Apbil 12, 1907] 



SCIENCE 



565 



than at maximum. Three stars will illus- 

 trate the results obtained. 



X Cygni 



We have then the following results: 



1. Binary systems, showing orbital revo- 

 lution. 



2. Colored stars, pointing to atmospheric 

 absorption. 



3. A regular variation in that color {i. e., 

 that absorption). 



Standard Photographic Magnitudes: Ed- 

 WAED C. Pickering. ^By title.) 



On the Proiahle Distance of Orion: Heney 



NoRKis Russell. 



Almost all the bright stars in Orion are 

 similar in spectrum and proper motion, 

 and are probably at roughly equal dis- 

 tances from us. Their very small proper 

 motions indicate that their parallax is too 

 small to be directly measured, and Gill's 

 observations of /3 Orionis confirm this 

 view. 



But many of these stars are double, and 

 some appear to be physical systems, show- 

 ing more or less motion— always very slow. 

 It is possible to derive a relation between 

 the parallax and mass of such systems and 

 the observed distance and rate of motion, 

 which, though not exact in any one case, 

 give good mean values when applied to 

 several stars. Tests of this relation on 



forty well-known binaries show that its 

 probable error for a single case is about 

 20 per cent. Applying it to nineteen pairs 

 which appear to belong to the Orion group, 

 and to be physical systems, we find for 

 their mean parallax ir=0".011/Vm, where 

 m is their average mass, and ir is subject to 

 considerable uncertainty owing to errors of 

 observation. 



There are many spectroscopic binaries in 

 Orion, but orbits have been published for 

 only two. These indicate that the average 

 mass of these stars is about ten times that 

 of the sun (subject to revision when more 

 data are available). This leads to a mean 

 parallax of 0".005 or a distance of 600 

 light years. 



For reasons already mentioned, this 

 should be regarded only as an indication 

 of the order of magnitude of the true dis- 

 tance. Errors of observation, and also 

 errors of judgment in including as phys- 

 ical systems pairs which may ultimately 

 turn out to be optical, would both tend to 

 make the calculated parallax too great. 



The Numier and Distribution of Stellar 

 Clusters and Neiulce: S. I. Bailey. 

 The visible universe beyond our solar 

 system is made up of two classes of ob- 

 jects, stars and nebula. All star-clusters 

 may be divided into two classes, irregular 

 and globular; the Pleiades serve to repre- 

 sent the former, the cluster in Hercules, 

 the latter. The term nebula may be ap- 

 plied to any nebulous object, whatever its 

 real nature, which can not be resolved into 

 stars. All nebulae may be divided con- 

 veniently into two classes, gaseous and 

 white, the former giving a spectrum of 

 bright lines, the latter an apparently con- 

 tinuous spectrum. The best known ex- 

 ample of the former is the great nebula in 

 Orion, of the latter, the great spiral in 

 Andromeda. 



Somewhat less than 700 resolvable star 



