ON THE STRUCTURE OF THE UNIVERSE u 



fitted into the general scheme of stellar evolution remains at 

 present one of the unsolved problems ot astronomy. The 

 suggestion has been made that they have been formed from 

 stars through collisions with other stars. One would naturally 

 expect such collisions to occur most frequently in the Milky 

 Way, where the stars are densest, but their huge velocities are 

 not thus accounted for. 



(3) The third class of nebulae, and the class which is by far 

 the most numerous, is that of the Spiral nebulae. Their dis- 

 covery was the one striking achievement of the great Parsons- 

 town reflector, constructed by Lord Rosse. Fath has estimated 

 that there are at least 160,000 nebulae of all kinds, so that the 

 number of spirals must be very great. By far the most con- 

 spicuous object of this class is the great nebula in Andromeda, 

 easily visible to the naked eye as a small blurred patch, very 

 different from a star in appearance. It was the only nebula 

 discovered before the invention of the telescope. Photographs 

 show it to consist of a bright central nucleus, with long, spiral, 

 nebulous arms wreathing around it. The spiral nebulae that we 

 know are placed at all inclinations to our solar system. The 

 Andromeda nebula is seen obliquely. Many, like the Whirlpool 

 Nebula, are seen perpendicular to the plane of the spiral arms ; 

 in this case, and in others, the two arms are clearly seen starting 

 out from opposite edges of the central nucleus. Some, again, 

 are viewed edge on, and in these the spiral arms are seen as a 

 narrow line, evidence that they lie in one plane. This line is 

 seen dark where it crosses in front of the central nucleus, owing 

 to the scattering which the light from the latter undergoes in 

 passing through the arms. This is the kind of nebula to which our 

 sidereal system has been compared. From a careful considera- 

 tion of the structure of the Milky Way, obtained by combining 

 all the available photographs, C. Easton has recently given a 

 hypothetical representation of it in the form of a spiral, in which 

 account has been taken of all its prominent features. This 

 attempt to account for the structure of the Milky Way on the 

 spiral hypothesis is very interesting and instructive. 



Now if the known spiral nebulae are distinct from our 

 sidereal system, if they are indeed " island universes " in space, 

 one would expect them to show a uniform distribution relative 

 to the galactic plane. If, on the other hand, they were found 

 to show, as do the planetary and irregular nebulae, a marked 



