O Stars: Definition

The defining characteristic of O stars is the presence of absorption lines of ionized helium; neutral helium lines are also present. On the main sequence the O stars represent an extreme of dwarf stars; they are the hottest with surface temperatures in the range of 30,000 K to 50,000 K, the heaviest with masses of 20 to 50 times solar, and intrinsically the brightest at luminosities of 50,000 up to one million tomes that of the Sun.

The spectra of O-Type stars shows the presence of hydrogen and helium. At these temperatures most of the hydrogen is ionized, so the hydrogen lines are weak. Both HeI and HeII (singly ionized helium) are seen in the higher temperature examples.

The radiation from O5 stars is so intense that it can ionize hydrogen over a volume of space 1000 light years across. One example is the luminous H II region surrounding star cluster M16.

O-Type stars are very massive and evolve more rapidly than low-mass stars because they develop the necessary central pressures and temperatures for hydrogen fusion sooner. Because of their early development, the O-Type stars are already luminous in the huge hydrogen and helium clouds in which lower mass stars are forming. They light the stellar nurseries with ultraviolet light and cause the clouds to glow in some of the dramatic nebulae associated with the H II regions.

Although various individual properties, such as temperature, may be exceeded by more exotic objects such as white dwarf or Wolf-Rayet stars, type O stars are extreme by the standards of ‘normal’ stars, and they are by far the rarest members of the main sequence.

In the solar vicinity, dwarf (luminosity class V) O type stars dominate the massive star population, leading to the conclusion that this is the most prolonged evolutionary phase in the lifetime of massive stars.

Included are blue stars with surface temperatures of 20,000 to 35,000K. The thermal energy is so great at these temperatures that most surface hydrogen is completely ionized so hydrogen (HI) lines are weak. Unionized helium (HeI) is visible but the stronger spectra are of mulitple ionized atoms (HeII, CIII, NIII,OIII,SiV). Note the rise of the spectral curve toward the blue from the blackbody radiation curve.

The radiation rises in intensity toward the blue end of the spectrum. The blackbody peak will be in the ultraviolet. Note that the hydrogen spectrum is seen, but it is red-shifted from its laboratory wavelength of 656 nm.

O Stars: Description

At these luminosities the consequences are the same as described for the hottest B stars: the time that O stars remain on the main sequence is less than one million years and as a result they are a rare kind of star. This, however, gives quite the wrong impression of their importance in the evolution of the galaxy. All thought there are currently only about 300,000 O type stars in the galaxy, which is a very small fraction of the total 200,000,000 stars, there has been time for 10,000 generation of O type stars in the lifetime of the galaxy. Therefor there have been about 3 billion O type stars formed from interstellar gas. As the O stars are more than 30 times heavier than the average star, they return most of their material back to the interstellar gas, we deduce that a large fraction (perhaps greater than one third) of all mater that currently makes up stars of all types has passed through earlier generations f O stars. The element-binding nuclear reactions that occur in O stars are therefore an important source of many of the heavy elements in our galaxy.

The O stars, being relatively young stars, are associated with the interstellar clouds of gas and dust in which star formation occurs. Together with the hotter B stars, they for OB Associations, which show that high-mass stars tend to form in groups rather than in isolation. Most O and B stars form in binary systems, many with components of similar mass. The more massive star of the binary evolves fastest and is likely to explode as a supernova, which can release its companion form orbit at high velocity. It is estimated that as many as 20 percent of the O stars are such runaways.

As with the hotter B stars, the higher luminosity of the O stars drive a high rate of mass loss via a stellar wind –from two millionths to 20 millionths of a solar mass per year in the O stars. Summed over all of the O stars in the galaxy, this can account for one-third of the gas being returned to space from stars (Red Giants and Planetary Nebula account for the rest.)

The high rate of mass loss from O stars has another effect of galactic significance If a binary system consisting of two O or B stars survives intact, the supernova explosions of one of its components, then the result can be a Neutron star orbiting through the stellar wind form the unevloved star. The gas falling onto the neutron star emits energetic x-rays. Although such systems are very rare -- probably only fifty in the entire galaxy – they account for a substantial fraction of the hard x-ray sources in the galaxy.

Rotation among O stars is similar to that found in B stars – they are rapid rotators and the fastest possess circumstellar disks, which produce emission line spectra leading to the Oe classification. These stars also have the highest rates of mass loss. Some of the Oe stars have, in addition to hydrogen and helium emission, strong emission lines from doubly ionized nitrogen. These are given the classification of Of and probably represent O stars that were once very massive (greater than 50 solar masses) which, like the Wolf-Rayet stars, have been stripped of the outer layers to reveal a nitrogen rich interior. About 15 percent of O and early B stars show emission lines.

O class main sequence stars or Blue Supergiants are the largest, the hottest, the rarest and the shortest lived stars.
They emit much of their radiation beyond the blue end of the spectrum in ultraviolet.

O Stars: Variability/Peculiarity

There are no large amplitude pulsating variables among the O stars, but most very in brightness by a few percent over months or years. There is also evidence of small changes from night to night in some stars.