Is there a specific limit that defines the boundaries of each classification category? For example, a main sequence G class star is between X and Y temperatures and has an absolute magnitude between A and B Solar units. Or, an O hypergiant is between X and Y temperatures and has an absolute magnitude between A and B Solar units. Along those lines. Essentially, something mathematical that can be used to plug in the characteristics of a star and will produce the classification. So, if I have a star that has a temperature of 5699K and a Mabs of 0.9M⊙, it pops out with a G2IV (the "2" is just a guess from eyeballing an HR chart). Or, are the classifications not that explicit?