The Wolf-Rayet stars are luminous, hot stars whose spectra are dominated by broad, strong emission lines associated with massive circumstellar shells expanding outwards with velocities on the order of 1000 - 2500 km/s. Wolf-Rayet stars are among the most luminous stars in the galaxy. Wolf-Rayet stars can be arranged into essentially two sequences, the nitrogen sequence and the carbon sequence. The nitrogen sequence (the WN stars) shows many emission lines of ionized nitrogen, whereas the carbon sequence (the WC stars) have spectra dominated by emission lines of ionized carbon.
HD 192163 is an example of a WN star. Notice the strong emission lines due to N III, N IV and N V. The spectral subtype (6, in the case of HD 192163) reflects the line strength ratios between different ionization stages of nitrogen. The spectrum also shows emission lines due to He II. As the hydrogen lines coincide closely in wavelength with every other He II line, it can be quite difficult to decide if hydrogen contributes to these emission lines. Notice that the He I 3888 line consists of a blue-shifted absorption component and a broad emission component centered on 3888 Å. This is a classic "P-Cygni" line profile formed in the massive stellar winds emanating from this object.
HD 193793 is an example of a WC star. In this spectral range, the spectra of these stars are dominated by a single broad, blended emission line of C III and C IV, although other much weaker emission lines can be seen.
The classification system for the WN stars has been revised recently by Smith et al. 1996 (MNRAS 281, 163) who have introduced a three-dimensional system. The WN subclass is based on the He II 5411/He I 5875 ratio. In addition, the WN stars are classified with respect to line broadening and the presence of hydrogen in the spectrum.