CHAPTERS ON THE STARS. j 7 



period is llh. 5m. 7s. Apparently about 65 per cent, of the whole 

 period is occupied by the increase of the light. This very slow rate of 

 increase is especially striking from the fact that in many cases in this 

 cluster the increase is extremely rapid, probably not more than ten 

 per cent, of the whole period. In one ease, No. 45, having a period of 

 14h. 8m., the rise from minimum to maximum, a change of two mag- 

 nitudes takes place in about one hour, and in certain cases, chiefly owing 

 to the necessary duration of a photographic exposure, there is no proof 

 at present that the rise is not much more rapid. 



"The marked regularity in the period of these stars is worthy of 

 attention. Several have been studied during more than a thousand, 

 and one during more than five thousand, periods without irregularities 

 manifesting themselves." 



It may be added that this regularity of the period, taken in con- 

 nection with the case of rj Aquilse, already mentioned, affords a strong 

 presumption that the variations in the light of these stars are in seme 

 way connected with the revolution of bodies around them, or of one star 

 round another. Yet it is- certain that the types are not of the Algol 

 class and that the changes are not due merely to one star eclipsing an- 

 other. That such condensed clusters should have a great number of 

 close binary systems is natural, almost unavoidable, we might suppose. 

 It will hereafter be shown to be probable that among the stars in gen- 

 eral single stars are the exception rather than the rule. If such be the 

 case, the rule should hold yet more strongly among the stars of a con- 

 densed cluster. 



Perhaps the most important problem connected with clusters is the 

 mutual gravitation of their component stars. Where thousands of 

 stars are condensed into a space so small, what prevents them from all 

 falling together into one confused mass? Are they really doing so, and 

 will they ultimately form a single body? These are questions which 

 can be satisfactorily answered only by centuries of observation; they 

 must, therefore, be left to the astronomers of the future. 



XEBUL.E. 



The first nebula, properly so-called, to be detected by an astronomi- 

 cal observer was that of Orion. Huyghens, in his 'Systema Saturnium,' 

 gives a rude drawing of this object, with the following description: 



"There is one phenomenon among the fixed stars worthy of men- 

 tion which, so far as I know, has hitherto been noticed by no one, and, 

 indeed, cannot be well observed except with large telescopes. In the 

 sword of Orion are three stars quite close together. In 1656, as I 

 chanced to be viewing the middle one of these with the telescope, in- 

 stead of a single star, twelve showed themselves (a not uncommon 

 circumstance). Three of these almost touched each other, and, with 



VOL. LVIII.— 2 



