The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (easy books to read in english TXT) 📕

Star Clusters.—On observing the heavens on a clear, dark night, there can be seen in different parts of the sky closely aggregated groups of stars called clusters. In some instances the component stars are so near together that the naked eye is unable to discern the individual members of the cluster. They then assume an indistinct, hazy, cloudlike appearance. Upwards of 500 clusters are known to astronomers, the majority of which are very remote. Many of them contain thousands of stars compressed into a very small space, and others are so distant that the largest telescopes are incapable of resolving their nebulous appearance into separate stars.

Star clusters have been arranged into two classes, ‘irregular’ and ‘globular;’ but no sharp line of demarcation exists between them, though each have their distinctive peculiarities. Irregular clusters consist of aggregations of stars brought promiscuously together, and presenting an appearance devoid of any structural arrangement. They are of different shapes and sizes, possess no distinct outline, and are not condensed towards their centre, like those that are globular. On examination, they present an intricate reticulated appearance; streams and branches of stars extend outwards from the parent cluster, sometimes in rows and sinuous lines, and, in other instances, diverging from a common centre, forming sprays. Sometimes the stars are seen to follow each other on the same curve which terminates in loops and arches of symmetrical proportions.

There are three conspicuous clusters in the northern sky that are visible to the naked eye—viz. the Pleiades in Taurus, the Great Cluster in the sword-handle of Perseus, and Praesepe in Cancer, commonly called the Beehive.

The cluster which from time immemorial has had bestowed upon it the chief attention of mankind are the beautiful Pleiades or Seven Sisters, and intertwined among its stars are the legendary and mythological beliefs of ancient nations and untutored tribes inhabiting the different regions of the globe. When viewed with a telescope of moderate size the cluster appears as a scattered group, and numerous stars become visible that are imperceptible to ordinary vision.

In the sword-handle of Perseus there is a cluster which, to the naked eye, appears as a small patch of luminous cloud. This inconspicuous object when observed with an instrument of moderate power is resolved into a magnificent assemblage of stars, and presents a spectacle which creates in the mind of the beholder mingled feelings of admiration and amazement. No telescope has yet penetrated its utmost depths, or revealed all the glories of this shining region, crowded with glittering points of light comparable in number to the pebbles strewn on the shore of a troubled sea.

The cluster Praesepe in Cancer is visible on a clear night to the unaided eye as a small nebula. This object attracted the attention of Galileo, to which he applied his newly invented telescope, and was delighted to find that his glass was capable of resolving it into a group of stars thirty-six in number, and all of comparatively large magnitude. The disappearance of Praesepe in consequence of the condensation of vapour in the atmosphere was regarded by the ancients as a sure indication of approaching rain. In the same constellation, near the Crab’s southern claw, there is another rich cluster, which consists of 200 stars of the ninth and tenth magnitudes.

In Sobieski’s Shield there is a magnificent fan-shaped cluster of minute stars with a prominent one in its centre; and in the constellation of the Southern Cross there is a cluster which, on account of the varied colours of its component stars, has been compared by Sir John Herschel to ‘a piece of rich fancy jewellery;’ eight of the principal stars being coloured red, green, and blue.

Globular Clusters.—These have been described by Herschel as ‘the most magnificent objects that can be seen in the heavens.’ They are all very remote, of a rounded form, and when viewed with a telescope present the appearance of ‘a ball of stars.’ In some clusters the constituent stars are distinguishable as minute points of light; in others, more remote, they are of a coarse granular texture, and in those still more distant they resemble a ‘heap of golden sand.’ Some clusters are situated at such a profound distance in space that it is impossible with the most powerful of telescopes to define their stellar structure; all that can be distinguished of these is a cloudy luminosity resembling in appearance an irresolvable nebula. Globular clusters usually present a radiated appearance. Rays, branches, and spiral-shaped streams of stars appear to flow from the circumference of some; and, in other instances, fantastic appendages of stars project outwards from the parent cluster. There doubtless exists much variety in the structural arrangement of these clusters, and an equal diversity in the magnitude and number of the stars which enter into their formation. The stars in some clusters may equal those of the first magnitude, and in others they may not exceed in dimensions the minor planets. In the telescope they vary in size from the eleventh to the fifteenth magnitude; the smaller stars occupy the centre of a cluster, whilst the larger ones are found near its circumference. Globular clusters are more condensed towards their centre than those of irregular shape, and some have a nucleated appearance. This apparent condensation is not altogether owing to the depth of star strata as viewed from the circumference of the cluster, but there appears to exist an attractive force (probably gravitational) which draws the stars towards its centre, and if this ‘clustering power’ were not opposed by some other counteracting force, those bodies would coalesce into one mass. It may be ‘that a centrifugal impulse predominates by which full-grown orbs are driven from the nursery of suns in which they were reared to seek their separate fortunes and enter on an independent career elsewhere.’

It is not known how the dynamical equilibrium of a star cluster is maintained; and on account of its extreme distance no motion is perceptible among its component stars. The laws by which those stellar aggregations are produced and governed are wrapped in obscurity, and the nature of the motions of their stars, whether towards concentration or diffusion, cannot at present be ascertained. If those globular clusters could be observed sufficiently near, they would most probably expand into vast systems of suns occupying immense regions of space.

The largest and most magnificent globular cluster in the heavens is ω Centauri, in the Southern Hemisphere. To the naked eye it resembles a round, indistinct, cometary object, about equal to a star of the fourth magnitude; but when observed with a powerful telescope it appears as a globe of considerable dimensions composed of innumerable stars of the thirteenth and fifteenth magnitudes, all exceedingly minute and gathered into small knots and groups. A remarkable cluster in Toucani is described by Sir John Herschel as ‘most magnificent; very large; very bright, and very much compressed in the middle.’ The interior mass consists of closely aggregated pale rose-coloured stars, surrounded by others of a pure white which embrace the remainder of the cluster. There is a fine globular cluster in Sagittarius between the Archer’s head and the bow. It was observed by Hevelius in 1665. The central portion is very much compressed, and consists of excessively minute stars enclosed by others of larger size. In Aquarius there is a magnificent ball of stars of a beautiful spherical form, which Sir J. Herschel compared to a heap of fine sand. Numerous other clusters are profusely distributed over the heavens, occupying regions in the profound depths of space which can only be reached by the aid of most powerful instruments.

The finest and most remarkable object of this class visible in the northern heavens is the Great Cluster which lies between η and ζ Herculis. It was discovered by Halley in 1714, who writes: ‘This is but a little patch, but it shows itself to the naked eye when the sky is serene and the moon absent.’ When observed with a powerful telescope its magnificence at once becomes apparent to the beholder. ‘Perhaps,’ says Dr. Nichol, ‘no one ever saw it for the first time through a telescope without uttering a shout of wonder.’ At its circumference the stars are rather scattered, but towards the centre they appear so closely aggregated that their combined effulgence forms a perfect blaze of light. Sir William Herschel estimated that there are 14,000 stars in the cluster, each a magnificent world but unaccompanied by any planetary attendants.