Stars are labeled occarding to a letter nomenclature. The classes are O (hotter), B, A, F, G, K, M (cooler). Each spectral class is divided into 10 subclasses, ranging from 0 (hottest) to 9 (coolest). Stars are furthermore categorised into six additional categories according to their luminosity, using 1a (most luminous supergiants), 1b (less luminous supergiants), II (luminous giants), III (normal giants), IV (subgiants), and V (main sequence and dwarfs).
Classes R, N, S, T, Q and W do exist too and are being used for relatively rare star types.
Class O - Dark Blue
Class O stars consist of ionized atoms (especially helium). They have temperatures between 28,000 and 50,000 Kelvin. An example of a Class O star is Mintaka, which is a O1-3III type.
Class B - Blue
Class B stars consist of neutral helium mixed with some hydrogen. They have temperatures between 10,000 and 28,000 Kelvin. An example of a class B star is Alpha Eridani A, which is a B3V-IV type.
Class A - Light Blue
Class A stars consist of strong hydrogen mixed with some ionized metals. They have temperatures between 7,500 - 10,000 Kelvin. An example of a class A star is Sirius A, which is a A0-1V type.
Class F - White
Class F stars consist of hydrogen and ionized metals mixed with calcium and iron. They have temperatures between 6,000 - 7,500 Kelvin. An example of a class F star is Procyon A, which is a F5V-IV type.
Class G - Yellow
Class G stars consist of ionized calcium mixed with both neutral and ionized metals. They have temperatures between 5,000 - 6,000 Kelvin. An example of a class G star is Sol, which is a G2V type.
Class K - Orange
Class K stars consist of neutral metals. They have temperatures between 3,500 - 5,000 Kelvin. An example of a class K star is Alpha Centauri B, which is a K0-3V type.
Class M - Red
Class M stars consist of ionized atoms (especially helium). They have temperatures between 2,500 - 3,500 Kelvin. An example of a class M star is Wolf 359, which is a M5-8V type.
