Spectral Types and Surface Temperatures. Spectral types are a measure of the surface temperatures of stars and are letter designations: O, B, A, F, G, K, and M. Type O stars have the highest surface temperatures and can be as hot as 30,000 Kelvins. On the other extreme, type M stars have the lowest surface temperatures and can be as cool as 3,000 K. Hertzsprung-Russell Diagram The most significant feature of the HR Diagram is that the stars are not distributed in it at random (showing any combinations of absolute magnitudes and spectral types), but rather group themselves only into certain parts or regions of the diagram thus forming several groups.