Evolutionary tracks of pure helium-star models at solar metallicity in the HR diagram, for six different initial masses (8, 10, 15, 20, 25, and 30 M_⊙), as indicated by the labels. The numbers in the parentheses denote the corresponding final masses. The positions where the surface mass-fraction of carbon is larger than 0.2 are marked by thick grey lines. The star symbol marks the end point of the evolution, which is the beginning of core oxygen-burning that occurs a few years before core collapse, for each evolutionary sequence, while the tilted rectangle denotes the position at 1000 yrs before the supernova explosion. The positions of Galactic WR stars with known distances given by Hamann et al. (2006) and Sander et al. (2012) are marked by open squares (WNL), filled squares (WNE), filled circles (WC), and triangles (WO).