156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



identical with that imagined by Laplace as resulting in the birth of 

 systems of planets and satellites, but on a far more stupendous scale. 

 The final product of the chain of events we have been considering 

 must be some type of star cluster — perhaps a globular star cluster, 

 or possibly an " island universe " similar to our galactic system. 

 The difficulties in the way of an exact mathematical investigation 

 into the history of the ejected gas, as the filaments condense around 

 nuclei and as these form stars and begin to move as detached bodies, 

 are enormous. On the other hand, the determination of the final 

 steady states possible for a system of stars created in this way is 

 quite simple. There is found to be only one type of final steady 

 state possible for a system of stars created out of a rotating mass 

 of gas, and this shows exactly the features presented by the system 

 of stars of which our sun is a member. The system of stars will be 

 of a flattened shape, symmetrical about the plane of greatest cross- 

 section (the galactic plane in our system) ; the velocities in any 

 small region of space will not be distributed at random, but will 

 show a preference for two opposite directions (" star streaming ") ; 

 these directions will be parallel to the plane of symmetry and per- 

 pendicular to the radius to the center of the system. This last direc- 

 tion is that given by Charlier for the direction of " star streaming " 

 in our system. Our system passes all tests for having been born out 

 of a spiral nebula the plane of which was what is now the plane of 

 the Milky Way; indeed, Easton and others have claimed to find 

 traces of the two spiral arms still surviving in the distribution of 

 stars in this plane, as though the final stead} 7 state had not yet been 

 reached. 



[Added February, 1922. — The test, however, is not a very stringent 

 one. For if a number of stellar systems, each one of which had been 

 born out of a rotating nebula in the way we have imagined, were to 

 fall together as the result of gravitational action and unite into a 

 single giant system of stars, it can be shown that this giant system, 

 if or when it attained a steady state, would show precisely these 

 same properties. Thus our test leaves it an open question whether our 

 universe has been born out of a single nebula or out of a great num- 

 ber of smaller nebulae. It probably accords best with present obser- 

 vational knowledge to suppose that our universe has been formed 

 by the intermingling of a large number of separate star groups each 

 of which is the product of a single spiral nubula. The globular 

 clusters may well be groups of this type which have not yet mingled 

 with the main mass of stars, while the moving star clusters, such as 

 the Taurus cluster, the Ursa-major cluster, and possibly also the 

 whole system of the B-type stars, may be groups, or the remains of 

 groups, which have fallen into the main mass and become inter- 



