[renormalised (Carl)]

Shelley, MS = Main Sequence or your ordinary, core hydrogen fusing, stars.

A.K.A Luminosity Class V...which is where you get part of the notation for the written spectral classes of stars, e.g. our Sun is a G2V class star. A hypergiant is class 0, supergiant I, bright giant II, giant III, subgiant IV, sub dwarf VI and the white dwarfs are VII.

Then you have the HR diagram's MK (Morgan-Keenan) sequence...W, O, B, A, F, G, K, M, L, T and now a new class Y, for the dimmest and coolest brown dwarfs. You also have subclasses of stars that are generally lumped in with M....C (carbon stars, that are made up of old classes R and N) and S (shell stars, which can also be C class stars as well). Each spectral class is also divided into 10 subclasses, from 0-9...G0-G9, for example (they can also be divided up into 10 sub-subclasses as well, e.g. G2.5, B1.9 etc). Traditionally, the hottest of the O class stars was O3, but that maybe revised to account for some of the very hottest and largest of MS stars, like those recently found in R136a in the LMC.

Also, with W class stars (the Wolf Rayet's), you have classes WC and WN, depending on whether they exhibit abundant carbon or nitrogen in their spectra. All W class stars have hydrogen deficient spectra as they've basically blown off their outer layers and are essentially the exposed cores of the original stars.