JUM3 1, 1803.1 



KNOWLEDGE 



109 



F 



WHAT IS A STAR CLUSTER? 



By A. C. Ranyakd. 



OKTY years ago, when it was generally believed tbat 

 star clusters were stellar universes similar to the 

 Galaxy which spans the heavens, it was assumed 

 that the individual stars of clusters would bear 

 comparison — as to magnitude and brightness — 

 ■with our sun. It was then pretty generally believed tbat 

 the stars of clusters were only rendered dim by distance, 

 and that they were separated from one another by 

 interspaces similar to the star depths which separate us 

 from the isolated stars dotted over the heavens, many of 

 which are evidently associated with the Milky Way ; but 

 as time went on, it was seen that the clusters and nebube 

 were in many instances evidently associated with the Milky 

 Way, and that the clusters which are grouped about the 

 Milky Way are probably at about the same distance from 

 us as the Milky Way, and as the numerous bright stars 

 which are evidently associated with it. Such considerations 

 made it evident that the individual stars of the clusters 

 associated with the Milky Way must be very small or very 

 dim compared with the brighter Milky Way stars. 



Determinations of parallax and the results derived from 

 the observed motions and brightness of double stars seem 

 to point to the conclusion tbat many of the fixed stars are 

 actually smaller than our sun, and that, as a rule, the stars 

 do not very greatly exceed the sun in mass or brightness. 

 We are, therefore, forced to conclude either that (1) the 

 Milky Way stars must be immensely large or intensely 

 bright compared with our sun and the other stars whose 

 mass and brightness we have been able to approximately 

 estimate, or (2) that the individual stars of clusters associ- 

 ated with the Milky Way must be very small or very dim 

 compared with our sun. 



stari It may be shown that (if there were no absorption 

 of light in space) our sun would appear to shine as a star of 

 about the sixth magnitude if it were removed to a distance 



Flo. 1. — UutoiK-lifd- ftelu-d iiluck lUiule from a rUotograph uf 

 the Cluster in Hercules, taken by the Brothers Henry on the 23rd 

 June, 18S6. 



The cluster in Hercules shown in our plate is just 

 visible to the naked 'eye, and its total light is probably 

 about equivalent to the light given by a sixth magnitude 



Fig. 2. — Untouched etched Block from the Henrv Pliotograpli of 

 23rd .Tune, ISSH, on the same scale as the Enlargements made 

 from tlu- Lick Photographs, shown in the Collotype PLite. 



corresponding to a parallax of a tenth of a second, that is, 

 if it were removed to a distance about seven and a half 

 times as great as the distance of our nearest stellar neigh- 

 bour, a Centauri. At such a distance our sun would give 

 us about as much light as we derive from the whole of the 

 stars of the Hercules cluster, and at such a distance a 

 sphere equal in diameter to the orbit of Neptune would 

 subtend a diameter of about six seconds, as seen from the 

 earth ; but the denser part of the Hercules cluster has a 

 diameter of about three minutes. Consequently, if a 

 quarter • of the photosphere of our sun were torn into small 

 fragments and scattered in space within a sphere of thirty 

 times the diameter of the orbit of Neptune, or nine 

 hundred times the diameter of the earth's orbit, we should 

 (supposing the fragments of photosphere to retain their 

 brightness and to be all placed square to the line of sight) 

 have a cluster of small stars which, as seen from a distance 

 seven and a half times as great as the distance of a. Cen- 

 tauri, would present an appearance similar in brightness 

 and diameter to the Hercules cluster. If the Hercules 

 cluster is at double the above distance, the diameter of 

 the cluster and of the individual stars would need to be 

 doubled, and their volume and mass would need to be 

 multiplied by eight. But the assumption that this cluster 

 and the other clusters scattered along the Milky Way 

 are at fifteen times the distance of a Centauri, forces 

 us to assume that the brighter stars associated with 

 the Milky Way are on a gigantic scale compared with 

 (X Centauri and the other stars whose mass and brightness 

 we have been able to estimate. If the Hercules cluster is 

 at only four or five times the distance of a Centauri, the 

 individual stars of which the cluster is composed must be 



* The solar disc from which we receiTe light only corresponds in 

 area to half the hemisphere turned towards us, and to a quarter of the 

 whole solar surface. 



