What do we mean by a star's spectral type, and how is spectral type related to surface temperature and color. Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look redder to us, and cooler stars look bluer. Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the coolest and M are the hottest. Hotter stars look redder to us, and cooler stars look bluer. Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look bluer to us, and cooler stars look redder. Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the coolest and M are the hottest. Hotter stars look bluer to us, and cooler stars look redder.