Published online by Cambridge University Press: 25 May 2016
We argue that the WR phenomenon is simply a consequence of very high mass-loss rates (i.e., it is independent of initial mass, composition, or evolutionary status) and that it occurs in very massive main-sequence stars. We explore this argument using new HST observations of R 136a, the compact cluster containing three high-mass WR stars on the main sequence. We suggest that the underlying cause of the WR phenomemon in these stars is mass-loss enhanced by rapid rotation. Finally, we explore the implications of luminous, main-sequence WR stars for interpreting unresolved WR star clusters and galaxies.