NEWS FEOM THE STARS ABBOT. 167 



or sixth magnitude can be seen with the unaided eye, according to 

 the clearness of the sky and its freedom from the glare of cities. A 

 diiference of five magnitudes means a difference of a hundredfold in 

 brightness. Thus sixth, eleventh, sixteenth, and twenty-first magni- 

 tude stars are respectively a hundred, ten thousand, a million, and a 

 hundred million times fainter than first-magnitude stars. Our sim 

 is about twenty-six magnitudes, or twenty-five billion times brighter 

 than zero magnitude stars like Vega. 



Do the stars increase in number without limit as we consider fainter 

 and fainter ones revealed by larger and larger telescopes? To 

 answer this question counts have been made of the actual numbers 

 in the whole sky for the brighter magnitudes, and then of numerous 

 patches of sky sufficient to give a fair average sample for the fainter 

 magnitudes. In this way it has been found that up to the tenth 

 magnitude the num- 

 ber of stars brighter 

 than a given mag:ni- t^ " v////Mmm/////mi *" C^-^ 



times as great as 



the number brighter (j 



than the magnitude ^— ^ 



next preceding. ^UN 



From this point on 



the increase grows « - 



less and less rapid, ~ - 



so that of s t a r s 



brighter than the Fig. 3. — The system of R R Draconis. Diagram by Shapley. 

 seventeenth magni- ^^""'^ Astrophysical Journal, vol. 37. 



tude, the estimated number according to Chapman and Melotte, is 

 only 55,000,000 instead of 1,800,000,000 as it would be if this constant 

 ratio of increase prevailed. Up to the present time no thorough 

 counts have been finished beyond the 17.5 magnitude, although by 

 the aid of j)botography it is possible to observe stars as faint as 

 the twenty-first magnitude with the great 60-inch reflector of Mount 

 Wilson Solar Observatory. Arranging the information given by the 

 counts in mathematical fashion, it appears that it is unlikely that 

 any very considerable increase in the number of the stars will be 

 found by observing stars fainter than the twenty-sixth magnitude, 

 however large the telescope available. 



Stars at this limit are about as much fainter than those of zero 

 magnitude as our sun is brighter, so that the brightest star (our 

 sun) is twenty-five billion times twenty-five billion (25X10^X25X10^) 

 times as bright as the stars of the faintest class which are probably 

 shining upon us in any considerable numbers. The total estimated 



