396 
ASTRONOMY: A. VAN MA AN EN 
mean +6.9, for these stars when at their minimum luster. The radial veloci- 
ties, however, for the three cases in which they have been determined, are 
small, in the mean 10 km. per second; this would indicate high luminosities. 
It should not be forgotten, however, that the relation between luminosity 
and radial velocity is based on stars of the same spectral type and it is ques- 
tionable if this relation may safely be applied to stars of all types, including 
planetaries and novae. 
The appearance of the nebulosity around Nova Persei and especially the 
little nebulous ring, which was photographed by Ritchey 6 and Pease 7 in 1917, 
opens the way for speculations concerning a possible relation between novae 
and planetary nebulae. To arrive at conclusions, however, further observa- 
tional data are necessary. 
The size of the planetary nebulae can be deduced from the above mentioned 
parallaxes, by which the diameters measured in arc can be converted into 
astronomical units or into light-years; the results are given in table 3; for the 
diameters in this table the major axes have been used. 
TABLE 3 
DIAMETER 
N. G. C. 
Angular 
In astronomical units 
In light-years 
2392 
46" 
2,100 
0.03 
6720 
80 
10,000 
0.16 
6804 
32 
1,450 
0.02 
6905 
47 
3,100 
0.05 
7008 
95 
5,900 
0.09 
7662 
31 
1,350 
0.02 
By comparing these values with the size of our solar system (the diameter of 
the orbit of Neptune is 60 astronomical units) it is clear that the planetary 
nebulae must be of enormous size. 
1 Maanen, A. van, these Proceedings, 3, 1917, (133-136). 
2 Newkirk, B. L., Lick Obs. Bui., Univ. CaL, Berkeley, 9, 1917, (100-107). 
3 Maanen, A. van, Pub. Astron. Soc. Pacific, San Francisco, 30, 1918, (308). 
4 Campbell, W. W., and Moore, J. H., these Proceedings. 1, 1915, (496-498). 
5 Ritchey, G. W., Pub. Astron. Soc. Pac, San Francisco, CaL, 29, 1917, (256). 
6 Adams, W. S., and Stromberg, G., Ml. Wilson Contr., No. 131; Astroph. X., Chicago, 45, 
1917, (293-305). 
'Pease, F. G., Ibid., 29, 1917, (256-257). 
