The varying degree of diffuseness in line character for stars earlier than class F5 presents an additional difficulty in their classification. On plates having a dispersion of around 30 A per mm the lines have such a varied appearance that it is almost impossible to classify the spectra on a uniform system. If the dispersion is reduced to lessen this effect, the lines in general become fainter.
The best compromise seems to be a dispersion of around 125 A per mm and greatly broadened spectra on high-contrast plates. Spectra of this dispersion can be classified with high accuracy for stars of classes O-B5 inclusive, if a fine-grain emulsion is used. The varying widths of the spectral lines are not very noticeable, except for a very few stars with exceedingly broad lines.
Spectra of classes B9-A2 are most difficult of all to classify accurately. All lines with the exception of the Balmer series are weak, and the broad-line stars show few spectral features that can be used. By the time class A3 is reached, numerous metallic lines make their appearance, and classification becomes progressively easier on passing toward lower temperature.
Dispersions higher than 125 A per mm can be used to classify the early-type stars, if a certain rough ratio is preserved between the dispersion and the spectrum width. For the highest accuracy the width of the spectrum should be about one-third the distance between H and H. With plates of higher dispersion a corresponding reduction in the magnifying power of the viewing eyepiece should be made. For spectra later than Fo a width of about one-sixth the distance between H and H is sufficient, unless the dispersion is less than 125 A per mm. Wide spectra for the late-type stars allow the use of the G band and other important blended features. The advantage of using broad spectra is somewhat similar to that of extrafocal images in stellar photometry.
|9 Sgr............||O5||O5||17:57||-24 22'||5.9||Oe5||1|
|HD 5005......||O6||.......||0:47||+56 5||7.7||B2||1|
|1 Ori C........||O6||O7||5:30||- 5 27||5.4||Oe5||2|
|HD 165052...||O7||O6||17:59||-24 24||6.8||Oe5||1|
|S Mon...........||O7||O7||6:35||+9 59||4.7||Oe5|
|Ori A.........||O8||O8||5:29||+9 52||3.7||Oe5|
|Ori..............||O9 V||O9||5:30||-5 59||2.9||Oe5||3|
|10 LAC.........||O9 V||O9||22:34||+38 32||4.9||Oe5||3|
|HD 188209...||O9 I||.......||19:49||+46 47||5.5||B0||4|
|HD 218915....||O9 I||.......||23:06||+52 31||7.1||B0||4|
|1. No emission lines visible on low-dispersion spectrograms. He II 4686 is much stronger than 4650.|
|2. The H lines are abnormally broad in comparison to other absorption lines.|
|3. Main-sequence star. Luminosity differences at O9 are shown by the following ratios: 4068: 4089, 4387: 4541, and 4650: 4686.|
|4. O-type supergiants.|