of absorption and due to their immense size, they burn their hydrogen fuel very quickly. This makes them the first stars to leave the main sequence.
Class B stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include magnesium and silicon. Since O and B stars are so powerful, they live for only a short time, and thus do not stray far from they area in which they formed. These stars tend to cluster together in what are called OB associations, which are associated with giant molecular clouds. These stars make up 0.13% of the main sequence of stars in the galaxy.
Class A stars are amongst the more common stars, making up 0.63% of all main sequene stars in the galaxy. They are most commonly white, either with a tinge of blue or green. They have strong hydrogen lines as well as lines of ionized metal. The lines of calcium is notably strengthened at this point.
Class F stars are still quite powerful but they tend to be main sequence stars. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their color is white with hints of yellow or green. They make up 3.1% of main sequence stars.
Class G stars are probably the best known star type, reason being that Earth's sun is a Class G. They have even weaker hydrogen lines than F, but along with ionized metals, they have neutral metals. G is host to the "Yellow Evolutionary Void", since supergiant stars tend to swing between O & B in the blue untraviolet and the K & M in the orange and red spectras. While they do this, they do not stay within the G classification for long, as this is an extremely unstable place for a subergiant to be. Class G's make up about 8% of the main sequence.
Class K stars are orangish in color which are slightly cooler than Earth's sun. Some K stars are giants and supergiants, such as the stars Arcturus, while others such as Alpha Centauri B are main sequence stars. They have extremely weak hydrogen lines and mostly neutral metals. By late Class K life, molecular bands of titanium oxide become present. These stars make up some 13% of main sequence stars.
Class M stars are by far the most common class of star, making up 76% of the main sequence of stars. These are mostly made up of Red Dwarf stars such as Proxima Centauri. M is also host to most giants and supergiants such as Antares and Betelgeuse, as well as the Mira variables. Class M also holds the hotter Brown Dwarfs that are within the spectrum. Titanium and Vanadium oxides can become present in this class of star.