O-Type Stars

An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (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. Due to their high mass, O-type stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Most of these stars are young massive main sequence, 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, such as the spiral arms of a spiral galaxy or a pair of galaxies undergoing collision and merger (such as the 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.

O-type stars are classified by the relative strength of certain spectral lines.[1] 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 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.

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 loops 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. 25–60M☉ stars may become yellow hypergiants before either exploding as a supernova or evolving back to hotter temperatures. Above about 60M☉, O-type stars evolve though a short blue hypergiant or luminous blue variable 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, asymptotic-giant-branch (AGB), and then post-AGB 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.