A Wolf-Rayet (abbreviated to WR) is a massive and hot star. It has already evolved as the combustion of the hydrogen at its core, at the root of the first period of every star’s stable and peaceful life, has already given way to the combustion of helium. It has lost a significant quantity of its mass in the form of stellar winds which have carried away its external layers, thus bringing to light the deeper layers with nuclear processed material. Hydrogen has completely disappeared from the surface of the majority of them.
Illustration : Nebula around Wolf-Rayet WR124 in our Galaxy (©ESO).