Wolf–Rayet stars, often abbreviated as
WR stars, are a rare heterogeneous set of stars with unusual
spectra showing prominent broad emission lines of ionised
helium and highly ionised
nitrogen or
carbon. The spectra indicate very high surface enhancement of
heavy elements, depletion of hydrogen, and strong
stellar winds.
The
surface temperatures of known Wolf–Rayet stars range from 20,000
K to around 210,000
K, hotter than almost all other kinds of stars. They were previously called
W-type stars referring to their
spectral classification.Classic (or
population I) Wolf–Rayet stars are
evolved, massive stars that have completely lost their outer
hydrogen and are
fusing helium or heavier elements in the core. A subset of the population I WR stars show hydrogen lines in their spectra and are known as WNh stars; they are young extremely massive stars still fusing hydrogen at the core, with helium and nitrogen exposed at the surface by strong mixing and radiation-driven mass loss.
A separate group of stars with WR spectra 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 reveal a bare carbon-oxygen core.All Wolf–Rayet stars are highly luminous objects due to their high temperatures—thousands of times the
bolometricluminosity 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.
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