The residuals without regard to sign average 0.87 mag., and there appears to be no systematic effect due either to type or luminosity. The scatter, however, is much greater for the spirals, especially in the later types, than for the elliptical nebulae. The limiting cases are explained by peculiar structural features. The nebulae which fall well above the line usually have bright stellar nuclei, and those which fall lowest are spirals seen edge-on in which belts of absorption are conspicuous.
TABLE VII
| Type | mT | log d | C* | d† |
|---|---|---|---|---|
| E0 | 11.40 | –0.204 | 10.38 | 1.2 |
| 1 | 11.43 | .177 | 10.54 | 1.3 |
| 2 | 11.52 | .088 | 11.08 | 1.6 |
| 3 | 11.99 | .133 | 11.33 | 1.8 |
| 4 | 11.95 | – .011 | 11.90 | 2.4 |
| 5 | 10.97 | + .090 | 11.42 | 1.9 |
| 6 | 10.93 | .220 | 12.03 | 2.5 |
| 7 | 11.02 | .360 | 12.82 | 3.7 |
| Sa | 11.69 | .333 | 13.35 | 4.7 |
| b | 11.55 | .471 | 13.90 | 6.0 |
| c | 11.74 | .540 | 14.44 | 7.7 |
| SBa | 11.66 | .267 | 13.00 | 4.0 |
| b | 11.48 | .317 | 13.16 | 4.3 |
| c | 11.87 | .509 | 14.41 | 7.6 |
| Irr | 11.34 | +0.469 | 13.68 | 5.4 |
* C = mT + 5 log d.
† log d = 0.2 (C — mT); mT = 10.0.
EFFECTS OF ORIENTATION
The effect of the orientation is appreciable among the spirals in general. In order to illustrate this feature, they have been divided into three groups consisting of those whose images are round or nearly round, elliptical, and edge-on, or nearly so. The mean values of mT + 5 log d were then computed and compared with the theoretical value, 13.0. The residuals are negative when the nebulae are too bright for their diameters and positive when they are too faint. The results are given in [Table IX], where mean residuals are followed by the numbers of nebulae, in parentheses, which are represented by the means.
The numbers of the barred spirals are too limited to inspire confidence in the results, but among the normal spirals there is conclusive evidence that the highly tilted and edge-on nebulae are fainter for a given diameter than those seen in the round. A study of the individual images indicates that the effect is due very largely to dark absorption clouds, which become more conspicuous when the nebulae are highly tilted. These clouds are generally, but not universally, peripheral features. An extensive investigation will be necessary before any residual effect due to absorption by luminous nebulosity can be established with certainty. Even should such exist, it clearly cannot be excessive.
Fig. 6.—Progressive characteristics in the sequence of types. The upper curve represents the progression in total magnitude with type for nebulae having maximum diameters of one minute of arc. The elliptical nebulae and the normal spirals are included as representing the normal sequence, but the barred spirals and the irregular nebulae are omitted. The figures give the number of objects observed in each type. Among the later elliptical nebulae the numbers are so small that means of adjacent types have been plotted. The lower curve represents the progression in diameter along the normal sequence for nebulae of the tenth magnitude.