556 The Genesis of Double Stars 



The coincidence between the spectroscopic and the photometric 

 evidence permits us to feel complete confidence in the theory of 

 eclipses. When then we find a star with a light-curve of perfect 

 regularity and with the characteristics of that of Algol, we are justified 

 in extending the theory of eclipses to it, although it may be too 

 faint to permit of adequate spectroscopic examination. This extension 

 of the theory secures a considerable multiplication of the examples 

 available for observation, and some 30 have already been discovered. 



Dr Alexander Roberts, of Lovedale in Cape Colony, truly remarks 

 that the study of Algol variables " brings us to the very threshold of 

 the question of stellar evolution 1 ." It is on this account that I 

 propose to explain in some detail the conclusion to which he and some 

 other observers have beeu led. 



Although these variable stars are mere points of light, it has 

 been proved by means of the spectroscope that the law of gravitation 

 holds good in the remotest regions of stellar space, and further it 

 seems now to have become possible even to examine the shapes of 

 stars by indirect methods, and thus to begin the study of their 

 evolution. The chain of reasoning winch I shall explain must of 

 necessity be open to criticism, yet the explanation of the facts by 

 the theory is so perfect that it is not easy to resist the conviction that 

 we are travelling along the path of truth. 



The brightness of a star is specified by what is called its " magni- 

 tude." The average brightness of all the stars which can just be seen 

 with the naked eye defines the sixth magnitude. A star which only gives 

 two-fifths as much light is said to be of the seventh magnitude ; while 

 one which gives 2 J times as much light is of the fifth magnitude, and 

 successive multiplications or divisions by 2 A- define the lower or higher 

 magnitudes. Negative magnitudes have clearly to be contemplated ; 

 thus Sirius is of magnitude - 1*4, and the sun is of magnitude - 26. 



The definition of magnitude is also extended to fractions ; for 

 example, the lights given by two candles which are placed at 100 ft. 

 and 100 ft. 6 in. from the observer differ in brightness by one- 

 hundredth of a magnitude. 



A great deal of thought has been devoted to the measurement of 

 the brightness of stars, but I will only describe one of the methods used, 

 that of the great astronomer Argelander. In the neighbourhood of the 

 star under observation some half dozen standard stars are selected of 

 known invariable magnitudes, some being brighter and some fainter 

 than the star to be measured ; so that these stars afford a visible scale 

 of brightness. Suppose we number them in order of increasing bright- 

 ness from 1 to 6 ; then the observer estimates that on a given night 

 his star falls between stars 2 and 3, on the next night, say between 



1 Proc. Roy. Soc. Edinburgh, xxiv. Pt. n. (1902), p. 73. 



