Wolf-Rayet star

A Wolf-Rayet star (WR star or just WR) is a kind of star that is very hot, with strong, broad helium emission lines (typical stars' prominent lines are all absorption). The broadening is presumed due to Doppler broadening of lines emitted from a high velocity stellar wind. Their spectrum indicates they are extremely hot, 30,000-200,000 K, and they are very luminous, at 10,000 to millions of times the Sun's bolometric luminosity, though most of the luminosity is in ultraviolet and X-rays. Two general classes of stars show these characteristics: some are presumed to be in a late evolutionary stage of very massive stars (initially above 20 solar masses), but others are presumed to have been Sun-like stars now in their planetary-nebula stage. A few hundred Wolf-Rayet stars have been identified within the Milky Way, the first having been noticed in 1867 in the constellation Cygnus. Being so bright, individual Wolf-Rayet stars have been identified in other galaxies, e.g., throughout the Local Group. Even if the stars aren't distinguished, the Wolf-Rayet characteristic spectrum is sometimes evident in distant galaxies, which are referred to as Wolf-Rayet galaxies.

The spectral class W (W-class and W-type) was for some time commonly used for them, but the stars are now merely referred to as Wolf-Rayet (WR), the type further classified into number of WR-specific spectral classes:

• carbon-rich Wolf-Rayet (WC) carbon line(s).
• nitrogen-rich Wolf-Rayet (WN) nitrogen line(s).
• late-type nitrogen-rich Wolf-Rayet (WNL) hydrogen line(s).
• early-type nitrogen-rich Wolf-Rayet (WNE) no hydrogen line(s).
• oxygen-rich Wolf-Rayet (WO) oxygen line(s).