BEYOND THE MILKY WAY 



By Thornton Page 

 Yerkes Observatory 



IWith 4 plates] 



Before we can appreciate this topic it is necessary to understand pre- 

 cisely what is meant by the term "Milky Way" and how we can know 

 when we are "beyond" it. Initially it is simple to define the Milky 

 Way as the faint band of light which can be seen on clear nights to 

 extend completely around the whole sphere of the sky and which tele- 

 scopes have resolved into myriads of faint stars. As we shall see, the 

 term has come to mean more than just what we can see ; to some extent 

 it is synonymous with the term "galaxy," which describes our concept 

 of an organized system of stars, diffuse clouds of gas, and other ma- 

 terial, inside of which we are located, and the study of which has oc- 

 cupied astronomers for several decades and will probably occupy them 

 for several decades to come. 



Why should these faint stars be distributed so peculiarly, as if in a 

 doughnutlike ring around us? A simple interpretation comes to 

 mind if we assume, for the moment, that all stars are intrinsically of 

 about the same brightness. Then these faint stars of the Milky Way 

 would be farther away than the brighter stars, their apparent bright- 

 ness being less the more distant they are. (The apparent brightness of 

 a luminous source is known from other considerations to be inversely 

 proportional to the square of the distance.) Although refinements 

 are necessary, this simple deduction from an unproved assumption is 

 one of the basic methods for measuring astronomical distances. We 

 must investigate further these distances and their measurement before 

 the term "beyond" can have any meaning. 



Astronomical distances are well recognized to be considerably larger 

 than the distances we are familiar with ; it is quite impossible to com- 

 prehend them in an absolute sense. But since we are concerned here 

 only with one thing being "beyond" another, an appreciation of the 

 relati/ve sizes of various distances is sufficient. To avoid large num- 

 bers, it is convenient to use large units of distance : the "astronomical 

 unit," which is about 100 million miles (accurately the distance to the 



• An address given before the Cleveland Astronomical Society on November 4, 1949. Reprinted by 

 permission from Popular Astronomy, vol. 57, No. 10, December 1949. 



165 



