How Stars Are Classified     The color of a star reflects its surface temperature. The temperature of a star, in turn, is influenced by the star’s size.     Smaller stars produce less energy, so they generally have cooler surface temperatures than larger stars. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white. Color is the most common way to classify stars. The class of a star is given by a letter, and each letter corresponds to a color and range of temperatures.     Classification of Stars By Color and Temperature  ... Show more

How Stars Are Classified    
The color of a star reflects its surface temperature. The temperature of a star, in turn, is influenced by the star’s size.    
Smaller stars produce less energy, so they generally have cooler surface temperatures than larger stars. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white.

Color is the most common way to classify stars. The class of a star is given by a letter, and each letter corresponds to a color and range of temperatures.    

Classification of Stars By Color and Temperature    
Class Color Temperature Range Sample Star    
O Blue 30,000 K or more Zeta Ophiuchi    
B Blue-white 10,000–30,000 K Rigel    
A White 7,500–10,000 K Altair    
F Yellowish-white 6,000–7,500 K Procyon A    
G Yellow 5,500–6,000 K Sun    
K Orange 3,500–5,000 K Epsilon Indi    
M Red 2,000–3,500 K Betelgeuse, Proxima Centauri    

A class G star such as our sun is about twice as hot as a class M star such as Betelgeuse. Class B has stars about twice as hot as the sun. An example of a star in this class is Rigel.

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