Close to δ Velorum (in the False Cross) is the star ο Velorum. Even to the naked eye the bright star (3½ mag.) is seen to be surrounded by a cluster of faint stars, and in a binocular it is a splendid sight. The number of stars is small, but they are bright, ten being above the ninth magnitude.

Lastly we may mention a remarkably fine cluster which is visible in a binocular though not to the naked eye, in the Centaur, among a stream of small stars between the Cross and η Argūs. Dunlop called it “a pretty large cluster of stars of mixt magnitudes”; Brisbane, “a prodigious number of small stars very close together.”[9] There are at least a hundred above the sixteenth magnitude.

It should be noticed that nearly all these clusters are in the Milky Way, and the rest are very near it, for this is characteristic of star-clusters like these, viz. irregular groups containing mixed magnitudes of stars. In a list of all bright objects of this kind, only two are found as far as 30° away from the middle line of the Galaxy, while 89 are within 30° north or south of it. There are besides 38 in the Clouds of Magellan, which resemble the Milky Way in constitution. Many clusters are simply unusually dense portions of the Milky Way, and we may almost say that this type forms part of it and of the Magellan Clouds.

XIII
GLOBULAR STAR-CLUSTERS

If Kappa Crucis is the finest irregular star-cluster in the sky, Omega Centauri is undoubtedly the largest and most splendid of all the globular star-clusters, for its diameter is more than twice the diameter of the famous northern cluster in Hercules. It is easily found, being nearly in line with δ and γ Centauri (the two conspicuous stars just north of the Cross) and a little further from γ than γ is from δ. The cluster looks to the naked eye just like a tailless comet, and was mistaken for one by the author when first seen. In a binocular it is quite round, the soft milky light growing gradually brighter towards the centre, but without the slightest suggestion of irregularity, and no appearance of stars. It must have been seen by the early navigators who named the southern constellations, but it was first discovered as a star-cluster by Halley in the island of St. Helena. Most amazing is its appearance in a telescope, for the milky disc breaks up into thousands of tiny points of light, densely crowded, all alike, innumerable.

“This most glorious object,” as Herschel calls it, “the noble globular cluster ω Centauri, beyond all comparison the richest and largest object of the kind in the heavens,” is evidently quite distinct from κ Crucis and clusters like those described in the last [chapter]. In form it is circular, and the condensation towards the centre suggests that it is spherical. There are some scattered members of the group lying outside the bright crowded sphere. The stars are also immensely more numerous and more closely packed than in the irregular clusters, their total number being estimated at 10,000, and 6000 have been actually counted on photographs, and all these in a space which looks little larger than that occupied by the sun in the sky. Another striking difference is that, instead of bright and faint stars mingled together, here they are all nearly alike and very minute. Curiously enough, it is found that they belong to two magnitudes, and two only, the thirteenth and fifteenth, and this seems to be a feature of all globular clusters, as well as the form and the dense crowding of the stars. Herschel at first thought the stars in ω Centauri “singularly equal, and distributed with the most exact equality, the condensation being that of a sphere equally filled.” But he immediately adds: “Looking attentively, I retract what is said about the equal scattering and equal sizes of the stars. There are two sizes ... without greater or less, and the larger stars form rings like lace-work on it.” In his later notes he is again doubtful, for he thinks that the effect may be optical, and the larger stars only knots of faint stars; but photography has settled the question in our day.

Yet another point of difference between globular and irregular clusters is that the latter often have wisps of nebulosity clinging about them, but globular clusters are entirely free from it.

THE STAR-CLUSTER 47 TOUCANI

From Sir John Herschel’s drawing