The number of nebulae of known distance is too small to serve as a basis for estimates of the range in absolute magnitude among nebulae in general. Further information, however, can be derived from a comparison of total apparent magnitudes with apparent magnitudes of the brightest stars involved, on the reasonable assumption, supported by such evidence as is available, that the brightest stars in isolated systems are of about the same intrinsic luminosity.
The most convenient procedure is to test the constancy of the differences in apparent magnitude between the brightest stars involved and the nebulae themselves, over as wide a range as possible in the latter quantities.
An examination of the photographs in the Mount Wilson collection has revealed no stars in the very faint objects or in the bright elliptical nebulae and early-type spirals. This was to be expected from the conclusions previously derived. Observations were therefore confined to intermediate- and late-type spirals and the irregular nebulae to the limiting visual magnitude 10.5. The Magellanic Clouds and N.G.C. 6822 were added to the nebulae in Holetschek’s list. Altogether, data were available for 32 objects, or about 60 per cent of the total number in the sky to the adopted limit. For this reason it is believed that the results are thoroughly representative.
TABLE XVI
Difference in Magnitude between Nebulae and Their Brightest Stars
| N.G.C. | ms | mT | ms – mT |
|---|---|---|---|
| Sb | |||
| 224 | 15.5 | 5.0 | 10.5 |
| 1068 | 17.5 | 9.1 | 8.4 |
| 2841 | >19.5 | 9.4 | >10.1 |
| 3031 | 18.5 | 8.3 | 10.2 |
| 3310 | >19.0 | 10.4 | > 8.6 |
| 3623 | >20.0 | 9.9 | >10.1 |
| 3627 | 18.5 | 9.1 | 9.4 |
| 4438 | >19.0 | 10.3 | > 8.7 |
| 4450 | 19.5 | 10.0 | 9.5 |
| 4736 | 17.3 | 8.4 | 8.9 |
| 4826 | >19.5 | 9.2 | >10.3 |
| 5055 | >19.0 | 9.6 | > 9.4 |
| 5746 | >19.5 | 10.4 | > 9.1 |
| 7331 | 19.0 | 10.4 | 8.6 |
| SBb | |||
| 4699 | >19.5 | 10.0 | > 9.5 |
| Sc | |||
| 253 | 18.3 | 9.3 | 9.0 |
| 598 | 15.6 | 7.0 | 8.6 |
| 2403 | 17.3 | 8.7 | 8.6 |
| 2683 | >20.0 | 9.9 | >10.1 |
| 2903 | 19.0 | 9.1 | 9.9 |
| 4254 | 18.5 | 10.4 | 8.1 |
| 4321 | 18.8 | 10.5 | 8.3 |
| 4414 | >19.5 | 10.1 | > 9.4 |
| 4490 | 18.8 | 10.2 | 8.6 |
| 5194 | 17.3 | 7.4 | 9.9 |
| 5236 | 18.6 | 10.4 | 8.2 |
| 5457 | 17.0 | 9.9 | 7.1 |
| Irr. | |||
| LMC | 9.5 | 0.5 | 9.0 |
| SMC | 12.0 | 1.5 | 10.5 |
| 3034 | >19.5 | 9.0 | >10.5 |
| 4449 | 17.8 | 9.5 | 8.3 |
| 6822 | 15.8 | 8.5 | 7.3 |
The data are listed in [Table XVI] and are shown graphically in [Figure 10]. The luminosities of the brightest stars are given in photographic magnitudes. For the Magellanic Clouds, M 33, and N.G.C. 6822, these were obtained from published star counts. For M 31, 51, 63, 81, 94, and N.G.C. 2403, they depend upon unpublished counts, for which the magnitudes were determined by comparisons with Selected Areas. For the remaining nebulae, the magnitudes of stars were estimated with varying degrees of precision, but are probably less than 0.5 mag. in error.
Fig. 10.—Relation between total magnitudes of extra-galactic nebulae and magnitudes of the brightest stars involved. Differences between total visual magnitudes of nebulae and the photographic magnitudes of the brightest stars are plotted against the total magnitudes. The dots represent cases in which the stars could actually be detected; the incomplete crosses represent cases in which stars could not be detected, and hence give lower limits for the magnitude differences. The diagonal line indicates the approximate limits of observation, fixed by the circumstance that, in general, stars fainter than 19.5 probably would not be detected on the nebulous background.
The sloping line to the right in [Figure 10] represents the limits of the observations, for, from a study of the plates themselves, it appeared improbable that stars fainter than about 19.5 could be detected with certainty on a nebulous background. Points representing nebulae in which individual stars could not be found should lie in this excluded region above the line, and their scatter is presumably comparable with that of the points actually determined below the line. When allowance is made for this inaccessible region, the data can be interpreted as showing a moderate dispersion around the mean ordinate
| (7) |