An O-type main-sequence star (O V) is a main-sequence (core hydrogen-burning) star of spectral type O and luminosity class V. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and 50,000 K. They are between 40,000 and 1,000,000 times as luminous as the Sun.
Spectral standard stars
Spectrum of an O5V star
Properties of typical O-type main-sequence stars[1][2]
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 Monocerotis (O7 V) and 10 Lacertae (O9 V).[3]
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.[4] The two MKK O9 V standards were Iota Orionis and 10 Lacertae. The revised Yerkes standards ("MK") presented listed in Johnson & Morgan (1953)[5] 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). An important review on spectral classification by Morgan & Keenan (1973)[6] 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 (1978)[7] 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). Walborn & Fitzpartrick (1990)[8] provided the first digital atlas of spectra for OB-type stars, and included a main-sequence standard for O3 V (HDE 303308). Spectral class O2 was defined in Walborn et al. (2002), with the star BI 253 acting as the O2 V primary standard (actually type "O2 V((f*))"). They also redefined HDE 303308 as an O4 V standard, and listed new O3 V standards (HD 64568 and LH 10-3058).[9]
Properties
These are exceedingly rare objects; it is estimated that there are no more than 20,000 class O stars in the entire Milky Way,[10] around one in 10,000,000 of all stars.
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.[11]
Their masses range between 15 and 90 M☉, but
their radii are more modest at around 10 R☉.
Surface gravities are around 10 times that of the Earth, which is relatively low compared to other main sequence stars.
Class O main sequence stars' surface temperatures fall between 30,000 and 50,000 K. They are intensely bright: their bolometric luminosities are between 30,000 and 1,000,000 L☉.
Visual absolute magnitudes range from about −4 (eqv. 3,400 times brighter than the sun) to about −5.8 (eqv. 18,000 times brighter than the sun).[11][12]
Their light-driven stellar winds have a terminal velocity around 2,000 km/s.[13]
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 have lower metallicity (which makes their interiors less opaque than typical stars in the Milky Way) and noticeably higher temperatures, with the most obvious cause being lower mass loss rates, reduced because of their lower opacity.[14]
Examples of 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.
CEN 16 is a young O8.5V main-sequence star in the Omega Nebula.[15] CEN 16 has one confirmed brown dwarf around it, designated CEN 16 b.[16] CEN 16 b is a large young brown dwarf it has a size of 7.5RJ and has a mass of 70MJ, CEN 16 b also has a temperature of 2,884 Kelvin.[16]
CEN 16 b was directly imaged and confirmed in 2024, CEN 16 b is relatively far away from its star with a semi-major axis of 883.5 Astronomical Units[16]
W 3(OH) is a O8V main-sequence star located 3,000 Parsecs away.[17] W 3(OH) has one substellar companion, W 3(OH) b which was discovered in 2021 by detecting the slowly evaporating companion of W 3(OH).[18] W 3(OH) b has an undefined mass, but it could be around 2-20MJ. W 3(OH) b has a semi-major axis of 2,000 Astronomical units, this far distance is probably why W 3(OH) b still exists [18]
^Garrison, R. F (1994). "A Hierarchy of Standards for the MK Process". The MK Process at 50 Years. A Powerful Tool for Astrophysical Insight Astronomical Society of the Pacific Conference Series. 60: 3. Bibcode:1994ASPC...60....3G.
^Morgan, William Wilson; Keenan, Philip Childs; Kellman, Edith (1943). "An atlas of stellar spectra, with an outline of spectral classification". Chicago. Bibcode:1943assw.book.....M.
^Johnson, H. L; Morgan, W. W (1953). "Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas". Astrophysical Journal. 117: 313. Bibcode:1953ApJ...117..313J. doi:10.1086/145697.
^Morgan, W. W; Abt, Helmut A; Tapscott, J. W (1978). "Revised MK Spectral Atlas for stars earlier than the sun". Williams Bay: Yerkes Observatory. Bibcode:1978rmsa.book.....M.
^Table 5, William D. Vacca; Catharine D. Garmany & J. Michael Shull (April 1996). "The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars". Astrophysical Journal. 460: 914–931. Bibcode:1996ApJ...460..914V. doi:10.1086/177020. hdl:2060/19970023476.
^Martins, F (2004). New atmosphere models for massive stars: Line-blanketing effects and wind properties of O stars (Thesis). Bibcode:2004PhDT........21M.
^Massey, Philip; Bresolin, Fabio; Kudritzki, Rolf P; Puls, Joachim; Pauldrach, A. W. A (2004). "The Physical Properties and Effective Temperature Scale of O-Type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds". The Astrophysical Journal. 608 (2): 1001–1027. arXiv:astro-ph/0402633. Bibcode:2004ApJ...608.1001M. doi:10.1086/420766. S2CID119373878.
