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 type | Mass (M☉) | Radius (R☉) | Luminosity (L☉) | Effective temperature (K) | Color index (B − V) |
|---|---|---|---|---|---|
| O3V | 120.00 | 15.00 | 1,400,000 | 44,900 | −0.330 |
| O4V | 85.31 | 13.43 | 1,073,019 | 42,900 | −0.326 |
| O5V | 60.00 | 12.00 | 790,000 | 41,400 | −0.323 |
| O6V | 43.71 | 10.71 | 540,422 | 39,500 | −0.321 |
| O7V | 30.85 | 9.52 | 317,322 | 37,100 | −0.318 |
| O8V | 23.00 | 8.50 | 170,000 | 35,100 | −0.315 |
| O9V | 19.63 | 7.51 | 92,762 | 33,300 | −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 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]
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]
Only two known O-type main-sequence stars have planets or brown dwarfs.
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 7.5 RJ and has a mass of 70 MJ, CEN 16 b also has a temperature of 2,884 Kelvin. [17] 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 [18]
W 3(OH) is a O8V main-sequence star located 3,000 Parsecs away. [19] 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). [20] W 3(OH) b has an undefined mass, but it could be around 2-20 MJ. 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 [21]
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.
A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. Proxima Centauri, the star nearest to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the fusing stars in the Milky Way.
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.
Saiph, designation Kappa Orionis and 53 Orionis, is a blue supergiant star and the sixth-brightest star in the constellation of Orion. Of the four bright stars that compose Orion's main quadrangle, it is the star at the south-eastern corner. A northern-hemisphere observer facing south would see it at the lower left of Orion, and a southern-hemisphere observer facing north would see it at the upper right. Parallax measurements yield an estimated distance of 650 light-years from the Sun. It is smaller, less luminous but hotter at its surface than Rigel with an apparent visual magnitude of 2.1. The luminosity of this star changes slightly, varying by 0.04 magnitudes.
Red supergiants (RSGs) are stars with a supergiant luminosity class and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.
A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blue supergiants.
A G-type main-sequence star, also often, and imprecisely, called a yellow dwarf, or G star, is a main-sequence star of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective temperature between about 5,300 and 6,000 K. Like other main-sequence stars, a G-type main-sequence star converts the element hydrogen to helium in its core by means of nuclear fusion, but can also fuse helium when hydrogen runs out. The Sun, the star in the center of the Solar System to which the Earth is gravitationally bound, is an example of a G-type main-sequence star. Each second, the Sun fuses approximately 600 million tons of hydrogen into helium in a process known as the proton–proton chain, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri, Tau Ceti, and 51 Pegasi.
A giant star has a substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung in 1905 or 1906.
A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.
A K-type main-sequence star, also referred to as a K-type dwarf, or orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow/white G-type main-sequence stars. They have masses between 0.6 and 0.9 times the mass of the Sun and surface temperatures between 3,900 and 5,300 K. These stars are of particular interest in the search for extraterrestrial life due to their stability and long lifespan. Many of these stars have not left the main sequence as their low masses mean they stay on the main sequence for up to 70 billion years, a length of time much larger than the time the universe has existed. Well-known examples include Toliman and Epsilon Indi.
An F-type main-sequence star is a main-sequence, hydrogen-fusing star of spectral type F and luminosity class V. These stars have from 1.0 to 1.4 times the mass of the Sun and surface temperatures between 6,000 and 7,600 K.Tables VII and VIII. This temperature range gives the F-type stars a whitish hue when observed by the atmosphere. Because a main-sequence star is referred to as a dwarf star, this class of star may also be termed a yellow-white dwarf. Notable examples include Procyon A, Gamma Virginis A and B, and KIC 8462852.
An A-type main-sequence star or A dwarf star is a main-sequence star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1.4 and 2.1 solar masses (M☉), have surface temperatures between 7,600 and 10,000 K, and live for about a quarter of the lifetime of our Sun. Bright and nearby examples are Altair (A7), Sirius A (A1), and Vega (A0). A-type stars do not have convective zones and thus are not expected to harbor magnetic dynamos. As a consequence, because they do not have strong stellar winds, they lack a means to generate X-ray emissions.
A B-type main-sequence star is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have strong neutral helium absorption lines, which are most prominent at the B2 subclass, and moderately strong hydrogen lines. Examples include Regulus, Algol A and Acrux.
Upsilon Orionis (υ Ori, υ Orionis) is a star in the constellation Orion. It has the traditional name Thabit or Tabit (ﺛﺎﺑﺖ, Arabic for "the endurer"), a name shared with Pi3 Orionis. It is a blue-white main sequence star of apparent magnitude 4.62 located over 3000 light-years distant from the Solar System. It is a suspected Beta Cephei variable.
Sigma Orionis or Sigma Ori is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The combined brightness of the component stars is magnitude 3.80.
HD 111968, also known by the Bayer designation n Centauri, is a single star in the southern constellation of Centaurus. It is a white-hued star that is faintly visible to the naked eye with an apparent visual magnitude of +4.25. The star is located at a distance of approximately 149 light years from the Sun based on parallax. The radial velocity of the star is poorly constrained, with an estimated value of 2.5 km/s.
A yellow supergiant (YSG) is a star, generally of spectral type F or G, having a supergiant luminosity class. They are stars that have evolved away from the main sequence, expanding and becoming more luminous.
Theta2 Orionis is a multiple star system in the constellation Orion. It is a few arc minutes from its more famous neighbour the Trapezium Cluster, also known as θ1 Orionis.
An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have surface temperatures in excess of 30,000 kelvins (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.
HD 93403 is a spectroscopic binary containing two highly luminous hot blue stars. It is 10,000 light years away in the Carina Nebula in the constellation Carina. It appears to have spectral type O5.5III, but this is composed of two spectra from a blue supergiant and blue main sequence star of spectral type O5.5I and O7V respectively. The two stars orbit every 15 days with a separation that varies from 93 R☉ to 149 R☉. The binary is shedding mass at the high rate of 0.0005 M☉ per year.
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