O-type star

These stars illuminate any surrounding material and are largely responsible for the distinct bluish-white and pink coloration of a galaxy's arms.

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.

[3] Star types O3 to O8 are classified as luminosity class sub-type "Vz" if they have a particularly strong 468.6 nm ionised helium line.

They have characteristic surface temperatures ranging from 30,000–52,000 K, emit intense ultraviolet ('actinic') light, and so appear in the visible spectrum as bluish-white.

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.

Although they have a wide range of distinct characteristics, it is not fully understood how they all form and develop; they are thought to have degenerate cores that will eventually be exposed as a white dwarf.

Before then the material outside that core is mostly helium with a thin layer of hydrogen, which is rapidly being lost due to the strong stellar wind.

[16] In the lifecycle of O-type stars, different metallicities and rotation rates introduce considerable variation in their evolution, but the basics remain the same.

[8] O-type stars start to move slowly from the zero-age main sequence almost immediately after they form, 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.

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.

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.

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

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-type stars emit copious amounts of ultraviolet radiation, which ionizes the gas in the cloud and pushes it away.

[18] O-type stars explode as supernovae when they die, releasing vast amounts of energy, contributing to the disruption of a molecular cloud.

[19] 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.

Relative size of O-type stars with other main-sequence stars
The Trifid Nebula (M20) is sculpted and lit by the luminous O7.5III star visible at its centre in this infrared image.
CNO cycle that powers massive O-type stars.
Structure of low-mass, intermediate-mass, and high-mass stars. "M" indicates units of solar masses .
sdO-type star cross-section showing inert core and helium shell burning
Evolutionary tracks on the HR diagram. The 15 M and 60 M tracks are typical of massive O-type stars.
The central star of NGC 6826 is a low-mass O6 star.
Melnick 42, one of the hottest O supergiants, lies in the Tarantula nebula .
Alnitak is a triple star system with an O9.7 supergiant and an O9 giant as well as a B0 giant. These stars illuminate the nearby Flame Nebula .
The brightest star in the Trapezium cluster is O7V star θ 1 Orionis C. The other three are B0.5 and B1 main-sequence stars.
The O-type star in Cepheus B, HD 217086, illuminates the molecular cloud with ultraviolet radiation, driving it back while compressing it, triggering the formation of new stars.