198 THE POPULAR SCIENCE MONTHLY. 



The name Hercules sufficiently indicates the mythological origin 

 of the constellation, and yet the Greeks did not know it by that name, 

 for Aratus calls it " the Phantom whose name none can tell." The 

 Northern Crown, according to fable, was the celebrated crown of Ari- 

 adne, and Lyra was the harp of Orpheus himself. 



With the aid of the map you will be able to recognize the principal 

 stars and star-groups in Hercules, and will find many interesting com- 

 binations of stars for yourself. An object of special interest is the 

 celebrated star-cluster 13 M. You will find it on the map between the 

 stars Eta (77) and Zeta (). While an opera-glass will only show it as 

 a faint and minute speck, lying nearly between two little stars, it is 

 nevertheless well worth looking for, on account of the great renown of 

 this wonderful congregation of stars. Sir William Herschel computed 

 the number of stars contained in it as about fourteen thousand. It is 

 roughly spherical in shape, though there are many straggling stars 

 around it evidently connected with the cluster. In short, it is a ball 

 of suns. The reader should not mistake what that implies, however. 

 These suns, though truly solar bodies, are probably very much smaller 

 than our sun. Mr. Gore has recently computed their average diam- 

 eter to be about forty-five thousand miles, and the distance separating 

 one from another to be 9,000,000,000 miles. Adopting Mr. Gore's 

 estimates of their size and distance, I have recently shown, in a news- 

 paper article, that, as seen from the center of the cluster, the nearest 

 stars would shine about fourteen hundred times as bright as Sirius, 

 while even the farthest stars in the cluster would be much brighter 

 than Sirius, so that a world situated there would enjoy a sort of per- 

 petual daylight, the illumination of its nocturnal sky being, perhaps, 

 as much as fifteen times greater than the light shed from the full moon 

 upon the earth. 



If you have a field-glass, by all means try it upon 13 M. It will 

 give you a more satisfactory view than an opera-glass is capable of 

 doing, and will magnify the cluster so that there can be no possibility 

 of mistaking it for a star. Compare this compact cluster, which only 

 a powerful telescope can partially resolve into its component stars, 

 with 7 M. and 24 M., described above, in order to comprehend the 

 wide variety in the structure of these aggregations of stars. 



The Northern Crown, although a strikingly beautiful constellation 

 to the naked eye, offers few attractions to the opera-glass. Let us 

 turn, then, to Lyra. I have never been able to make up my mind 

 which of three great stars is entitled to precedence Vega, the lead- 

 ing brilliant of Lyra, Arcturus in Bootes, or Capella in Auriga. They 

 are the three leaders of the northern firmament, but which of them 

 should be called the chief, is very hard to say. At any rate, Vega 

 would probably be generally regarded as the most beautiful, on account 

 of the delicate bluish tinge in its light, especially when viewed with 

 a glass. There is no possibility of mistaking this star on account of 



