Their stellar winds have a terminal velocity around 2,000 km/s. The most luminous class O stars have mass loss rates of more than a millionth M☉ each year, although the least luminous lose far less. 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. Absolute magnitudes range from about −4, 3,400 times brighter than the sun, to about −5.8, 18,000 times brighter than the sun.Ĭlass 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. Surface gravities are around 10,000 times that of the Earth, relatively low for a main sequence star. Their radii are more modest at around 10 R☉. Their bolometric luminosities are between 30,000 and 1,000,000 L☉. Class O main sequence stars are between 15 and 90 M☉ and have surface temperatures between 30,000 and 50,000 K. 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. 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), HD 47839 (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. They are between 40,000 and 1,000,000 times as luminous as the Sun. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) star of spectral type O and luminosity class V.
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