Which class of stars are considered the coolest and are red in color?

The class of stars that are considered the coolest and appear red in color is the M class. M-type stars have surface temperatures that range from about 2,400 to 3,700 Kelvin, making them significantly cooler than stars in other classifications. The cooler temperature results in a reddish hue, which is why these stars are often referred to as red dwarfs.

This classification is part of the Morgan-Keenan (MK) system, which categorizes stars based on their spectral characteristics, including color and temperature. In contrast, other classes like B, A, and F have higher temperatures and exhibit blue to white colors. This fundamental distinction in surface temperature is what primarily characterizes M-type stars as the coolest, thus leading to their identification as red stars in the spectrum of stellar classifications.