Define main sequence star11/18/2023 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. Style="background-color:# Template:Color temperature"|33,300 Style="background-color:# Template:Color temperature"|35,100 Style="background-color:# Template:Color temperature"|37,100 Style="background-color:# Template:Color temperature"|39,500 Style="background-color:# Template:Color temperature"|41,400 Style="background-color:# Template:Color temperature"|42,900 Style="background-color:# Template:Color temperature"|44,900 Spectrum of an O5V star Properties of typical O-type main-sequence stars Spectral
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