Input (grade 13): Most stars are currently classified using the letters O, B, A, F, G, K and M, where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class.
Output (grade 3): Spectral types are named with a letter: M, K, G, F, A, B and O. M stars are the coldest stars and O stars are the hottest.