Wolf-Rayet stars


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Wolf–Rayet stars

A small group of very luminous very hot stars, with temperatures possibly as high as 90 000 K, that have anomalously strong and broad emission lines of ionized helium, carbon, oxygen, and nitrogen but few absorption lines. Since their discovery (by C.J.E. Wolf and G. Rayet, 1867), more than 300 have been found in our Galaxy and its neighbors. The majority either have lines of He, C, and O – WC stars – or He and N – WN stars ; both types have anomalously low abundances of hydrogen. A further rare group, called WO stars , have recently been identified with very strong O emission lines. The emission lines are thought to arise in an expanding stellar atmosphere moving at very high speeds of up to 3000 km s–1 so that the star is continuously and rapidly losing mass. The average mass for Wolf–Rayets is 10 solar masses. About half are known to be binary stars, usually with O or B stars as companions, an example being Gamma Velorum (WC8 + O7).

These unusual properties provide clues that Wolf–Rayets are the centers of very massive Of stars stripped of their outer envelopes with the products of interior nuclear burning being revealed. Because the Wolf–Rayets are the more evolved members of each binary, they must originally have been the more massive partner, a star of at least 20 solar masses. Half that mass has thus been lost in their stellar winds, whose high outflow rate would strip this mass off the star in only 100 000 years. This gas is in fact often visible as a ring nebula surrounding the Wolf–Rayet star. In a close binary the companion's gravity may assist the stripping, but for single stars the cause of the high mass loss is still uncertain, although radiation pressure probably plays a major role.

Wolf-Rayet Stars

 

a class of stars characterized by very high temperatures and luminosities. They are distinguished from other stars by the presence of wide emission bands of hydrogen, helium, oxygen, carbon, and nitrogen in their spectra. This class of stars was named after the French astronomers C. Wolf and G. Rayet, who were the first to note the peculiarities in their spectra (1867). About 130 Wolf-Rayet stars are known in our galaxy; their average luminosity is 4,000 times that of the sun. More than 50 Wolf-Rayet stars have been found in the Magellanic clouds, the closest stellar systems to our galaxy. Their luminosity is equal to or slightly greater than that of similar stars of our galaxy. In the Milky Way, Wolf-Rayet stars are found mostly in regions of the spiral arms and are often linked with gas and dust clouds and clusters of normal hot stars. The effective temperature of Wolf-Rayet stars exceeds 50,000° C. Their radii are 10 to 15 times the radius of the sun, and their masses are on the order of 10 times the mass of the sun. Many Wolf-Rayet stars are close binary stars. The companion usually belongs to the normal hot stars of spectral class O. The width of the emission bands in the spectra of Wolf-Rayet stars reaches 50-100 angstroms. It is caused possibly by the radial expansion of the spherically symmetrical envelope formed as a result of the constant ejection of matter from the surfaces of Wolf-Rayet stars. Another hypothesis explains the diffusion of the lines by electron shells expanding in all directions. It is quite possible that the large-scale turbulence of matter plays a significant role in the atmosphere of WolfRayet stars. It is also possible that the expulsion of matter from the surface can occur as a result of rotation instability. This instability is increased by the presence of a massive companion.

REFERENCES

Vorontsov-Vel’iaminov, B. A. Gazovye tumannosti i novye zvezdy. Moscow-Leningrad, 1948.
Zvezdnye atmosfery. Moscow, 1963. (Translated from English.)

V. P. ARKHIPOVA

References in periodicals archive ?
Wolf-Rayet stars are evolved massive stars which are fusing helium or heavier elements in the core.
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