STELLAR EVOLUTION JEANS. 155 



ter with high central condensation behaving precisely as imagined by 

 Laplace — rotating and throwing off their excess of angular mo- 

 mentum as the}'' cool by the ejection of matter in their equatorial 

 planes. 



There is, however, almost incontrovertible evidence that the nebulas 

 which have just been described are nothing but ordinary spiral nebulae 

 seen edgewise, for observation discloses a continuous sequence of 

 nebulae the shapes of which bridge completely the gap between 

 the lenticular nebulae, in which we are looking at right angles to the 

 axis of rotation, and the familiar spiral nebula in which we look ap- 

 proximately along this axis. The characteristic nebula shows a nu- 

 cleus which we can now identify with the lenticular figure demanded 

 by theory, having two arms emerging symmetrically from opposite 

 points of the nucleus. If our identification is correct these arms 

 must be formed out of the matter already discharged from the nu- 

 cleus. It has in point of fact been found by van Maanen and Kostin- 

 sky that the matter in the arms appears to be in motion approxi- 

 mately along the arms and in the outward direction. 



Any external gravitational field, whether of the universe as a whole 

 or of neighboring stars or nebulae, would produce a tidal field similar 

 to that produced by the sun and moon on the surface of our earth, a 

 field specified mathematically by a second harmonic. This field, no 

 matter how small in amount, would suffice to destroy the exact circu- 

 lar shape of the " equator " of the nucleus and so would concentrate 

 the emission of matter at two opposite points on this equator. Thus 

 it is easy to understand why the nebulae, as a rule, exhibit two sym- 

 metrical arms emerging from antipodal points. It is very much less 

 easy to understand why these arms should be of the universal spiral 

 form — the absence of any explanation of this form must be regarded 

 as a serious drawback to our interpretation of the spiral nebulae. It 

 is readily proved that the ejected filaments of matter, whatever the 

 shape they assume, could not remain of uniform line-density. Such a 

 distribution of density would be unstable, and it can be proved that 

 nuclei would form at approximately equal distances, around which 

 the matter of the arms would condense. In this way it is possible to 

 explain the nuclei and condensations which are observed in the arms 

 of the spiral nebulae. It is also found possible to calculate the amount 

 of matter which will condense around each nucleus ; the mass of each 

 is found to be of the order of magnitude of the known masses of the 

 stars. 



In this way I have been led to conjecture that the spiral nebulae 

 are whirling masses of gas which, owing to their rapidity of rotation, 

 throw off gaseous stars much as a " Catherine-wheel " firework throws 

 off sparks. If so, the condensations in the arms of these nebulae are 

 stars in the process of birth. Dynamically the mechanism is almost 



