Wolf-Rayet stars represent an evolutionary phase in the lives of massive stars during which they undergo heavy mass loss. They are characterized by an extraordinary spectrum which is dominated by emission lines of highly ionized elements.
These extremely hot (up to ~50,000K) and very luminous (1E5 to 1E6 Lsol) are very rare, reflecting their short lifespan.
Wolf-Rayet Stars are supernovas waiting to happen. They are essentially the naked cores of massive stars from which extreme stellar winds have stripped off the atmosphere. As they age, they move towards the lower left of the H-R Diagram, becoming smaller and hotter before ending in a final apocalyptic fling as a Type IIb supernova.
Their surface composition is extremely exotic, being dominated by helium rather than hydrogen, and typically showing broad wind emission lines of elements like carbon (WC type), nitrogen (WN type), or oxygen: the products of core nucleosynthesis. The presence or absence of hydrogen, respectively, is used to distinguish the so-called late type WN stars (WNL) from the early (WNE) types.
Intense stellar winds drive mass loss rates of several 1E4 up to 1E5 Msol per year; the latter are at least three or four times that expected for other hot, O-type or B-type stars.
Text by Chris Clowes
Initially published on 31 December 2007.