The other wonderful cluster is that known as 47 Toucani, which lies close to the smaller Magellanic Cloud. It is smaller in apparent size than Omega Centauri, but Dr. Gould, observing it at Cordoba, speaks of it as “one of the most impressive and perhaps the grandest of its kind in either hemisphere,” and he estimates its magnitude at four and a half, as seen with the naked eye. It is thus described by Sir John Herschel: “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, I think, has a pale pinkish or rose color, 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.” Sir John Herschel’s drawing of this cluster reminds one of a swarm of bees, and perhaps suggested to Tennyson the lines:
“Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”
There are other interesting specimens of the globular class in the Southern Hemisphere, but not of such large apparent dimensions as those already described. Of these may be mentioned 22 Messier, which lies about midway between the stars Mu and Sigma Sagittarii. It is described by Sir John Herschel as a fine globular cluster, with stars of two magnitudes, namely eleven or twelve, and fifteen or sixteen, the larger being visibly reddish, and he suggested that it consists of “two layers, or one shell over another.” Owing to the comparative brightness of the larger components, this cluster forms a good object for small telescopes. I saw the brighter stars well with a 3-inch refractor in the Punjab sky, but, of course, the greater portion of the cluster has a nebulous appearance in a telescope of this size.
Between Alpha and Beta Scorpii there is a condensed globular cluster. With small telescopes it very much resembles a telescopic comet, but with larger instruments its true character is revealed. Sir William Herschel considered it “the richest and most condensed mass of stars in the firmament.” In May, 1860, a “temporary star” of the seventh magnitude suddenly appeared in the centre, almost blotting out the cluster by its superior light. The star faded away before the end of June of the same year, and has not been seen with any certainty since. It has been suggested that this temporary star lay between the cluster and the earth, but it seems to me much more probable that the outburst took place in the cluster itself, and that it was possibly caused by a collision between two of the component stars, or by a swarm of meteors rushing with a high velocity through the cluster.
The beauty and sublimity of the spectacle presented by these globular clusters, when viewed with a powerful telescope, is such as can not be adequately described, and it has been said that when seen for the first time, “few can refrain from a shout of rapture.” The component stars, although distinctly visible as points of light, defy all attempts at counting them, and seem literally innumerable. Placed like a mass of glittering diamond-dust on the dark background of the heavens, they impress us forcibly with the idea that if each of these lucid points is a sun, the thousands which seem massed together in so small a space must be in reality either relatively close and individually small, or else the system of suns must be placed at a distance almost approaching the infinite.
The distance of these globular clusters from the earth is, however, certainly very great. Attempts to accurately determine their position in space have not been attended with success. As the component stars are at practically the same distance from the eye, we have no comparison stars to measure from, and their exact distance, therefore, remains unknown. We may, however, estimate their probable distance with some show of plausibility. We may assume that the stars of the Hercules cluster would, if concentrated in a point, shine as a star of about the fourth magnitude. As the components are of about the twelfth and thirteenth magnitudes, this would imply that the cluster consists of about 2,500 stars. With the data assumed, we may therefore conclude that the components of the Hercules cluster are suns of comparatively small size, separated by considerable distances, but apparently massed together by the effect of distance.
Among less condensed star clusters there are many interesting objects. The Pleiades have been already referred to. On a photograph of this remarkable group, taken at the Paris Observatory, over 2,000 stars can be counted of all degrees of brilliancy, from those visible without optical aid down to points of light so faint as to be invisible to the eye in the telescope with which they were photographed. Here we have a cluster of probably larger size than that in Hercules, probably at a greater distance from the earth, and with its larger components of considerably greater mass than our sun.
Near the bright star Pollux, I see a small cluster of stars of about the seventh and eight magnitudes, which, with a binocular field-glass, very much resembles the Pleiades as seen with the naked eye. A smaller cluster (known as 39 Messier) may be seen near the star Pi Cygni.
The well-known Chi Persei may be also seen with an opera-glass, but a telescope is necessary to show the component stars to advantage, and the larger the telescope the greater the number of faint stars in these wonderful objects.