type SpectralClass
Spectral classes, hot to cool, then the ones that are not on that sequence.
D is a white dwarf and L/T/Y are brown dwarfs — neither belongs on the
OBAFGKM temperature ladder, which is why classIndex returns null for them
and interpolating a temperature from the ladder is not a thing callers may do.
Type
'O' | 'B' | 'A' | 'F' | 'G' | 'K' | 'M' | 'L' | 'T' | 'Y' | 'W' | 'C' | 'S' | 'D'