An Etymological Dictionary of Astronomy and Astrophysics

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 10⁶ 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. For prominent Galactic O stars see → HD 93129.

See also: O, letter of alphabet used in the Harvard spectral classification; → star.