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An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 52,000 K. They are between 30,000 and 1,000,000 times as luminous as the Sun. These stars are rare; it is estimated that there are no more than 20,000 in the entire Milky Way. Examples include σ Orionis A and 10 Lacertae.
Spectral Standard Stars
The "anchor" standards which define the MK classification grid for O-type main-sequence stars, i.e. those standards which have not changed since the early 20th century, are 15 Monocerotis (O7 V) and 10 Lacertae (O9 V). The Morgan–Keenan–Kellerman (MKK) "Yerkes" atlas from 1943 listed O-type standards between O5 and O9, but only split luminosity classes for the O9s. The two MKK O9 V standards were Iota Orionis and 10 Lacertae. The revised Yerkes standards ("MK") presented listed in Johnson & Morgan (1953) presented no changes to the O5 to O8 types, and listed 5 O9 V standards (HD 46202, HD 52266, HD 57682, 14 Cephei, 10 Lac) and 3 O9.5 V standards (HD 34078, Sigma Orionis, Zeta Ophiuchi). An important review on spectral classification by Morgan & Keenan (1973) listed "revised MK" standards for O4 to O7, but again no splitting of standards by luminosity classes. This review also listed main-sequence "dagger standards" of O9 V for 10 Lacertae and O9.5 V for Sigma Orionis.
O-type luminosity classes for subtypes earlier than O5 were not defined with standard stars until the 1970s. The spectral atlas of Morgan, Abt, & Tapscott (1978) defined listed several O-type main-sequence (luminosity class "V") standards: HD 46223 (O4 V), HD 46150 (O5 V), HD 199579 (O6 V), HD 47839 (O7 V), HD 46149 (O8 V), and HD 46202 (O9 V). Walborn & Fitzpartrick (1990) 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*))"). They also redefined HDE 303308 as an O4 V standard, and listed new O3 V standards (HD 64568 and LH 10-3058). The effective temperatures of O3 V stars are likely near 50,000 Kelvin.