Passage: 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<unk>Keenan<unk>Kellerman (MKK) "Yerkes" atlas from 1943 listed O-type standards between O5 and O9, but only split luminosity for the O9s. The two MKK O9 V standards were Iota Orionis and 10 Lacertae. The revised Yerkes standards ("MK") presented in Johnson & Morgan (1953) listed no changes to the O5 to O8 types, and listed 5 O9 VHD standards (HD 46202, 5226, 57682, 14 HD, 10 Lacertae) and 3 O5 VHD (O078, 34 Lacertae). The Morgan<unk>Keenan<unk>Kellerman (MKK) "Yerkes" atlas from 1943 listed O-type standards between O5 and O9, but only split luminosity for the O9s. The two MKK O9 V standards were Iota Orionis and 10
MKK O9 V