From UFStarfleet Wiki
Each spectral class is divided into 10 subclasses, ranging from 0 (hottest) to 9 (coolest). Stars are also divided into six catagories according to luminosity; 1a (most luminous supergiant), 1b (less luminous supergiant), II (luminous giants), III (normal giants), IV (subgiants) and V (main sequence and dwarfs). For instance, Pinastri is classified as G2V, which means that is a relatively hot G-Class (or G-Type) main sequence star. In addition, classes R, N, S, T, Q and W are relatively rare star types not found in the main sequence.
Class O - Dark Blue
Temperature: 28,000-50,000 Kelvin
Composition: Ionized atoms, especially helium.
Example: Mintaka (O1-3III)
Class B - Blue
Temperature: 10,000-28,000 Kelvin
Composition: Natural helium, some hydrogen.
Example: Alpha Eridani A (B3V-IV)
Class A - Light Blue
Temperature: 7,500-10,000 Kelvin
Composition: Strong hydrogen, some ionized metals
Example: Sirius A (A0-1V)
Class F - White
Temperature: 6,000-7,500 Kelvin
Composition: Hydrogen and ionized metals, calcium and iron.
Example: Procyon A (F5V-IV)
Class G - Yellow
Temperature: 5,000-6,000 Kelvin
Composition: Ionized calcium, both neutral and ionized metals
Example: Pinastri (G2V-LV)
Class K - Orange
Temperature: 3,500-5,000 Kelvin
Composition: Neutral metals.
Example: Alpha Centauri B (K0-3V)
Class M - Red
Temperature: 2,500-3,500 Kelvin
Composition: Ionized atoms, especially helium
Example: Wolf 359 (M5-8V)
Hertzsprung-Russell Star Classification Chart
This diagram plots the spectral class or temperature of stars against their absolute magnitude (brightness or luminosity). About 90% of the stars can be found in the Main Sequence, and remain there during their long lifetime of burning hydrogen. When a star has used up all of the hydrogen in its care, it leaves the main sequence and becomes a Red Giant; very massive stars may become Red Supergiants.