subtypes - (n.)

designated by Arabic numerals. Further, stars are sorted by intrinsic brightness into luminosity classes designated by the first five Roman numerals. In turn, these are subdivided into a small number of subclasses designated by the first few letters of the lower case English alphabet; for example, the Sun is a G2 V star (also sometimes denoted as a dwarf G2 star) and Betelgeuse (a Orionis) is classified as M2 lab (i.e., intermediate between la and lb).