XIISTAR-CLUSTERS
Endlessly varied is the grouping of the stars. We find solitary stars, twin stars, large stars with small companions, bright stars with faint companions, stars alike in colour and type, stars in strongly contrasted colours, one young, the other aged, stars with invisible companions, couples close enough to touch one another, and couples so distant that the satellite takes more than a century to revolve round its primary. One or even both components of a bright pair may be themselves divisible into a closer pair, sometimes into a whole group of stars.
More beautiful and wonderful still are the clusters of stars of various tints and magnitudes, where scores, even hundreds, are gathered together in what to the eye is a tiny patch, though it may in fact take light many years to cross from one side of it to the other.
The finest example in the heavens of this kind of cluster is KappaCrucis, near β in the Southern Cross.[8]It is just visible to the naked eye as a small star, and in a binocular the main star is seen to be surrounded by a number of others; in a telescope it is a glorious sight. Orange and red stars are easily distinguished in the brilliant throng, even if we have not Herschel’s eye for colour and fail to discriminate the “greenish-white, green, red, blue-green, and ruddy” which made up what he likened to “a superb piece of fancy jewellery.” He charted over a hundred stars of all magnitudes from 7 to 17. Herschel’s observations at the Cape were made between 1834 and 1838. When Mr. Russell charted the stars of Kappa Crucis at the Sydney Observatory in 1872, he found 25 that had not been recorded by Herschel, although the great reflector was much larger than the Sydney instrument; many of Herschel’s stars had drifted, and five could not be found at all. If changes so striking as this take place in less than forty years, “it is evident,” as Russell observed, “that more attention should be bestowed on clusters.”
Quite a dozen star-clusters of this kind are visible to the naked eye in the southern hemisphere, the most striking being in Scorpio, where it shines as a conspicuous silvery spot just beyond the Scorpion’s tail, midway between κ Scorpii and γ Sagittarii. It is named M 7, which means the seventh in Messier’s list of clusters and nebulae. In a binocular it is seen to be a group of very many stars, some close together, others scattered. Lacaille, with his little half-inch telescope, counted from 15 to 20 stars, and Herschel, with his large reflector, estimated the number at 60. At Cordoba Observatory no less than 139 were catalogued.
M 6, a little north and west of M 7, is also visible to the naked eye as a nebulous patch, and is a fine cluster. About 50 stars between magnitudes eight and twelve have been photographed, and there are doubtless many more of lesser brightness.
Look also just above the naked-eye double ζ in Scorpio, and see what appears to be a hazy star. A binocular separates this into a number of small stars, and 150 have been photographed, of magnitudes seven to twelve. It is known as h 3652 or N.G.C. 6231, the former being thenumber in Sir John Herschel’s catalogue, the latter in Dreyer’s New General Catalogue of star-clusters and nebulae.
A little south of Sirius is a patch of nebulous light which shows as stars in a binocular. This was registered by Messier in 1764 as a “mass of small stars,” and is known as M 41. Webb saw the brighter stars arranged in curves and a ruddy star near the centre. One hundred and forty-four stars were registered by Gould, of which only five are as bright as eighth magnitude.
In the neighbourhood of the Cross there are quite a number of large bright clusters, for they must be large and bright above the average to be seen by the naked eye. A line passing through the shorter arm of the False Cross—i.e.from δ Velorum to ι Carinae, the naked-eye double—and continued for an equal distance beyond, leads to a white oval patch which is plainly visible to the naked eye, and in the binocular appears like a few stars sparkling on a nebulous background. With higher powers the background also is resolved into stars, of which there are some two hundred of the fifteenth magnitude and brighter up to the eighth. This is N.G.C. 3114.
In the same direction is θ Carinae, a bright Orion-type star withnumerous small stars crowding close to it. This is a very lovely group in a good binocular. It contains about twenty stars of magnitudes between three and eight, and with high powers appears as a brilliant, loosely scattered cluster covering a portion of sky equal in breadth to twice the sun’s apparent diameter.
Very similar is a cluster close to the bright and richly coloured star X Carinae. It looks like an elliptical nebula in a binocular, with a few stars scattered over it. Two hundred have been photographed.
Near ε Carinae, the beautiful ruddy star at the foot of the False Cross, is yet another most beautiful cluster, which contains about fifty stars of the ninth magnitude and brighter. It is visible to the naked eye, but when a telescope is turned upon it the brilliancy is startling. Radiant stars are scattered all over the field.
Just east of π Puppis, the top of the poop of Argo, is a fourth-magnitude star C of a bright orange colour, and round it is a cluster which Gould describes as “extremely impressive to the naked eye.” Ninety-two stars show on his photograph.
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 inthe 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.