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Astronomers have developed a method of classifying stars based on their color and some other characteristics. The star classifications are O, B, A, F, G, K, M (you can remember that with the handy mnemonic, “Oh be a fine girl, kiss me”.) O stars are the most extreme group of all. They have the highest temperatures, the most luminosity, and the most mass (oh, and the shortest lives).
An O star appears blue to the eye, and can have a surface temperature of more than 41,000 Kelvin; its color would be better described as ultraviolet, but we can’t see that color with our eyes. The surface temperature of an O star is so great that hydrogen on the surface of the star is completely ionized, but other elements are more visible, like Helium, Oxygen, Nitrogen, and Silicon.
O stars are very massive and evolve very rapidly. Shortly after they form as a protostar, they already have the pressure and temperatures in their cores to begin hydrogen burning. The O stars light up their stellar nurseries with ultraviolet light and cause the clouds of nebula to glow. You can thank O stars for illuminating the beautiful nebula photographs captured by Hubble. O stars burn through their fuel quickly, and can detonate as supernovae in just a few million years.
Some O stars include Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis.