Part 7

+--------------+----------+----------+-----------------------------+ | Name. | Maximum | Minimum | Remarks. | | |Magnitude.|Magnitude.| | +--------------+----------+----------+-----------------------------+ |Z Sculptoris | 6 | 8 | Period unknown. | | | | | | |RY Sagittarii | 6.5 | Fainter | Usually steady, but subject| | | |than 11.5 | to a sudden drop at | | | | | irregular intervals. | | | | | Worth watching. | | | | | | |RT Capricorni | 6.5 | About 8 | Period probably irregular. | | | | | Very few observations | | | | | published. | | | | | | |S Phoenicis |{ 7.2 | 8.7 }| Observers differ. | | |{ or | or }| Period irregular. | | |{ 6.8 | 8.4 }| | | | | | | |W Ceti | 6.5 | 12 | Period perhaps 366 days | | | | | | |T Ceti | 5.4 | 6.9 | Irregular. | | | | | | |S Leporis | 6.5 | 7.5 | Irregular. | | | | | | |U Hydrae | 4.5 | 6.3 | Irregular. Red | | | | | “carbon star.” | | | | | | |θ Apodis | 5.5 | 6.6 | Period probably irregular. | | | | | | |R Apodis | 5.5 | 6.2 | Suspected. | | | | | | |T Indi | 7.2 | 8.9 | No regular period found | +--------------+----------+----------+-----------------------------+

XII

STAR-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 Kappa Crucis, near β in the Southern Cross. 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.”

It is easy to remember the names of the stars in the Southern Cross. Begin at the foot, which is obviously the brightest, and count round the Cross in clockwise direction α, β, γ, δ. κ is beyond β in a line with γ, β.

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 the number 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 with numerous 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.” There are at least a hundred above the sixteenth magnitude.

These two astronomers observed at Paramatta, New South Wales, in the early part of the nineteenth century.

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.

From Sir John Herschel’s drawing]

Smaller than ω Centauri, but even more beautiful in the telescope, is the cluster 47 Toucani, which to the unaided eye appears like a fourth-magnitude star near the smaller Cloud of Magellan. The long curve of Grus followed southwards leads to it. Nearly as many stars as in ω Centauri, or about 9500, are here massed into a still smaller space, so the cluster is brighter, and is “compressed to a blaze of light” at the centre. The two sets of stars, which are mingled together throughout, are of thirteenth to fifteenth and of seventeenth magnitudes respectively. Herschel saw the inner denser part rose-coloured while the outer was white, but the present writer could not see this nor find anyone to confirm it to-day, possibly because the refracting telescopes now so often used do not show colour so well as large reflectors like Herschel’s. A double star of 11th magnitude, which is conspicuous in Herschel’s drawing, is doubtless far outside the cluster, and only appears projected against it by perspective.

Also named ξ Toucani.