O-type main-sequence star

Walborn & Fitzpartrick (1990)[8] provided the first digital atlas of spectra for OB-type stars, and included a main-sequence standard for O3 V (HDE 303308).

Spectral class O2 was defined in Walborn et al. (2002), with the star BI 253 acting as the O2 V primary standard (actually type "O2 V((f*))").

Of the few there are, all class O stars are very young – no more than a few million years old – and in our galaxy they all have high metallicities, around twice that of the sun.

Surface gravities are around 10 times that of the Earth, which is relatively low compared to other main sequence stars.

Class O main sequence stars' surface temperatures fall between 30,000 and 50,000 K. They are intensely bright: their bolometric luminosities are between 30,000 and 1,000,000 L☉.