ISLAND GALAXIES — DOUGLAS 195 



With the whole gahixy of stars^ — our siin and the thousand million 

 other suns which stud the heavens all around us. 



There are known to be many thousand spiral nel)uhK', and if each 

 be comj^arable in size to our whole stellar galaxy it is obvious that 

 they are not within it. They are in fact island galaxies. The term 

 is here used to denote exactly the same thing as the term " island 

 universes," which has become so connnon aii expression in American 

 astronomical writing. Since " universe " is defined as " all that 

 exists, the creation and the Creator," its us6 in the plural seems unfor- 

 tunate, especially as the word "galaxy" is quite adequate. Digni- 

 fying our stellar system by the name "'' tlie galax}^ " — not merely 

 because it is the system to which our sun belongs but for the more 

 logical reason that as yet no other aggregation of stars is known to 

 be quite as large — it then becomes natural to divide all nebukie into 

 two main classes, termed, respectively, the " galactic " and the 

 "extragalactic " nebulae. 



Our galaxy is a gigantic aggregation of stars and gaseous nebulee. 

 It comprises all the stars visible to the naked eye and the many 

 thousands more revealed by the telescope when used visually. These 

 numbers are multiplied manyfold by the use of photography when 

 stars so faint or remote as to be invisible leave the impress of their 

 images upon the sensitive plate after many hours of exposure. The 

 study of these photographs, counting the numbers of the stars of 

 different magnitude or brightness and comparing the numbers in 

 different parts of the sky, has shown that it is possible to make an 

 estimate of their number and a representation of their distribution 

 in space. 



Even with the unaided eye it is evident that the distribution of 

 stars is not spherically symmetrical. All along a great circle in 

 the heavens the stars are more numerous than elsewhere, and this en- 

 circling band is called the Milky Way. The photographic plates 

 reveal the same concentration, and so the Milky Way is called the 

 galactic plane, while the directions at right angles to this plane, 

 where the stars are less numerous and on the average less distant, 

 are termed the galactic poles. Our sun happens to be situated not 

 far from the center of this great lens-shaped cluster of stars. The 

 dimensions of the galaxy are so vast as to be best appreciated when 

 expressed in light-years, the unit so frequently employed by astron- 

 omers, equivalent to nearly 6,000,000,000,000 miles. Our galaxy is 

 approximately 100,000 light-years across the galactic plane and about 

 one-fifth as much measured toward the galactic poles. 



In this vast region, at great distances one from another, there are 

 30,000 million stars, according to the most recent calculations of 

 Scares and van Ehijn as reported by Dr. C. G. Abbot of the Smith- 

 sonian Institution, Washington. As these are by no means equi- 



