310 FKOM NEBULA TO NEBULA 



all others put together that the inference is unescapable 

 that they are the advanced stages of a cosmic process by 

 which all nebulae are being molded. They are simply sys- 

 tems of planetary bodies enshrouded in star-dust ex- 

 pelled by the explosion of their central sun or suns, which 

 they are gradually redding up by their gravitational suc- 

 tion while regularly rounding their orbits. To visualize 

 such a spiral nebula you need only imagine our sun to ex- 

 plode profoundly, and then picture in your mind's eye 

 what convolutions should take place in the nebula by the 

 action of the planets. The exaggerated sizes predicated 

 of some of the nebulae are no hindrance to Nature's dis- 

 ciplinary powers. Be a nebula as large as it may, it still 

 has its master stellar resultant, it still remains subser- 

 vient to the inexorable law of equilibrium, the law of the 

 balance-arm, Kepler's laws, the law of the logarithmic 

 spiral, the law of gravitation all one and the same in es- 

 sence. By way of exemplifying these remarks let me 

 cull still another extract from Professor Eddington's 

 book (p. 241) : 



It is generally believed that the spirals predominate enor- 

 mously over the other classes of nebulae ; and, as the whole num- 

 ber of nebulae bright enough to be photographed has been es- 

 timated by E. A. Fath at 160,000, they must form a very num- 

 erous class of objects. They are seen by us at all inclinations, 

 some, like the Whirlpool Nebula, in full front view, whilst others 

 are edge-on to us and appear as little more than a narrow line. 

 An example of the latter kind is also illustrated in Plate IV. In 

 all cases, where it is possible to discriminate the details, the spiral 

 is seen to be double-branched, the two arms leaving the nucleus 

 at opposite points and coiling around in the same sense. From 

 the researches of E. v. d. Pahlen, it appears that the standard 

 form is a logarithmic spiral. The arms, however, often present 

 irregularities, and numerous knots and variations of brightness 

 occur. Unlike the planetary and extended nebulae the spectrum 

 shows a strong continuous background; bright lines and bands 

 are believed to occur, at least in the Great Andromeda Nebula; 

 but they are of the character of those found in some of the early 

 type stars, and are distinct from the emission lines of the gaseous 

 nebulae. 



The distribution of spiral nebulae presents one quite unique 

 feature ; they actually shun the galactic regions and preponderate 

 in the neighborhood of the galactic poles. The north galactic pole 



