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 S Monocerotisand 10 Lacertae. The Morgan–Keenan–Kellerman "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 presented listed in Johnson & Morgan presented no changes to the O5 to O8 types, and listed 5 O9 V standards and 3 O9.5 V standards. An important review on spectral classification by Morgan & Keenan 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 defined listed several O-type main-sequence standards: HD 46223, HD 46150, HD 199579, HD 47839, HD 46149, and HD 46202. Walborn & Fitzpartrick 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., with the star BI 253 acting as the O2 V primary standard )"). They also redefined HDE 303308 as an O4 V standard, and listed new O3 V standards.
Properties
These are 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. Class Omain sequence stars are between and have surface temperatures between 30,000 and 50,000 K. Their bolometric luminosities are between. Their radii are more modest at around. Surface gravities are around 10,000 times that of the Earth, relatively low for a main sequence star. Absolute magnitudes range from about −4, 3,400 times brighter than the sun, to about −5.8, 18,000 times brighter than the sun. 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. O-type main sequence stars in the Large Magellanic Cloud, with lower metallicity, have noticeably higher temperatures, with the most obvious cause being lower mass loss rates. The most luminous class O stars have mass loss rates of more than each year, although the least luminous lose far less. Their stellar winds have a terminal velocity around 2,000 km/s.
Prominent O-class main sequence stars
θ Muscae is a naked-eye Wolf-Rayet star, but the majority of the visible light is produced by an O-class main sequence companion and an OB supergiant.
