Hopmann[15] has revised the scale of magnitudes by photometric measures of the comparison stars used by Holetschek. New magnitudes were thus obtained for 85 individual nebulae and from these were derived mean correction tables applicable to the entire list. The revised magnitudes are used throughout the following discussion. Hopmann’s corrections extend to about 12.0 mag., and have been extrapolated on the assumption that they are constant for the fainter magnitudes. The errors involved are unimportant in view of selective effects which must be present among the observed objects near the limit of visibility.
The nebulae were classified and their diameters measured from photographs of about 300 of them taken with the 60-inch and 100-inch reflectors at Mount Wilson. Most of the others are included in the great collection of nebular photographs at Mount Hamilton, which have been described by Curtis;[16] and, through the courtesy of the Director of the Lick Observatory, it has been possible to confirm the classification inferred from the published description by actual inspection of the original negatives.
Types, diameters, and total visual magnitudes are thus available for some 400 of the nebulae in Holetschek’s list. The few unclassified objects are all fainter than 12.5 mag. The data are listed in Tables I–IV, in which the N.G.C. numbers, the total magnitudes, and the logarithms of the maximum diameters in minutes of arc are given for each type separately. A summary is given in [Table V], in which the relative frequencies and the mean magnitudes of the various types will be found.
RELATIVE LUMINOSITIES OF THE VARIOUS TYPES
The frequency distribution of magnitudes for all types together and for the elliptical nebulae and the spirals separately is shown in [Table VI] and [Figure 1]. With the exception of the two outstanding spirals, M 31 and M 33, the apparent luminosities are about uniformly distributed among the different types. The relative numbers of the elliptical nebulae as compared with the spirals decrease somewhat with decreasing luminosity, but this is very probably an effect of selection. The elliptical nebulae are more compact than the spirals and become more stellar with decreasing luminosity. For this reason some of the fainter nebulae are missed when small-scale instruments are used, although the same luminosity spread over a larger area would still be easily detected. The effect is very pronounced on photographic plates. It accounts also for the slightly brighter mean magnitude of the elliptical nebulae as compared with the spirals in [Table V].
TABLE I
Elliptical Nebulae
| N.G.C. | mT | log d |
|---|---|---|
| E0 (17) | ||
| 404 | 11.1 | +0.11 |
| 474 | 12.6 | – .40 |
| 1407 | 10.9 | .15 |
| 3348 | 11.8 | – .15 |
| 3379* | 9.4 | + .30 |
| 4283 | 12.2 | – .52 |
| 4486* | 9.7 | + .30 |
| 4494* | 10.1 | – .15 |
| 4552* | 9.9 | + .23 |
| 4589 | 11.4 | – .30 |
| 4648 | 12.3 | .52 |
| 5044 | 11.8 | .30 |
| 5216 | 13.3 | .70 |
| 5273 | 12.1 | .52 |
| 5557 | 12.3 | .40 |
| 5812 | 12.0 | – .40 |
| 5846 | 10.9 | 0.0 |
| Mean | 11.40 | –0.204 |
| E1 (13) | ||
| 467 | 13.0 | –0.70 |
| 596 | 11.8 | .22 |
| 1400 | 11.1 | .22 |
| 2880 | 12.0 | .52 |
| 3226 | 12.0 | .10 |
| 3962 | 11.8 | – .30 |
| 4278* | 10.8 | .0 |
| 4374* | 9.9 | + .08 |
| 4472 | 8.8 | + .30 |
| 4478 | 11.5 | – .10 |
| 4636 | 10.9 | + .08 |
| 5813 | 12.6 | – .30 |
| 7626 | 12.3 | –0.30 |
| Mean | 11.43 | –0.177 |
| E2 (14) | ||
| 221* | 8.8 | +0.42 |
| 1453 | 11.9 | – .10 |
| 2672 | 12.8 | – .40 |
| 3193 | 12.1 | 0.0 |
| 3599 | 12.0 | –0.30 |
| 3608 | 11.6 | .22 |
| 3640 | 11.1 | – .05 |
| 4261 | 11.1 | + .20 |
| 4291 | 12.3 | – .52 |
| 4377 | 11.9 | – .05 |
| 4406* | 10.0 | + .30 |
| 4476 | 12.8 | – .30 |
| 4649* | 9.5 | + .30 |
| 5127 | 13.3 | –0.52 |
| Mean | 11.52 | –0.088 |
| E3 (10) | ||
| 1052 | 11.8 | –0.15 |
| 1600 | 12.7 | + .17 |
| 3222 | 13.3 | – .15 |
| 4319 | 12.8 | – .52 |
| 4365 | 11.4 | + .04 |
| 4386 | 12.3 | – .52 |
| 5322* | 9.6 | + .15 |
| 5982 | 11.4 | .0 |
| 7562 | 12.8 | – .22 |
| 7619 | 11.8 | – .15 |
| Mean | 11.99 | –0.133 |
| E4 (13) | ||
| 584 | 10.9 | +0.30 |
| 1700 | 12.5 | – .10 |
| 2974 | 11.8 | .15 |
| 3605 | 12.5 | – .52 |
| 3610 | 11.8 | + .15 |
| 3894 | 12.8 | – .05 |
| 4125* | 10.3 | + .30 |
| 4378 | 12.1 | – .15 |
| 4382* | 10.0 | + .48 |
| 4551 | 12.8 | + .04 |
| 4742 | 12.3 | .0 |
| 5576 | 12.3 | – .15 |
| 7454 | 13.3 | 0.0 |
| Mean | 11.95 | –0.011 |
| E5 (6) | ||
| 720 | 10.9 | + .11 |
| 2693 | 12.3 | – .15 |
| 3377 | 10.9 | + .17 |
| 4473 | 10.3 | .11 |
| 4621* | 10.0 | + .30 |
| 4660 | 11.4 | 0.0 |
| Mean | 10.97 | +0.090 |
| E6 (7) | ||
| 821 | 11.8 | 0.0 |
| 2768 | 10.7 | + .18 |
| 3613 | 11.8 | .25 |
| 4179 | 11.8 | .34 |
| 4435* | 10.5 | .11 |
| 4546* | 10.3 | .18 |
| 4697* | 9.6 | +0.48 |
| Mean | 10.93 | +0.220 |
| E7 (5) | ||
| 3115* | 9.5 | +0.60 |
| 4111 | 10.1 | .54 |
| 4270 | 12.1 | .0 |
| 4570 | 11.1 | .38 |
| 5308 | 12.3 | +0.28 |
| Mean | 11.02 | +0.360 |
| Peculiar (8) | ||
| 185 | 12.3 | +0.48 |
| 205* | 9.3 | .90 |
| 524† | 11.9 | .41 |
| 3607† | 9.9 | .11 |
| 3998† | 12.1 | + .23 |
| 4459‡ | 11.3 | – .22 |
| 5485‡ | 12.3 | .05 |
| 5739 | 13.3 | –0.40 |
The various types are homogeneously distributed over the sky, their spectra are similar, and the radial velocities are of the same general order. These facts, together with the equality of the mean magnitudes and the uniform frequency distribution of magnitudes, are consistent with the hypothesis that the distances and absolute luminosities as well are of the same order for the different types. This is an assumption of considerable importance, but unfortunately it cannot yet be subjected to positive and definite tests. None of the individual similarities necessarily implies the adopted interpretation, but the totality of them, together with the intimate series relations among the types, which will be discussed later, suggests it as the most reasonable working hypothesis, at least until inconsistencies should appear.
TABLE II
Barred Spirals
| N.G.C. | mT | log d |
|---|---|---|
| SBa (26) | ||
| 936 | 11.1 | +0.48 |
| 1023* | 10.2 | .78 |
| 2732 | 12.3 | .11 |
| 2781 | 12.3 | .11 |
| 2787 | 11.4 | .36 |
| 2859 | 11.1 | .28 |
| 2950 | 11.6 | .15 |
| 3384* | 10.7 | .48 |
| 3412* | 11.2 | + .40 |
| 3418 | 13.1 | .0 |
| 3458 | 12.8 | – .22 |
| 3945 | 11.5 | + .20 |
| 4026 | 11.1 | .48 |
| 4203 | 11.1 | .36 |
| 4346 | 12.0 | .20 |
| 4371 | 12.0 | .18 |
| 4421 | 12.8 | .17 |
| 4442 | 10.9 | .50 |
| 4477 | 10.9 | .40 |
| 4596 | 12.0 | .25 |
| 4643 | 11.1 | .26 |
| 4754 | 10.9 | .48 |
| 5473 | 12.0 | + .08 |
| 5574 | 13.0 | – .05 |
| 5689 | 12.0 | + .30 |
| 5701 | 12.3 | +0.17 |
| Mean | 11.66 | +0.267 |
| SBb (16) | ||
| 1022 | 11.8 | +0.04 |
| 2650 | 12.8 | .0 |
| 3351* | 11.4 | + .48 |
| 3400 | 12.5 | – .10 |
| 3414 | 11.5 | + .26 |
| 3504 | 11.4 | .30 |
| 3718 | 11.8 | +0.48 |
| 4102 | 12.0 | +0.36 |
| 4245 | 11.1 | .15 |
| 4394 | 11.5 | .60 |
| 4548 | 11.1 | .60 |
| 4699* | 10.0 | .57 |
| 4725* | 9.2 | .70 |
| 5218 | 12.8 | .25 |
| 5566 | 11.1 | .20 |
| 7723 | 11.8 | +0.18 |
| Mean | 11.48 | +0.317 |
| SBc (15) | ||
| 613 | 10.6 | +0.60 |
| 779 | 12.1 | .48 |
| 3206 | 13.3 | .45 |
| 3344 | 11.4 | .60 |
| 3346 | 12.3 | .40 |
| 3625 | 13.3 | .0 |
| 3686 | 12.0 | .30 |
| 3769 | 12.8 | .43 |
| 3953 | 11.1 | .74 |
| 3992 | 11.5 | .85 |
| 4303* | 10.6 | .78 |
| 4579* | 9.7 | .45 |
| 5383 | 12.6 | .40 |
| 5921 | 12.8 | .70 |
| 7479 | 12.1 | +0.48 |
| Mean | 11.87 | +0.509 |
| Peculiar (2) | ||
| 2782 | 12.3 | +0.26 |
| 4314 | 11.1 | +0.34 |