311 
277 - 280 ] Disturbances from Passing Stars 
they must originally (still speaking statistically) have been greater than they 
now are. 
The average periods of binaries are so large in comparison with the criti 
cal period of 55 days, that the argument can be put in a very simple form. 
In a binary of period 100 years, which is about the average period of observed 
binaries, the velocity of each component relative to the centre of gravity of 
the system is about four kilometres a second. This system is disturbed by 
stars and other binary systems moving with velocities relative to it of the 
order of forty kilometres a second, and it is excessively improbable that such 
disturbance can either leave the relative velocity at the low figure of four 
kilometres a second or decrease it still further. The normal event is for the 
relative velocity to be increased, and this may either break up the system by 
causing its components to describe hyperbolic orbits or may lessen the 
period if they continue to describe elliptic orbits. Either event results in a 
decrease of the average period of the whole system of binaries. 
279. We accordingly conclude that the average period of the binaries in 
the sky is at present being reduced by their encounters with other stars. 
Hence the average of these periods in the past must have been greater than 
the present average of about 100 years, and we must suppose that they had 
some other origin than fission of the kind we have had under consideration. 
The most probable account of the long-period binaries would seem to be 
that they represent the remains of independent condensations in the parent 
nebula which failed to get clear of one another’s gravitational fields and have 
been describing orbits about one another ever since, their periods being on the 
average continually lessened by encounters with other stars. 
On this view binary systems fall into two distinct classes— 
(i) Systems formed by rotational fission, whose periods are short but 
lengthening. 
(ii) Systems formed out of independent condensations in the parent 
nebula, whose periods are long but shortening. 
The former class have, on the average, periods of less than 55 days, and the 
latter class periods of more than 55 days. The two classes correspond broadly 
to the two observational classes of spectroscopic and visual binaries. 
The Genesis of Triple and Multiple Systems. 
280. After a binary system has been formed by fission, each of its two 
components may undergo a further shrinkage, under the same conditions of 
approximate constancy of angular momentum as produced fission in the parent 
star, and these conditions may produce fission in the components, thus gene 
rating triple or multiple systems. Actually the angular momenta of the two 
components will not remain absolutely constant, but will be diminished to a