5 Conclusion

The principal opportunity and expediency of use of the present technique for spectral classification of O-M stars up to V=22 mag was shown in the example of two young open clusters NGC 2244 and NGC 2264. It was proved also by results of comparison of spectral estimates obtained on the basis of UBV photometry with other independent estimates of spectra available in MK system (Fig. 2, Tables 3-9).

The developed technique can be used for spectral classification of stars not only in directions of young open clusters, but also in directions of more extended stellar groups (star-forming regions, OB - associations). The present technique, comparing to Q-method which is applicable only to young O-A0 stars, is more universal as far as it permits us to carry out spectral classification in much more useful O-M spectral intervals.

The main advantage of the present technique in relation to other multicolour photometric methods of spectral classification consists of allowing us to carry out extensive spectral classification programs of O-M stars up to V=22.0 mag using for this purpose only three-colour UBV photometric data. An important conclusion made was that for faint star spectral classification there is practically no necessity to use additional data, in particular on proper motions. In this case spectral classification can be carried out with the use of one main criterion which characterizes distribution of dust substance in a given direction.