It is shown that the O- and B-type stars can be classified in a two-dimensional scheme by two different methods. In one method V, B, L and U are used. the region L contains several hydrogen lines ands is therefore used as an indicator for luminosity, whereas the region U provides information concerning the pectral type. This method is based on the same principle as the methods of Stroemgren and Chalonge. In the other method V,B,U and W are used, but now U is an indicator of luminosity and the colour U-W is used for the determination of the spectral type. The principle of this method was first found by Mlle L. Divan.
Considering the large difference in character of the two methods, it is surprising that the results of the two-dimensiomal classification show a remarkable agreement. The large discrepancies between the directly observed luminosity and the luminosity derived by Strömgren's method from Hbeta or Hlambda, the so-called cosmical scatter, are reproduced exactly by both our methods.
A similar agreement is found for deviations of spectral type from the MK type. This proves that both the method of Strömgren-Chalonge and the method of Divan are basically correct, in so for as they give definite information concerning the state of a stellar atmosphere, i,e., electron pressure and temperature. The cosmical scatter must be caused by some unknown factor, which affects the relation between these two parameters and the total stream of light emitted by a star. Blaauw has found that in the Scorpio-Centaurus group age differences appear to be correlated with differences in luminosity. Similar effects are shown by our luminosity criteria, which indiates that at least part of the cosmical scatter may be the effect of different ages.
Provisional results for 120 stars, mostly in the Scorpio-Centaurus group and in the cluster NGC 6231, are given in Table 5. V and B-V in this table are expressed in magnitudes, whilst throughout the article the quantities [B-U], [U-W] and [B-L] represent logarithms of intensity ratios corrected for interstellar reddening.
|Lambda eff. (Å)||5441||4298||3837||3623||3235|