244 CARNEGIE INSTITUTION OF WASHINGTON. 



iimgnitudes, the distance in parsecs, d, being related to the apparent 

 visual magnitude, m, by 



log d=0.2 (m+13.8). 



Results derived from a photographic study of the globular cluster 

 N. G. C. 7006 include a comparison of near and distant systems, a 

 record of new nebulae, and new data on the form, distance, and dimen- 

 sions of this very distant globular cluster. A count of 13,500 stellar 

 images on five photographs of N, G. C. 7789 shows the system to be an 

 open cluster, containing something more than 1,000 stars, situated 

 in a rich galactic field. It is about 20 parsecs in linear diameter, if a 

 previous estimate of the parallax (0''0003) be accepted, and most of its 

 stars are brighter than absolute photographic magnitude +6.5. 



Cepheid and Cluster Variables. 



The discovery of variable stars in the faint globular cluster M 72 was 

 mentioned in last year's repoit. With the assistance of Miss Ritchie 

 and Miss Mayberry, Mr. Shapley has now derived and pubhshed the 

 periods and light-curves of 26 of these stars, which are by far the faint- 

 est and most distant variables studied up to the present time. Without 

 exception they are Cepheids of the cluster type, the periods ranging 

 between 0.33 and 0.66 day. The median photographic magnitudes are 

 all near the mean value 16.8; the average range of brightness is 1 

 magnitude. A new determination gives 83,000 light years for the 

 distance of the cluster. 



Since the last report the photometric parallaxes of 9 Cepheid vari- 

 ables of the galactic sj^stem have been derived, 5 of them belonging 

 to the cluster type. The variable W Vh-ginis, period 17.113 days, is 

 remarkable for its great distance (20,000 light years), and especially 

 for its high galactic latitude, which is typical of cluster variables but 

 quite anomalous for long-period Cepheids. The period of SW Tauri 

 is very uncommon, falUng between the values typical for long and short 

 period Cepheids; the star's position with respect to the galactic plane 

 suggests affiliation with the short-period group. 



With respect to the theory of Cepheid variation, Mr. Shapley finds 

 that the observed period-luminosity law requires that the average 

 heat-content per unit-mass should be practically identical for all 

 Cepheids with periods longer than 3 days. Eddington's theory of the 

 radiative equilibrium of a giant star is assumed for the computation of a 

 theoretical period-luminosity curve. 



In addition to the 98 new variables reported last year. Miss Ritchie 

 has found 7 new variable stars on Mr. Shapley's photographs of clus- 

 ters; two of the new variables are in the distant system N. G. C. 7006. 

 Miss Ritchie has also derived a series of magnitudes of Barnard's sus- 

 pected variable near Messier 11. 



