Visual absolute magnitudes range from about −4 (eqv. They are intensely bright: their bolometric luminosities are between 30,000 and 1,000,000 L ☉. Surface gravities are around 10 times that of the Earth, which is relatively low compared to other main sequence stars.Ĭlass O main sequence stars' surface temperatures fall between 30,000 and 50,000 K. Their radii are more modest at around 10 R ☉. Their masses range between 15 and 90 M ☉, but 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. 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. They also redefined HDE 303308 as an O4 V standard, and listed new O3 V standards ( HD 64568 and LH 10-3058). (2002), with the star BI 253 acting as the O2 V primary standard (actually type "O2 V((f*))"). Spectral class O2 was defined in Walborn et al. 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). 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), S Monocerotis (O7 V), HD 46149 (O8 V), and HD 46202 (O9 V). O-type luminosity classes for subtypes earlier than O5 were not defined with standard stars until the 1970s. This review also listed main-sequence "dagger standards" of O9 V for 10 Lacertae and O9.5 V for Sigma Orionis. 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. 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 Lacertae) and 3 O9.5 V standards ( HD 34078, Sigma Orionis, Zeta Ophiuchi). The two MKK O9 V standards were Iota Orionis and 10 Lacertae. 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. those standards which have not changed since the early 20th century, are S Monocerotis (O7 V) and 10 Lacertae (O9 V). The "anchor" standards which define the MK classification grid for O-type main-sequence stars, i.e. Spectrum of an O5V star Properties of typical O-type main-sequence stars Spectral
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