Wolf-Rayet stars reach surface temperatures near 210,000 kelvin
Most stars show dark absorption lines in their spectra. In 1867, Charles Wolf and Georges Rayet found three stars in Cygnus whose spectra glowed with bright, broad emission bands instead. These Wolf-Rayet stars turned out to be among the hottest known, with surfaces from 20,000 up to about 210,000 kelvin, blasting their outer layers into space.
The two astronomers made the discovery with the 40 cm Foucault telescope at the Paris Observatory. Explaining the bright bands took decades. Edward Pickering thought they came from an unusual form of hydrogen, but they were eventually traced to helium, an element first detected only in 1868. Carlyle Smith Beals later identified carbon, oxygen and nitrogen lines as well. By 1929 the sheer width of the bands was understood as Doppler broadening: gas was rushing along our line of sight at 300 to 2,400 km per second, driven off the star by the pressure of its own radiation.
The classic Wolf-Rayet stars are massive, evolved stars that have lost all their outer hydrogen and now fuse helium or heavier elements in their cores. A subgroup, the WNh stars, are young, extremely massive stars still burning hydrogen, whose intense mixing and winds expose helium and nitrogen at the surface. Separately, some central stars of planetary nebulae, once Sun-like stars, show similar spectra after shedding their atmospheres to bare carbon-oxygen cores.
In 1938 the International Astronomical Union split them into WN types, dominated by nitrogen lines, and WC types, dominated by carbon and oxygen. A hotter WO class, marked by ionised oxygen, followed from 1969 onward.
All are extraordinarily luminous, from thousands of times the Sun's output for the planetary-nebula stars to over a million for the WNh group, yet they are not especially bright to the eye, since most of their energy emerges as ultraviolet light. Even so, the naked-eye systems Gamma Velorum and Theta Muscae each contain one, and two of the most massive stars known, R136a1 and BAT99-98 in the 30 Doradus region, belong to the class.
Source: Wolf–Rayet star
