68 ILLINOIS ACADEMY OF SCIENCE 
suggestion of Kapteyn (‘‘Scientia,’’ Vol. XIV, N. XXXII-6, 
pp. 345-357) that the irregular nebula of Orion has qualities 
that fit it for an initial place in the helium series of stars, 
seems to tally well with the view, entertained earlier in this 
paper, that center-to-center collisions of stars, or other massive 
bodies, would give rise to irregular or radiant nebulze. When 
one considers the prodigious velocities at which the great 
stars must collide, it is perhaps not too much to suspect that 
extreme dissociation attends the collision, and that this may be 
sufficient to give rise to the spectral effects which appear in 
the hydrogen-helium-nebulium nebulz, and these may lead on 
to the spectral characters that appear in the helium stars and 
in the series that follows them, This speculative conjecture 
seems to find a measure of support in the succession of spectra 
cbserved in the Nove. If this be warranted, the origin of this 
class of stars falls into the final place in the series of stellar 
approaches, the supremely close approaches, the head-on col- 
lisions. This brings the whole into a series of harmonious 
relationships. The dynamics of the last term, however, are of 
the collision-rebound or gaseous type, not of the orbital. A 
center-to-center collision is, however, an event much less fre- 
quent than are such approaches as are held to give rise to 
spiral nebulz, and yet the stars greatly outnumber the known 
spiral nebulz. Perhaps a reconciliation of this discrepancy 
may be found in the probable fact that the stars endure longer 
than the nebulz, and in the multiplication of stars by spira- 
loidal separation into great knots that later condense into suns 
of a smaller order. 
A further respect in which the stars seem to fall concor- 
dantly in with the orbital scheme of evolution, though not them- 
selves of the orbital genus, seems to be disclosed by the recent 
discovery that the velocities of the stars are correlated with 
their spectral types, and these, in the opinion of some, are 
correlated with their ages, and in the opinion of others, with 
their masses. In either case, the relation is referred to the 
mutual influence of the stars in their approaches to one another 
as time goes on, these approaches not being usually near 
enough to call forth dispersive action. This class of approaches 
may be assigned a place at the distal end of the series of ap- 
proaches, while the less distant order of approaches form the 
smaller spiral nebulz, the still nearer approaches the larger 
