Star cluster

“ It is probable that the fainter stars have no real connection with the cluster itself, which is merely seen upon a background of more distant star-dust. Modern photographs have shown that this cluster is involved in a great nebula, which stretches in curious wisps and straight lines from star to star, and surrounds the whole group. The Pleiades make a brilliant object for a small telescope with a low magnifying power, but are too scattered for an instrument of any size to be effective upon them. The finest of all irregular star-clusters is that known as the Sword-handle of Perseus. ”

“ The familiar group of the winter sky known as the Pleiades is a loose cluster, showing relatively very few stars even in telescopes or on photographic plates. The "Beehive," or cluster known as Praesepe in Cancer, and a double group in the sword-handle of Perseus, both just visible to the naked eye, are excellent examples of star clusters of the average type. When the moon is absent, they are easily recognized without a telescope as little patches of nebulous light; but every increase of optical power adds to their magnificence. ”

“ 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. ”

“ Each star, if isolated, would be surrounded with its own gaseous envelope. These gaseous envelopes might, in the case of a cluster, form over the whole, or a part, of the cluster a continuous mass of gas. So long as such a cluster was within a certain distance from us, the light from the stellar masses would predominate over that of the gaseous envelope. The spectrum would, therefore, be an ordinary stellar spectrum. Suppose such a cluster to be removed farther and farther from us, the light from each star would be diminished in proportion of the inverse square of the distance ”

“ A star cluster will result from an immensely widespread nebula breaking up into a host of separate nuclei, each of which becomes a star. The irregular manner in which the materials would, in many cases, be widely distributed through space after collision would prevent a nebula from condensing into a single mass. Subordinate centres of attraction would be established, as was long ago shown by Sir William Herschel in his famous memoir on the formation of stars ”

“ You will recall the winding streams in the Great Nebula of Orion. Another star cluster worth a moment's attention is No. 1479, above and to the left of Sirius. We had better use the five-inch for this, as many of the stars are very faint. Not far away we find the double star μ, whose components are of the fifth and eighth magnitudes, distance 2.8", p. 343°. The small star is pale blue. Cluster No. 1512 is a pleasing object with our largest aperture. In No. 1511 we have a faint nebula remarkable for the rows of minute stars in and near it. The star γ is an irregular variable. ”

“ It seems tempting to pursue the thought here suggested, and to reflect that the whole Milky Way may, after all, be merely a star cluster, comparable in size with some of the other star clusters which we see, and that, viewed from a remote point in space, the Milky Way would seem to be but one of the many clusters of stars containing our sun as an indistinguishable unit. ”

“ It is, therefore, a Star Cluster, one of countless numbers, and containing our Sun as a single unit.
It has as yet been found impossible to determine even approximately the distance of these Star Clusters.
[Pg 425]
NEBULÆ
From Stars we pass insensibly to Nebulæ, which are so far away that their distance is at present quite immeasurable. All that we can do is to fix a minimum, and this is so great that it is useless to express it in miles. Astronomers, therefore, take the velocity of light as a unit. ”

“ That the assumption of uniform distribution of stars in space could not be true in detail was evident to Herschel from the beginning. A star cluster, for example, in which many thousands of faint stars are collected together in a very small space on the sky, would have to be interpreted as representing a long projection or spike full of stars, extending far beyond the limits of the adjoining portions of the sidereal system, and pointing directly away from the position occupied by the solar system. ”

“ A system of two stars that revolve about their common center of mass is called a binary star. A group of three or more stars so close together that they appear as a single star is called a multiple star. A group of stars physically close together is called a star cluster. A variable star changes in magnitude. A star which suddenly becomes many times brighter than previously, and then gradually fades, is called a nova. ”

“ Cluster work is not altogether unlike "parallax hunting" in its preliminary stage of securing the photographic observations. The object is to obtain an absolutely faithful picture of a star group, just as it exists in the sky. We have every reason to suppose that a very large number of stars condensed into one small spot upon the heavens means something more than chance aggregation. The Pleiades group (page 10) contains thousands of massive stars, doubtless held together by the force of their mutual gravitational attraction. If this be true, there must be complex orbital motion in the cluster ”

“ 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. ”

“ A star-cluster is a bunch or collection of stars separated from the great mass of stars which stud the heavens. The Pleiades, or 'seven stars,' as they are familiarly called, form a cluster, of which six of the components are easily seen by the naked eye, while five others may be distinguished by a good eye. ”

“ We find every variety in the clusters; sometimes the stars are remarkable for their brilliancy, sometimes for their enormous numbers, and sometimes for the remarkable form in which they are grouped. Sometimes a star cluster is adorned with brilliantly-coloured stars; sometimes the luminous [Pg 462] points are so close together that their separate rays cannot he disentangled; sometimes the stars are so minute or so distant that the cluster is barely distinguishable from a nebula. ”

“ It would now, with its companion-atoms, represent a planetary system, and the molecules in a gas would stand in about the same relationship to each other as the fixed' stars over us, which pursue their unknown courses; the little air-bubble in the glass of water becomes a star-cluster like the one in which our sun is situated. The circle of little bubbles around the margin of this glass would represent such a gathering of star-clusters as we now see before us in the milky way. ”

“ When examined with sufficient telescopic power, a great many of these luminous patches are perceived to be composed of clusters of little stars, which in a smaller telescope are invisible separately, but whose rays of light blend together so as to produce a confused luminous appearance. Others, however, cannot be resolved into individual stars even with the best telescopes, and in many cases the spectroscope gives direct evidence that the nebula has a constitution altogether different from that of a star-cluster. We thus distinguish between the nebulae proper and the star-clusters ”

“ Some of them have been resolved into star-clusters, but many of them are masses of incandescent gas. Of the so-called fixed stars, many form binary or multiple systems, the members revolving in orbits under each other's attractions, while other more scattered groups are moving clusters, travelling in parallel paths through space like flocks of birds. Variable stars and extinct or dark stars are also known. The fixed stars preserve, at least to unaided vision, an unalterable relation to each other, because of their vast distance from the earth. ”

“ It may be that the telescope, on account of its small size, cannot separate the objects sufficiently, or it may be that the imperfections of the telescope do not present the star as a point of light, but rather as a more or less diffused, luminous disc. In either case it may happen that a star overlaps other stars in its immediate neighbourhood, and consequently an object which is really a cluster of separate stars may fail altogether to present the appearance of a cluster. ”

“ He rechecked all his figures and retraversed the chart, only to have his needle pierce again the same tiny, unmarked dot. He stared at it for a full minute, studying the map all around his marker.
"Star Cluster AC 257-4736," he ruminated. "The smallest, most insignificant, least-known star cluster he could find, and my largest possible error can't put it anywhere else. Kind of thought it might be in a cluster, but I never would have looked there. No wonder it took a lot of stuff to trace his beam. It would have to be four numbers Brinnell harder than a diamond drill to work from there." ”

“ In a recent photograph of this cluster 50,000 stars are shown in an area of the sky which would be entirely covered by the full moon.
Photographs of this swarm of suns fail to do it justice because of its density. In order to give some idea of the wonderful structure of such a cluster a photograph of the globular cluster ω Centauri in the Southern Hemisphere is here shown. It is estimated that this cluster contains in the neighborhood of ten thousand stars, and within its confines Professor Bailey discovered 128 variable stars. ”

“ Although these loosely associated star clusters do not show such evidence in favor of family connection as the more closely compacted globular clusters, still we can hardly escape from the conviction that their apparent aggregation is really due to some physical bond of union, and not merely the result of a fortuitous scattering of stars at different distances in the line of sight. ”

“ 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. ”

“ In the Small Cloud, the single stars are proportionally more numerous, since 200 have been counted, among which three are of the sixth magnitude, while it only includes thirty-seven of the nebulæ and seven star-clusters. These immense aggregations, the elements of which are themselves swarms of suns, remind us of the largest, in appearance at least, of all the clusters which the eye contemplates in the depths of the sky—the Milky Way. ”

“ For each star has to be treated singly and located at its proper distance by elaborate measurements; progress is very laborious. But when we determine the distance of this remote cluster, we secure at one scoop the distances of many thousands of stars. The distance being known, the apparent magnitudes can be turned into true magnitudes, and statistics and correlations of absolute brightness and colour can be ascertained. Even before the distance is discovered we can learn a great deal from the stars in clusters which it is impracticable to find out from less remote stars. ”

“ There can be little doubt that these stars belong to the Milky Way cluster, so that their presence is a property of the cluster rather than of the galactic plane in general. Spiral nebulae have the remarkable characteristic of avoiding the galactic plane, and it has been suggested that the space outside the limits of the stellar universe is filled with them. ”

“ Of such star-clusters there exist a great many, which, in their turn again, form a still grander system, to which we also belong, and the boundaries of which are indicated by the outermost limits of the milky way.
How many of such galaxies may be hidden in the vastnesses beyond, in the bosom of infinite space, we will never know, for the light can only reach us from limited distances. Whatever may be beyond that very farthest nebula, the pale light of which has taken fifteen thousand years to reach us, is concealed from us forever. ”

“ Like bricks already baked, of various qualities, shapes and colour, that are no longer formless clay but have become fit units of a future wall, each of them having a fixed and distinctly assigned space to occupy in some forthcoming building, are these seemingly adult worlds. The astronomer has no means of recognizing their relative adolescence, except perhaps by making a distinction between the star clusters with the usual orbital motion and mutual gravitation, and those termed, we believe, irregular star-clusters of very capricious and changeful appearances. ”