Variable Stars 
23 
cases, however, the two components alternately pass in front of one another in 
their orbits as seen from the earth, and so undergo eclipse at regular intervals. 
At these moments of eclipse we do not see the total light from both con 
stituents, so that the brightness of the system appears to undergo a diminution. 
The light received from such a system accordingly varies, and the system is 
described as an “eclipsing variable,” or an “eclipsing binary.” It will be 
understood that physically an eclipsing binary is precisely similar to a non 
eclipsing binary; the* difference between them results solely from the accident 
of the earth being nearly in the plane of the orbit in the case of the eclipsing 
binary, and well out of this plane in the case of the non-eclipsing binary. 
Variable Stars. 
24 . While the variation in the light of an eclipsing variable is in a 
sense accidental, and in no way physically inherent in the system itself, 
there are other classes of variable stars in which the light variation must be 
ascribed to actual changes in the light-emission of the stars themselves. The 
general characteristic of these true variables is that the quality of the light 
changes as well as the amount received. In extreme cases the light varies 
through a considerable range of colour as the star varies, while the amount of 
visual light may vary by a factor as great as 4000 to 1 , although the ratio is 
greatly reduced when the invisible heat-radiation of the star is added to its 
visible light radiation (cf. §48 below). 
The Cepheid variables already mentioned form the most interesting class 
of true variables. Both these and other variables are of somewhat rare occur 
rence in space, but their importance is not to be measured by the frequency of 
their occurrence, and we shall discuss them further in Chap, xv, below. 
Groups of Stars. 
Triple and Multiple Systems. 
25 . We have already noticed that nearly one-third of the nearest stars are 
binary, and one of these systems, the nearest of all, constitutes in effect a triple 
system, namely, the binary a Centauri, accompanied in its journey through 
space by its distant companion Proxima Centauri. The period of the two 
components of a Centauri is 78'83 years, while its distance shews that the 
period of Proxima about this binary system is to be reckoned in millions of 
years. 
This provides a rather extreme instance of a triple system, but there are 
innumerable instances of more normal triple systems in which the periods are 
shorter, as also of multiple systems in which more than three components 
journey together through space, describing orbits meanwhile under their 
mutual gravitational attractions. Russell found that of 800 double stars no 
fewer than 74, or 925 per cent, of the whole, formed part of triple or multiple