O-type main-sequence star
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 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun. 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 (O7 V) and (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 . 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 (, , , , 10 Lacertae) and 3 O9.5 V standards (, 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: (O4 V), (O5 V), (O6 V), (O7 V), (O8 V), and (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 (). Spectral class O2 was defined in Walborn et al. (2002), with the star acting as the O2 V primary standard (actually type "O2 V((f*))"). They also redefined as an O4 V standard, and listed new O3 V standards ( and ). These are exceedingly rare objects; it is estimated that there are no more than 20,000 class O stars in the entire Milky Way, around one in 10,000,000 of all stars.
