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A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen, atomic
helium, and ionized helium; also known as O-type star. This is the earliest spectral
type and the only main sequence star in which ionized helium is present. The effective
temperatures of these stars range from about 30,000 K to 50,000 K, their luminosities
from 50,000 to 1,000,000 times that of solar luminosity, and their masses from about
20 to 100 solar masses. The hottest O-type stars display high ionization emission
features such as N III and He II, Of star. They are divided into subtypes O2, the
hottest, to O9.7, the coldest. O-type stars are relatively rare, for each star of
100 solar masses there are 10^6 stars of solar mass. They are relatively short-lived
since they spend only a few million years on the main sequence. The brightest O-type
star in the sky visible with naked eye is Alnitak.
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