ASTRONOMY: A. VAN MAANEN 
395 
of Table IV of Publications of the Astronomical Laboratory at Groningen, No. 27, 
the apparent photographic magnitudes, m, were derived. 
The values are given in the third column of table 2 ; by means of the formula 
M = m + 5 -H5 log ir, we can derive the absolute magnitudes M, which are 
given in the fourth column. 
The mean absolute magnitude is +9.1 and the deviations from this mean are 
small; this may be due to a small dispersion or to the choice of objects; the 
material, however, is still insufficient to discern which of the two possibilities 
is the cause. 
This faint absolute magnitude, +9.1, is noteworthy because the spectra of 
these objects consist in many cases mainly of bright lines, whereas, with the 
stars at large, bright-line spectra are usually associated with high luminosities ; 
TABLE 1 
N. G. C. 
EXPOSURE TIME 
NUMBER 
EXPOSURES 
NUMBER OF COMP. 
STARS 
7T REL. 
P. E. 
2392 
15 m 
18 
8 
+0"020 
±0'f003 
6720 
25 
14 
9 
+0.002 
0.005 
6804 
25 
18 
8 
+0.020 
0.003 
6905 
20 
18 
9 
+0.013 
0.002 
7008 
20 
20 
9 
+0.014 
0.004 
7662 
25 
16 
8 
+0.021 
±0.004 
TABLE 2 
N. G. C. 
7T ABS. 
m 
M 
2392 
+0*022 
10.0 
+ 6.7 
6720 
+0.008 
14.7 
+ 9.2 
6804 
+0.022 
13.4 
+ 10.1 
6905 
+0.015 
14.5 
+ 10.4 
7008 
+0.016 
12.8 
+ 8.8 
7662 
+0.023 
12.9 
+ 9.7 
for the Wolf-Rayet stars for instance, whose spectra resemble those of the 
planetaries in many respects, we find a mean absolute magnitude not far from 
0. 3 On the other hand, assuming the relation between luminosity and radial 
velocity found for the stars to apply to the planetary nebulae, the high radial 
velocities observed for these objects, about 29 km. per second, 4 are in good 
agreement with their low intrinsic brightness. Thus, for 13 K and M stars of 
mean absolute magnitude 10.0, Adams and Stromberg found a mean radial 
velocity of 30 km. per second. 5 
One class of stars, which seems to take an intermediate position, is that 
of the novae, whose spectra pass through the planetary spectrum towards that 
of the Wolf-Rayet stars. The parallax-material, which is extremely scarce as 
yet and includes only 3 novae, points to absolutely faint magnitudes, in the 
