An O-type star is a hot, blue-white

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

spectral type 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 ...

O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000

kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...

(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. Stars of this type are very rare, but because they are very bright, they can be seen at great distances and four of the 90 brightest stars as seen from Earth are O type.Those four stars are Gamma Velorum, Alnitak (Zeta Orionis), Mintaka (Delta Orionis), and Zeta Puppis. Due to their high mass, O-type stars end their lives rather quickly in violent

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

explosions, resulting in

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...

s or

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

s. Most of these stars are young massive

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

, giant, or supergiant stars, but the central stars of planetary nebulae, old low-mass stars near the end of their lives, also usually have O spectra. O-type stars are typically located in regions of active

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

, such as the spiral arms of a

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Antennae Galaxies). These stars illuminate any surrounding material and are largely responsible for the distinct coloration of a galaxy's arms. Furthermore, O-type stars often occur in multiple star systems, where their evolution is more difficult to predict due to mass transfer and the possibility of component stars exploding as supernovae at different times.

Classification

O-type stars are classified by the relative strength of certain spectral lines. The key lines are the prominent He⁺ lines at 454.1 nm and 420.0 nm, which vary from very weak at O9.5 to very strong in O2–O7, and the He⁰ lines at 447.1 nm and 402.6 nm, which vary from absent in O2/3 to prominent in O9.5. The O7 class is defined where the 454.1-nanometer He⁺ and 447.1-nanometer He⁰ lines have equal strength. The very hottest O-type stars have such weak neutral He lines that they must be separated on the relative strength of the N²⁺ and N³⁺ lines. The luminosity classes of O-type stars are assigned on the relative strengths of the He⁺ emission lines and certain ionised nitrogen and

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

lines. These are indicated by the "f" suffix on the spectral type, with "f" alone indicating N²⁺ and He⁺ emission, "(f)" meaning the He emission is weak or absent, "((f))" meaning the N emission is weak or absent, "f*" indicating the addition of very strong N³⁺ emission, and "f+" the presence of Si³⁺ emission. Luminosity class V, main-sequence stars, generally have weak or missing emission lines, with giants and supergiants showing increasing emission line strength. At O2–O4, the distinction between main sequence and supergiant stars is narrow and may not even represent true luminosity or evolutionary differences. At intermediate O5–O8 classes, the distinction between O((f)) main sequence, O(f) giants, and Of supergiants is well-defined and represents a definite increase in luminosity. The increasing strength of Si³⁺ emission is also an indicator of increasing luminosity and this is the primary means of assigning luminosity classes to the late O-type stars. Stars of types O3 to O8 are classified as luminosity class sub-type Vz if they have a particularly strong 468.6 nm ionised helium line. The line's presence is thought to indicate extreme youth; the "z" stands for zero-age. To help with the classification of O-type stars, standard examples are listed for most of the defined types. The following table gives one of the standard stars for each spectral type. In some cases, a standard star has not been defined. For spectral types O2 to O5.5, supergiants are not split into Ia/Iab/Ib sub-types. Subgiant spectral types are not defined for types O2, O2.5, or O3. Bright giant luminosity classes are not defined for stars hotter than O6.

Characteristics

O-type stars are hot and luminous. They have characteristic surface temperatures ranging from 30,000 to 52,000 K, emit intense

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

light, and so appear in the

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called '' visible light'' or simply light. A typical human eye will respond to ...

as bluish-white. Because of their high temperatures the luminosities of main sequence O-type stars range from 10,000 times the Sun to around 1,000,000 times, giants from 100,000 times the Sun to over 1,000,000, and supergiants from about 200,000 times the Sun to several million times. Other stars in the same temperature range include rare O-type subdwarf ( sdO) stars, the central stars of planetary nebulae (CSPNe), and

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

s. The white dwarfs have their own spectral classification scheme, but many CSPNe have O-type spectra. Even these small low-mass subdwarfs and CSPNe have luminosities several hundred to several thousand times that of the Sun. sdO-type stars generally have somewhat higher temperatures than massive O-type stars, up to 100,000K. O-type stars represent the highest masses of stars on the main sequence. The coolest of them have initial masses of around 16 times the Sun. It is unclear what the upper limit to the mass of an O-type star would be. At solar

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

levels, stars should not be able to form with masses above 120–150 solar masses, but at lower metallicity this limit is much higher. O-type stars form only a tiny fraction of main-sequence stars and the vast majority of these are towards the lower end of the mass range. The most massive and hottest types O3 and O2 are extremely rare, were only defined in 1971 and 2002 respectively, and only a handful are known in total. Giant and supergiant stars are somewhat less massive than the most massive main sequence O-type stars due to mass loss, but are still among the most massive stars known. The formation rate of class O stars cannot be observed directly, but

initial mass function In astronomy, the initial mass function (IMF) is an empirical function that describes the initial distribution of masses for a population of stars. The IMF is an output of the process of star formation. The IMF is often given as a probability d ...

s (IMF) can be derived that model observations of existing star populations and particularly young star clusters. Depending on the chosen IMF, class O stars form at a rate of one in several hundred main sequence stars. Because the luminosity of these stars increases out of proportion to their masses, they have correspondingly shorter lifespans. The most massive spend less than a million years on the main sequence and explode as supernovae after three or four million years. The least luminous O-type stars can remain on the main sequence for around 10 million years, but cool slowly during that time and become early B-type stars. No massive star remains with spectral class O for more than about 5–6 million years. Although sdO and CSPNe stars are low-mass stars billions of years old, the time spent in this phase of their lives is extremely short, of the order of 10,000,000 years. The present day mass function can be directly observed, and in the solar neighbourhood less than one in 2,000,000 stars is class O. Differing estimates find between 0.00003% (0.00002% if white dwarfs are included) and 0.00005% of stars being of class O. It has been estimated that there are around 20,000 massive O-type stars in the galaxy. The low-mass sdO and CSPNe O-type stars are probably more common, although less luminous and therefore harder to find. Despite their short lifetimes, they are thought to be normal stages in the evolution of common stars only a little more massive than the Sun.

Structure

O-type main-sequence stars are fueled by

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

, as all main-sequence stars are. However, the high mass of O-type stars results in extremely high core temperatures. At these temperatures, hydrogen fusion with the

CNO cycle The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, ...

dominates the production of the star's energy and consumes its nuclear fuel at a much higher rate than low-mass stars which fuse hydrogen predominantly with the proton–proton cycle. The intense amount of energy generated by O-type stars cannot be radiated out of the core efficiently enough, and consequently they experience

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

in their cores. The

radiative zone A radiation zone, or radiative region is a layer of a star's interior where energy is primarily transported toward the exterior by means of radiative diffusion and thermal conduction, rather than by convection. Energy travels through the radiati ...

s of O-type stars occur between the core and

photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

. This mixing of core material into the upper layers is often enhanced by fast rotation, and has a dramatic effect on the evolution of O-type stars. They start to slowly expand and show giant or supergiant characteristics while still burning hydrogen in their cores, then may remain as blue supergiants for much of the time during helium core burning. Subdwarf O star schematic cross section

sdO-type stars and CSPNe have a substantially different structure, although they have a wide range of characteristics and it is not fully understood how they all form and develop. They are thought to have degenerate cores that will eventually become exposed as a white dwarf. Outside the core the stars are mostly helium with a thin layer of hydrogen which is rapidly being lost due to the strong stellar wind. There may be several different origins for this type of star, but at least some of them have a region where helium is being fused in a shell, which enlarges the core and powers the high luminosity of these small stars.

Evolution

In the lifecycle of O-type stars, different metallicities and rotation rates introduce considerable variation in their evolution, but the basics remain the same. O-type stars start to move slowly from the zero-age main sequence almost immediately, gradually becoming cooler and slightly more luminous. Although they may be characterised spectroscopically as giants or supergiants, they continue to burn hydrogen in their cores for several million years and develop in a very different manner from low-mass stars such as the Sun. Most O-type main-sequence stars will evolve more or less horizontally in the HR diagram to cooler temperatures, becoming blue supergiants. Core helium ignition occurs smoothly as the stars expand and cool. There are a number of complex phases depending on the exact mass of the star and other initial conditions, but the lowest mass O-type stars will eventually evolve into red supergiants while still burning helium in their cores. If they do not explode as a supernova first, they will then lose their outer layers and become hotter again, sometimes going through a number of

blue loop In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russell ...

s before finally reaching the Wolf–Rayet stage. The more-massive stars, initially main-sequence stars hotter than about O9, never become red supergiants because strong convection and high luminosity blow away the outer layers too quickly. 25–60 stars may become

yellow hypergiant A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually lumi ...

s before either exploding as a supernova or evolving back to hotter temperatures. Above about 60, O-type stars evolve though a short

blue hypergiant A hypergiant (stellar classification#Yerkes spectral classification, luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergian ...

luminous blue variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large ...

phase directly to Wolf–Rayet stars. The most massive O-type stars develop a WNLh spectral type as they start to convect material from the core towards the surface, and these are the most luminous stars that exist. Low to intermediate-mass stars age in a very different way, through red-giant,

horizontal-branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proce ...

, asymptotic-giant-branch (AGB), and then

post-AGB The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late ...

phases. Post-AGB evolution generally involves dramatic mass loss, sometimes leaving a planetary nebula, and leaving an increasingly hot exposed stellar interior. If there is sufficient helium and hydrogen remaining, these small but extremely hot stars have an O-type spectrum. They increase in temperature until shell burning and mass loss ceases, then they cool into white dwarfs. At certain masses or chemical makeups, or perhaps as a result of binary interactions, some of these lower-mass stars become unusually hot during the horizontal branch or AGB phases. There may be multiple reasons, not fully understood, including stellar mergers or very late thermal pulses re-igniting post-AGB stars. These appear as very hot OB stars, but only moderately luminous and below the main sequence. There are both O (sdO) and B (sdB) hot subdwarfs, although they may develop in entirely different ways. The sdO-type stars have fairly normal O spectra but luminosities only around a thousand times the Sun.

Examples

O-type stars are rare but luminous, so they are easy to detect and there are a number of naked eye examples.

Main-sequence

Trapezium cluster optical and infrared comparison

9 Sagittarii 9 Sagittarii (9 Sgr) is a massive binary star in the constellation Sagittarius. It has an apparent magnitude of 5.97. Both components are highly luminous O-type main-sequence stars. Surroundings 9 Sgr is a naked eye star lying in the ...

10 Lacertae 10 Lacertae (10 Lac) is a star in the constellation Lacerta. With an apparent magnitude of 4.9, it is located around distant in the small Lacerta OB1 association. It is a hot blue main-sequence star of spectral type O9V, a massive sta ...

AE Aurigae AE Aurigae (abbreviated as AE Aur) is a runaway star in the constellation Auriga; it lights the Flaming Star Nebula. Description AE Aurigae is a blue O-type main sequence dwarf with a mean apparent magnitude of +6.0. It is classifi ...

BI 253 BI 253 is an O2V star in the Large Magellanic Cloud and is a primary standard of the O2 type. It is one of the hottest main-sequence stars known and one of the most-massive and most-luminous stars known. Discovery BI 253 was first ...

* Delta Circini * HD 93205 (V560 Carinae) * Mu Columbae *

Sigma Orionis Sigma Orionis or Sigma Ori (σ Orionis, σ 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 Alni ...

* Theta1 Orionis C *

VFTS 102 VFTS 102 is a star located in the Tarantula nebula, a star forming region in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. The peculiarity of this star is its projected equatorial velocity of ~ (about ), making it the ...

Zeta Ophiuchi Zeta Ophiuchi (ζ Oph, ζ Ophiuchi) is a single star located in the constellation of Ophiuchus. It has an apparent visual magnitude of 2.6, making it the third-brightest star in the constellation. Parallax measurements give an estimate ...

Giants

Iota Orionis Iota Orionis (ι Orionis, abbreviated ι Ori) is a multiple star system in the equatorial constellation of Orion the hunter. It is the eighth-brightest member of Orion with an apparent visual magnitude of 2.77 and also the brightest ...

* LH54-425 * Meissa * Plaskett's star *

Xi Persei Xi Persei (ξ Persei, abbreviated Xi Per, ξ Per), known also as Menkib , is a star in the constellation of Perseus. Based upon parallax measurements taken during the Hipparcos mission, it is approximately 1,200 light-years from the S ...

* Mintaka * HD 164492 A

Supergiants

* 29 Canis Majoris * Alnitak * Alpha Camelopardalis *

Cygnus X-1 Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the ...

Tau Canis Majoris Tau Canis Majoris (τ CMa, τ Canis Majoris, 30 CMa) is a multiple star system in the constellation Canis Major. It is approximately 5,000 light years distant from Earth and is the brightest member of the open cluster NGC 2362. Sys ...

* Zeta Puppis

Central stars of planetary nebulae

NGC 2392 The Eskimo Nebula (NGC 2392), also known as the Clown-faced Nebula, Lion Nebula, or Caldwell 39, is a bipolar double-shell planetary nebula (PN). It was discovered by astronomer William Herschel in 1787. The formation resembles a person's head s ...

(O6) * IC 418 (O7fp) * NGC 6826 (O6fp)

Subdwarfs

HD 49798 HD 49798 is a binary star in the constellation Puppis about 650 parsecs from Earth. It has an apparent magnitude of 8.3, making it one of the brightest known O class subdwarf stars. HD 49798 was discovered in 1964 to be a rare hydrogen- ...

(sdO6p)

Location

Spiral arms

O-type main-sequence stars tend to appear in the arms of spiral galaxies. This is because, as a spiral arm moves through space, it compresses any molecular clouds in its way. The initial compression of these molecular clouds leads to the formation of stars, some of which are O- and

B-type star A B-type main-sequence star (B V) 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 st ...

s. Also, as these stars have shorter lifetimes, they cannot move great distances before their death and so they stay in or relatively near to the spiral arm in which they formed. On the other hand, less massive stars live longer and thus are found throughout the galactic disc, including in between the spiral arms.

O/OB associations

Stellar association A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally ...

s are groups of stars that are gravitationally unbound from the beginning of their formation. The stars in stellar associations are moving from one another so rapidly that gravitational forces cannot keep them together. In young stellar associations, most of the light comes from O- and B-type stars, so such associations are called OB associations.

Molecular clouds

The birth of an O-type star in a molecular cloud has a destructive effect on the cloud, but also may trigger the formation of new stars. O-type stars emit copious amounts of

radiation, which ionizes the gas in the cloud and pushes it away. O-type stars also have powerful

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...

s, with velocities of thousands of kilometers per second, which can blow a bubble in the molecular cloud around the star. O-type stars explode as supernovae when they die, releasing vast amounts of energy, contributing to the disruption of a molecular cloud. These effects disperse the remaining molecular material in a star-forming region, ultimately stopping the birth of new stars, and possibly leaving behind a young

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

. Nevertheless, before the cloud is disrupted, the sweeping up of material by an expanding bubble (called Collect and Collapse) or the compression of existing cloudlets (called Radiation Driven Implosion) may lead to the birth of new stars. Evidence of triggered star-formation has been observed in a number of star-forming regions, such as Cepheus B and the Elephant's Trunk nebula (where it may account for 14–25% of stars formed).

Notes

References

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