CHAPTERS ON THE STARS. g 



The system of oc Centauri is interesting from the shortness of the 

 period, the brightness of the stars and the fact that it is the nearest 

 star to ns so far as known. We reproduce a diagram of the apparent 

 orbit from Dr. See's work. The period of revolution found by Dr. 

 See is eighty-one years. The major axis of the apparent orbit is 32"; 

 of the minor axis 6". 



The pairs of which, so far as known, the period of revolution is the 

 shortest, are these: 



Years. 



5 Pegasi ; R. A. =21h. 40m. ; Dec.= + 25°11' ; Period = ll. 42. 



6 Equulei; " =21h. 10m. ; " 

 B, Sagittarii;" =18h. 56m. ; " 

 ft Argus; '' = 7h. 47m. ; " 

 85 Pegasi ; " - 23h. 57m. ; " 



TRIPLE AND MULTIPLE SYSTEMS. 



Systems of three or more stars so close together that there must be 

 a physical connection between them are quite numerous. There is 

 every variety of such systems. Sometimes a small companion of a 

 brighter star is found to be itself double. A curious case of this sort 

 is that of y Andromedse. This object was observed and measured by 

 Struve as an ordinary double star, of which the companion was much 

 smaller than the principal star. Some years later Alvan Clark found 

 that this companion was itself a close double star, of which the com- 

 ponents, separated by about 1", were nearly equal. Moreover, it 

 was soon found that these components revolved round each other in 

 a period not yet accurately determined, but probably less than a cen- 

 tury. Thus we have a binary system revolving round a central star, 

 as the earth and moon revolve round the sun. 



In most triple systems there is no such regularity as this. The 

 magnitudes and relative positions of the components are so varied that 

 no general description is possible. Stars of every degree of brightness 

 are combined in every way. Observations on these systems extend over 

 so short an interval that we have no data for determining the laws 

 of motion that may prevail in any but one or two of the simplest cases. 

 They are, in all probability, too complicated to admit of profitable 

 mathematical investigation. There is, therefore, little more of inter- 

 est to be said about them. 



There is a very notable multiple system known as the Trapezium of 

 Orion, from the fact that it is composed of four stars, one of which is 

 plainly visible to the naked eye, while the others may he well seen in 

 the smallest telescope. There are also two other very faint stars, each 

 of which seems to be a companion of one of the bright ones. This sys- 

 tem is situated in the great nebula? of Orion, to be described in the next 



