232 POPULAR SCIENCE MONTHLY. 



to which our attention is directed in the present work. If we are asked 

 what they are, we may reply that the stars are suns. But we might 

 equally well say that the sun is one of the stars; a small star, indeed, 

 surrounded by countless others, many of which are much larger and 

 brighter than itself. We shall treat our theme as far as possible by 

 what we may call the natural method, beginning with what, being most 

 obvious to the eye, was first noticed by man, or will be first noticed by 

 an observer, and tracing knowledge up step by step to its present state. 



Several features of the universe of stars will be evident at a glance. 

 One of these is the diversity of the apparent brightness, or, in technical 

 language, of the magnitudes of the stars. A few far outshine the great 

 mass of their companions. A greater number are of what we may call 

 medium brightness; there is a yet larger number of fainter ones, and 

 about one half of all those seen by a keen eye under favorable con- 

 ditions are so near the limit of visibility as to escape ordinary notice. 

 Moreover, those which we see are but an insignificant fraction of the 

 number revealed by the telescope. The more we increase our optical 

 power, the greater the number that come into view. How many mil- 

 lions may exist in the heavens it is scarcely possible even to guess. The 

 photographic maps of the heavens now being made probably show 

 fifty millions, perhaps one hundred millions or more. 



Another evident feature is the tendency of the brighter stars to 

 cluster into groups, known as constellations. The latter are extremely 

 irregular, so that it is impossible to decide where one constellation 

 should end and another begin, or to which constellation a certain star 

 may belong. Hence, we can neither define the constellations nor say 

 what is their number, and the division of the stars among them is a 

 somewhat arbitrary proceeding. 



A third feature is the Milky Way or Galaxy, which, to ordinary 

 vision, appears as an irregular succession of cloud-like forms spanning 

 the heavens. We now know that these seeming clouds are really con- 

 geries of stars too small to be individually visible to the naked eye. 

 We shall hereafter see that the stars of the Galaxy form, so to speak, 

 the base on which the universe appears to be constructed. Each of 

 these three features will be considered in its proper place. 



II. Magnitudes of the Staks. 

 The apparent brightness of a star, as we see it from the earth, 

 depends upon two causes — its intrinsic brilliancy or the quantity of 

 light which it actually emits, and its distance from us. It follows that 

 if all the stars were of equal intrinsic brightness we could determine 

 their relative distances by measuring the respective amounts of light 

 which we receive from them. The quantity of light in such a case 

 varies inversely as the square of the distance. This will be made 



