THE BOUNDARIES OF ASTRONOMY. 101 



In the first place, let us distinctly understand what is meant by our 

 sidereal system. We have already dwelt on the isolated position of 

 the sun and the attendant planets. The grandest truth in the whole 

 of astronomy is that which asserts that our sun is only a star separated 

 by the most gigantic distances from the other stars around. Our sun, 

 indeed, appears to be but one of the vast host of stars which form the 

 milky way. We need not here enter into the often-discussed question 

 as to whether the nebulae are, generally speaking, at distances of the 

 same order as the stars. There seems to be no doubt that some of the 

 nebulne are quite as near to us as some of the stars. At all events, for 

 our present purpose, we may group the milky way, the nebulae, the 

 stars, and the clusters, all into one whole which we call our sidereal 

 system. Is this sidereal system as thus defined an isolated object in 

 space ? are its members all so bound together by the law of universal 

 gravitation that each body, whatever be its movements, can only de- 

 scribe a certain path such that it can never depart finally from the 

 system '? This is a question of no small importance. It presents 

 features analogous to certain very interesting problems in biology 

 which the labors of Mr. Wallace have done so much to elucidate. We 

 are told that the fauna and flora of an oceanic island, cut off from 

 the perpetual immigration of new forms, often assumes a very re- 

 markable type. The evolution of life under such circumstances pro- 

 ceeds in a very different manner to the corresponding evolution in an 

 equal area of land which is connected with the great continental 

 masses. Is our sidereal system to be regarded as an oceanic island 

 in space, or is it in such connection with the systems in other parts of 

 space as might lead us to infer that the various systems had a common 

 character ? 



The evidence seems to show that the stars in our system are prob- 

 ably not permanently associated together, but that in the course of 

 time some stars enter our system and other stars leave it, in such a 

 manner as to suggest that the bodies visible to us are fairly typical of 

 the general contents of the universe. The strongest evidence that can 

 be presented on this subject is met with in the peculiar circumstances 

 of one particular star. The star in question is known as No. 1830 

 of Groombridge's catalogue. It is a small star, not to be seen with- 

 out the aid of a telescope. This star is endowed with a very large 

 proper motion. It would not be correct to say that its proper motion 

 exceeds that of any other known star, but it certainly has the largest 

 visible proper motion of any star of which the distance is known. The 

 proper motion of 1830 Groombridge amounts to over seven seconds 

 annually. It would take between two and three centuries to move 

 over a distance in the heavens equal to the apparent diameter of the 

 moon. The distance of this star is much greater than might have 

 been anticipated from its very large proper motion. The estimates 

 of the distance present some irregularities, but we shall probably be 



