O-type main-sequence star


An O-type main-sequence star is a main-sequence—core hydrogen-burning—star of spectral type O. The spectral luminosity class is typically although class O main sequence stars often have spectral peculiarities due to their extreme luminosity. These stars have between 15 and 90 times the mass of the Sun and surface temperatures between 30,000 and. They are between 40,000 and 1,000,000 times as luminous as the Sun.

Spectral standard stars

Spectral
type		Mass Radius Luminosity Effective
temperature
Color
index
O3V120.0015.001,400,00044,900−0.330
O4V85.3113.431,073,01942,900−0.326
O5V60.0012.00790,00041,400−0.323
O6V43.7110.71540,42239,500−0.321
O7V30.859.52317,32237,100−0.318
O8V23.008.50170,00035,100−0.315
O9V19.637.5192,76233,300−0.312

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 and .
The Morgan–Keenan–Kellerman "Yerkes" atlas from 1943 listed O-type standards between O5 and O9, but only split luminosity classes for the O9s. The two MKK O9 V standards were Iota Orionis and. The revised Yerkes standards presented listed in Johnson & Morgan presented no changes to the O5 to O8 types, and listed 5 O9 V standards and 3 O9.5 V standards. An important review on spectral classification by Morgan & Keenan 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 defined listed several O-type main-sequence standards: , , , , , and . Walborn & Fitzpartrick provided the first digital atlas of spectra for OB-type stars, and included a main-sequence standard for O3 V. Spectral class O2 was defined in Walborn et al., with the star acting as the O2 V primary standard )"). They also redefined as an O4 V standard, and listed new O3 V standards.

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, 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. Their masses range between, but their radii are more modest at around. 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. Visual absolute magnitudes range from about −4 to about −5.8.
Their light-driven stellar winds have a terminal velocity around 2,000 km/s. The most luminous class O stars have mass loss rates of more than each year, although the least luminous lose far less. O-type main sequence stars in the Large Magellanic Cloud have lower metallicity and noticeably higher temperatures, with the most obvious cause being lower mass loss rates, reduced because of their lower opacity.

Examples of O-class main sequence stars