But it was too late to reassign the letters. The sequence could be cut even more finely. Cannon subdivided each letter with numbers from 0 to 9, so that a spectrum whose appearance placed it halfway between standard G 0 and K 0 stars was called G 5. Using this scheme, Cannon led the classification at Harvard of 325,300 spectra recorded on wide-field photographs. A few other spectral types don't fit the sequence but instead parallel it.
Emission lines indicate an especially large, thick shroud of hot gas surrounding a star.
Some giant stars at the cool end of the spectrum have an excess of carbon. In other words, a carbon star's atmosphere is a red filter. The rare type- S stars are also red giants. They parallel type M but show strong bands of zirconium oxide and lanthanum oxide instead of an M star's titanium oxide. We can imagine that planets of S stars, bathed in chemically peculiar stellar winds, might be encrusted with gems of cubic zirconia.