Wolf–Rayet star

Wolf–Rayet stars (often referred to as WR stars) are a heterogeneous set of stars with unusual spectra showing prominent broad emission lines of highly ionised helium and nitrogen or carbon. The spectra indicate very high surface temperatures of 30,000 K to around 200,000 K, surface enhancement of heavy elements, and strong stellar winds.

Classic (or Population I) Wolf–Rayet stars are evolved, massive stars, O-type stars over 20 solar masses when they were on the main sequence, that have now completely lost their outer hydrogen and are fusing helium or heavier elements in the core. A subset of WR stars are the central stars of planetary nebulae (CSPNe), post Asymptotic Giant Branch stars that were similar to the Sun while on the main sequence, but have now ceased fusion and shed their atmospheres to show a bare carbon-oxygen core. Another group (type WNh) show hydrogen lines in their spectra and are young extremely massive stars still fusing hydrogen at the core, with nitrogen mixed to the surface and strong radiation-driven mass loss. They are all highly luminous due to their high temperatures, thousands of times the bolometric luminosity of the Sun (L_☉) for the CSPNe, hundreds of thousands L_☉ for the Population I WR stars, to over a million L_☉ for the WNh stars, although not exceptionally bright visually since most of their radiation output is in the ultraviolet.