160 Transactions. — Miscellaneous. 



Art. XV. — On the Origin of Double Stars. 



By Professor A. W. Bickerton. 



[Read before the Philosophical Institute of Canterbury, 5th August, 1880.] 



Plate IIa. 



In the former general papers on Partial Impact the probable formation of 

 double stars by its means has been referred to with increasing emphasis. 

 In the present maturing state of theory, probably, if we except temporary 

 stars and some nebulae, there is no phenomenon that receives such an abso- 

 lutely satisfactory explanation as does the origin of associated binaries. 

 There are three possible explanations of the origin of double stars. 1st. 

 They may have been associated at the birth of the visible Universe, either 

 as stars or nebulae which afterwards condensed. 2nd. It is possible when 

 three stars approach comparatively near each other, that by their mutual 

 attraction one may have its proper motion increased at the expense of the 

 other two. These latter may by their lessened motion become an associated 

 pair. 3rd. A partial impact, in which the coalesced part is neither so small 

 as to allow of escape of the non-colliding parts, nor so large as to produce 

 complete coalescence. Between these two extremes all impacts must pro- 

 duce bodies free of each other, but associated by gravitation. This "par- 

 tial impact" may have taken place when the two stars were in a nebulous 

 state, and they may have coalesced into stars since ; the reasoning which 

 applies to stellar, also applies to nebular impacts, but is not capable of 

 such complete demonstration. 



It is the origin of binaries by impact that is studied in this paper, and I 

 believe it will ultimately be found to account for nine-tenths of the double 

 stars.* In all partial impacts, as above limited, the collision is attended 

 with the formation of a central gaseous mass expanding into a nebula, and 

 two parts which pass on, — these parts in this case form the pair of binaries 

 under discussion. Eecent criticism has shown it necessary to again most 

 emphatically call attention to the fact that in such cosmical collisions there 

 is no loss of momentum m the portion of the bodies that is not in actual 

 collision, except that due to the work of sheering, and this I shall show to 

 be insignificant. 



The coalesced part will, however, exercise an increased attraction, which 

 will prevent the non-colliding parts from attaining their former proper 



* Mr. Croll in a foot-note to his paper on the origin of the sun's heat, calls atten- 

 tion to a paper by Mr. Johnson Stoney, in which he states that double stars may be due 

 to a partial entanglement of two colliding stars. 



