October 1, 1894.J 



KNOWLEDGE. 



233 



star of about the sixth maguitude. When examined with 

 a good telescope it is at once resolved into a multitude of 

 small stars, which can be individually seen, and even 

 counted with large telescopes. The number of stars 

 included in the cluster was estimated at fourteen thousand 

 by Sir William Herschel, but the real number is probably 

 much smaller. Were the number so great as Herschel 

 supposed, I find that the cluster would form a much 

 brighter object. It has been well photographed at the 

 Paris and Lick Observatories and by Dr. Roberts.* Its 

 globular shape is evident at a glance, and we cannot doubt 

 that the stars comprising it form a gigantic system, 

 probably isolated in space. 



Most people might think that this cluster was a mass of 

 double and multiple stars, but this is not so ; the com- 

 ponent:;, close as they are, are too far apart to be considered 

 as true double stars. Mr. Burnham, the famous double 

 star observer, finds one close double star near the centre, 

 and notes the remarkable absence of close double stars in 

 bright and apparently compressed clusters. 



In the same constellation, Hercules, between the stars 

 Eta and Iota, but nearer the latter, will be found another 

 object of the globular class, but not so bright, or so easily 

 resolvalile into stars as the cluster described above. It is 

 known as 92 ^lessier. Buifham, examining it with a nine- 

 inch mirror, thought the component stars brighter but 

 more compressed than in IH Messier. This object was 

 photographed by Dr. Eoberts, in May, 18!)1, with a 

 twenty-inch reflecting telescope, and an exposure of one 

 hour. Dr. Roberts says : " The photograph shows the 

 cluster to be involved in dense nebulosity, which on the 

 negative almost prevents the stars being seen through it, 

 and on the print quite obscures the stars. The stars in 

 this, as in ail other globular clusters, are arranged in 

 various patterns, and many of them appear to be nebulous." 



Another fine globular cluster is that known as 5 Messier. 

 It lies closely north of the fifth magnitude star 5 Serpentis. 

 It was discovei-ed by Kirch in 1702, and was observed in 

 1764 by Messier, who found he could see it with a telescope 

 of one foot in length, but could not resolve it into stars. 

 Sir William Herschel, with his forty-foot telescope, could 

 count about two hundred stars, but could not distinguish 

 the stars near the central blaze. Sir John Herschel de- 

 scribes it as an excessively compressed cluster of a globular 

 form, with stars of the eleventh to the fifteenth magnitude 

 condensed into a blaze at the centre. Lord Rosse found 

 it more than seven or eight minutes of arc in diameter, 

 with a nebulous appearance in the centre. This cluster 

 was photographed by Dr. Roberts with a twenty-iuch 

 reflector, in April, 1892. The photograph shows the stars to 

 about the fifteenth magnitude, and "the cluster is involved 

 in dense nebulosity about the centre. The nebulosity 

 hides the stars even on the negative.'' 



Another fine object of this class is that known as 15 

 Messier, in Pegasus, discovered by Maraldi in 174.5. Sir 

 John Herschel describes it as a remarkable globular 

 cluster, very bright and large, and blazing in the centre. 

 Webb found it a glorious object with a nine and one-third 

 inch mirror. It was photographed by Dr. Roberts in 

 November, 18110, with an exposure of two hours. He says : 

 "The photograph confirms the general descriptions, and the 

 negative shows, separately, the stars of which the cluster is 

 composed distinctly through the nebulosity to the centre. 

 Many of the stars have a nebulous appearance, and they 

 are arranged in curves, lines and patterns of various forms, 

 with lanes or spaces between them." 



The cluster known as 3 Messier, in Canes Venatici, is 



* See Kkowledge for May aud June, 1893. 



another fine object of the globular class. Sir John Her- 

 schel described it as a remarkable object, exceedingly bright 

 aud very large, with stars of the eleventh maguitude. 

 Admiral Smyth thought that it contained at least a thousand 

 stars. Buffham found it resolved even in the centre with a 

 nine-inch mirror. It was photographed by Dr. Roberts in 

 May, 1891, with an exposure of two hours, and the photo- 

 graph confirms the general descriptions given of the cluster, 

 though " the print fails to show the stars that on the 

 negative crowd the space covered by the dense nebulosity." 

 Dr. Roberts remarks that " nebulosity seems invariably to 

 be present in globular clusters." 



We may also mention the globular cluster known as 

 2 Messier, situated a little north of the star Beta Aquarii. 

 It was discovered by Maraldi in 1746. Sir John Herschel 

 compared it to a mass of luminous sand, and estimated the 

 stars to be of the fifteenth magnitude. Sir William Herschel, 

 with his forty-foot telescope could " actually see aud distin- 

 guish the stars even in the central mass." I estimate 

 that this cluster contains about four thousand stars, or 

 more than the number of stars visible to the naked eye at 

 one time. 



In the southern hemisphere there are some mag- 

 nificent examples of globular clusters, and, indeed, this 

 hemisphere seems to be richer in these objects than the 

 northern sky. Among these southern clusters is the truly 

 marvellous object known as Omega Centauri. Its apparent 

 size is very large — about two-thirds of the moon's 

 diameter — and it is distinctly visible to the naked eye as a 

 hazy star of the fourth magnitude, aud I have often so 

 seen it in the Punjab sky. Sir John Herschel, observing 

 it with a large telescope at the Cape of Good Hope, de- 

 scribes it as " beyond all comparison the richest and 

 largest object of its kind in the heavens. . . . All clearly 

 resolved into stars of two sizes, viz., thirteen and fifteen 

 . . . the larger lying in lines and ridges over the smaller ; 

 . . . the larger form rings like lace-work on it." This 

 wonderful object has recently been photographed by Dr. 

 Gill at the Royal Observatory, Cape of Good Hope,' and 

 also at Arequipa, Peru, with a telescope of thirteen inches 

 aperture. On the latter photograph the individual stars can 

 be distinctly seen and counted. The enumeration has been 

 undertaken by Mr. and Mrs. Baily, and a mean of their 

 counts gives six thousand three hundred and eighty-nine 

 for the number of stars in the cluster ; but they consider 

 that the real number is considerably greater. 



Another wonderful object is that known as 47 Toucani, 

 which lies near the smaller magellanic cloud. Sir John 

 Herschel describes it as " a most magnificent globular 

 cluster. It fills the field with its outskirts, but within its 

 more compressed part I can insulate a tolerably defined 

 circular space of 90" diameter, wherein the compression is 

 much more decided, and the stars seem to run together, and 

 this part has, I think, a pale pinkish or rose colour . . . 

 which contrasts, evidently, with the white light of the rest. 

 The stars are equal, fourteen magnitude, immensely 

 numerous and compressed . . . Condensation in three 

 distinct stages ... A stupendous object." 



There are a number of other globular clusters of smaller 

 size in the southern hemisphere, but the above are the 

 most remarkable. 



The actual dimensions of these globular clusters is an 

 interesting question. Are they composed of stars com- 

 parable in size and mass with our sun '.' or are the 

 component stars really small and comparatively close 

 together ? This is a difficult question to answer, as the 



t See pliotugraiili of (his Cluster taken at the Cape Observatory, 

 published in Knowledge for June, 1893. 



