50 F. W. Yery — Nebulosity around Nova Persei. 



tions which can be urged against them, either from rational 

 considerations, or on the ground of observational data, espe- 

 cially availing ourselves of every scrap of observation which 

 can support either of the hypotheses. 



(1) The lirst explanation, attributing the phenomenon to 

 colliding meteor swarms, has been chiefly advocated by Sir 

 Norman Lockyer. The certain existence of meteor swarms in 

 free space, and the partial agreement between the spectra of 

 some meteorites and nebulae, are strong points in favor of the 

 general theory. The fatal objections to the explanation in the 

 present case are : {a) That the expansion of the nebula has 

 proceeded at too rapid a rate. Let us suppose that two spher- 

 ical swarms of equal diameter and equal velocities of 100 

 kilometers per second, are traveling in opposite directions 

 along a common diameter, and that the circle of the intersect- 

 ing surfaces moves outward with the velocity of light, which 

 it must do if the nebular expansion has been produced in 

 this way. In one year each body will have moved through 

 3156x10° km., and the line of intersection will have expanded 

 through nine and one-half billion kilometers (English numera- 

 tion). Allowing that the nova is in the center of its sphere, 

 and measuring from this center, the angular aperture of the 

 lenticular volume bounded by the intersecting swarm-surfaces, 

 after one year, is less than 3°, and if the nova's parallax is 

 O'^'OS, the swarm will stretch two-thirds of the way to the sun ! 

 Even the broad celestial spaces are not wide enough to con- 

 tain such monsters, {b) The fading of the nebula has been 

 too rapid, for there is no reason to suppose that the meteoric 

 particles are confined to a thin superficial shell and that the 

 space within is relatively vacant, (c) As it would take over 

 100,000 years for the motion to pass from the circumference to- 

 the center of the sphere, any connection between the actual 

 nova and the nebula would be impossible on this hypothesis. I 

 showed in a previous paper* that the outburst of the nova 

 itself cannot be due to colliding meteor swarms. It is, if any- 

 thing, more completely demonstrable that the nebula has no 

 such origin. 



Sir Norman Lockyer explains the nebula around Nova 

 Persei as " a nebula [previously too faint to be detected] 

 invaded, not by one, but by many swarms [of meteors], under 

 such conditions that the collision effects vary very greatly in 

 intensity. . . . The most violent one . . . constitutes Nova 

 Persei. The least violent ones occurring in other parts of the 

 disturbed nebula, almost immeasurably removed, i. e., more 

 than 700 solar distances away, we only learn of from the recent 

 photographs."! The distances are here greatly underesti- 



*Tliis Journal (4), xiii, 114. f Nature, Ixv, 134, Dec. 12, 1901. 



