Wolf-Rayet star

A Wolf-Rayet star (WR star or just WR) is a type of star that is very hot, with strong, broad helium emission lines. 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. Some such stars are central stars of a planetary nebula and others are presumed to be in a late evolutionary stage of very massive stars, initially above 20 solar masses. More than 200 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, and in distant galaxies, their characteristic spectrum is sometimes prominent, and the galaxy is referred to as a Wolf-Rayet galaxy. Some subtypes based on spectral features:

• carbon-rich Wolf-Rayet (WC) carbon line(s)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).