Star Trek Spectral Classification

"Stars can generally be classified by their color. A purple star is always Class O, a yellow star is always Class G. However, there can be vast differences within each class. Most Class M stars are tiny and don't emit much light... however, many K- and G-type stars can evolve into red giants. That makes them Class M, but they are NOTHING like the tiny stars that make up the vast majority of Class M stars. Since most stars spend most of their lives on the Main Sequence, the descriptions on this page can be applied to the majority of stars. The oddballs and outsiders don't necessarily fit the exact parameters here (and will be discussed elsewhere), but even the oddballs conform to the interstellar color scheme, so even a blue oddball is class B."