Final answer:
Spectral types are a classification system used to categorize stars based on their temperatures, with the sequence in order of decreasing temperature being O, B, A, F, G, K, M, L, T, and Y.
Step-by-step explanation:
Spectral types are a classification system used to categorize stars based on their temperatures, using the characteristics of their spectra. The sequence of spectral types in order of decreasing temperature is O, B, A, F, G, K, M, L, T, and Y. The spectral classes O and B represent the hottest stars, while the spectral classes L, T, and Y represent the coldest star-like objects known as brown dwarfs.