Transcript: The original spectral classification sequence formed by Annie Cannon was alphabetical based on the strength of the hydrogen absorption features, but it turns out that stellar temperature, a more fundamental quantity, has a complex and not direct relationship with the strength of the hydrogen features. And so the original sequence got reordered with time. The current spectral sequence of stars going from hotter stars to cooler stars goes O, B, A, F, G, K, M. There’s been an extension in more recent years to the classes R, N, and S.