Variable Stars 555 



his observations to the Royal Society, and suggested that the variation 

 in brilliancy was due to periodic eclipses by a dark companion star, 

 a theory now universally accepted as correct. The Royal Society 

 recognised the importance of the discovery by awarding to Goodricke, 

 then only 19 years of age, their highest honour, the Copley medal. 

 His later observations of fi Lyrae and of B Cephei were almost as 

 remarkable as those of Algol, but unfortunately a career of such 

 extraordinary promise was cut short by death, only a fortnight after 

 his election to the Royal Society 1 . 



It was not until 1889 that Goodricke's theory was verified, when 

 it was proved by Vogel that the star was moving in an orbit, and 

 in such a manner that it was only possible to explain the rise and 

 fall in the luminosity by the partial eclipse of a bright star by a 

 dark companion. 



The whole mass of the system of Algol is found to be half as 

 great again as that of our sun, yet the two bodies complete their 

 orbit in the short period of 2 d 20 h 48 m 55 s . The light remains 

 constant during each period, except for 9 h 20 m when it exhibits a 

 considerable fall in brightness 2 ; the curve which represents the 

 variation in the light is shown in Fig. 7 below. 



The spectroscope has enabled astronomers to prove that many 

 stars, although apparently single, really consist of two stars circling 

 around one another 3 ; they are known as spectroscopic binaries. 

 Campbell of the Lick Observatory believes that about one star in six 

 is a binary 4 ; thus there must be many thousand such stars within 

 the reach of our spectroscopes. 



The orientation of the planes of the orbits of binary stars appears to 

 be quite arbitrary, and in general the star does not vary in brightness. 

 Amongst all such orbits there must be some whose planes pass nearly 

 through the sun, and in these cases the eclipse of one of the stars by 

 the other becomes inevitable, and in each circuit there will occur two 

 eclipses of unequal intensities. 



It is easy to see that in the majority of such cases the two com- 

 ponents must move very close to one another. 



1 Diet, of National Biography; article Goodricke (John). The article is by Miss Agnes 

 Clerke. It is strange that she did not then seem to he aware that he was a deaf-mute, 

 but she notes the fact in her Problems of Astrophysics, p. 337, London, 1903. 



2 Clerke, Problems of Astrophysics, p. 302 and ch. xvm. 



3 If a source of light is approaching with a great velocity the waves of light are 

 crowded together, and conversely they are spaced out when the source is receding. Thus 

 motion in the line of sight virtually produces an infinitesimal change of colour. The 

 position of certain dark lines in the spectrum affords an exceedingly accurate measurement 

 of colour. Thus displacements of these spectral lines enables us to measure the velocity 

 of the source of light towards or away from the observer. 



4 Aatrophysical Journ. Vol. xin. p. 89, 1901. See also A. Koberts, Nature, Sept. 12, 

 1901, p. 468. 



