Star type M, the red stars, are the coolest stars with an average surface temperature of under 3,500 K. This type of star has a 0.3 solar mass (M☉) and an average radius of 0.4 solar radius (R☉). Examples of red stars are Betelgeuse and Antares.
Stars are classified by the elements they absorb and their temperature. The seven main types of stars in decreasing temperature are O, B, A, F, G, K and M. O, B and A star types have approximate surface temperatures of 7,500 to over 25,000 K and are blue. F star types have an approximate surface temperature of 6,000 to 7,500 K and are blue to white.
G star types range in temperature from 5,000 to 6,000 K and are white to yellow. K star types range in temperature from 3,500 to 5,000 K and are orange to red. The last start type is M, and they have an approximate surface temperature of under 3,500 K and are red.