CHAPTERS ON THE STARS. 13 



an observable inequality. Such an inequality can arise only through 

 the action of a neighborhood of a mass at least comparable with that 

 of our sun. A new field of astronomical research is thus opened, the 

 exploration of which must occupy many years. The ultimate result 

 may be to make as great an addition to our knowledge of the heavens 

 as has been made during the last century by the telescope. 



STAR-CLUSTERS. 



A star-cluster is a bunch or collection of stars separated from the 

 great mass of stars which stud the heavens. The Pleiades, or 'seven 

 stars,' as they are familiarly called, form a cluster, of which six of the 

 components are easily seen by the naked eye, while five others may be 

 distinguished by a good eye. 



About 1780 Michell, of England, raised the question whether, sup- 

 posing the stars visible to the naked eye to be scattered over the sky 

 at random, there would be a reasonable possibility that those of the 

 Pleiades would all fall within so small a space as that filled by the 

 constellation. His correct conclusion was in the negative. It follows 

 that this cluster does not consist of disconnected stars at various dis- 

 tances, which happen to be nearly in a line from our system, but is 

 really a collection of stars by itself. Besides the stars visible to the 

 naked eye, the Pleiades comprise a great number of telescopic stars, of 

 which about sixty have been catalogued and their relative positions de- 

 termined. The principal star of the cluster is Alcyone or i] Tauri, which 

 is of the third magnitude. The five which come next in the order of 

 brightness are not very unequal, being all between the fourth and fifth 

 magnitudes. Six are near the sixth magnitude. The remainder, so far 

 as catalogued, range from the seventh to the ninth. 



In this case there is a fairly good method of distinguishing between 

 a star which belongs to the cluster and one which probably lies beyond 

 it. This test is afforded by the proper motion. All the stars of the 

 group have a common proper motion in the same direction of about 

 seven seconds per century. The first accurate measures made on the 

 relative positions of the stars of the cluster were those of Bessel, about 

 1830. In recent years several observers have made yet more accurate 

 determinations. The most thorough recent discussion is by Elkin. 

 One result of his work is that there is as yet no certain evidence of any 

 relative motion among the stars of the group. They all move on to- 

 gether with their common motion of seven seconds per century, as if 

 they were a single mass. 



A closer cluster, which is plainly visible to the naked eye and looks 

 like a cloudy patch of light, is Prassepe in Cancer. It is very well seen 

 in the early evenings of winter and spring. Although there is nothing 

 in the naked-eye view to suggest a star, it is found on telescopic ex- 



