IN STARRY SKIES

Evening Post, Volume CX, Issue 110, 5 November 1925, Page 4

IN STARRY SKIES

CELESTIAL PARTNERSHIPS

DOUBLE AND MULTIPLE STARS

(By "Omega Centauri.

There is not the slightest doubt that partnerships and unions of various kinds are extremely common amongst the stars. This fact has been recognised for a very long time, but the cosmical importance- of such associations has but lately been appreciated, and is just now becoming more and more manifest every day. There is still, however, great diversity of opinion as to cause and origin. How have such stellar connections been brought about? Were the pairs and groups produced by fission, by condensation round neighbouring nuclei, or by ■stellar encounter 1 When we examine the heavens, even with the naked eye, we can hardly fail to notice that, in numerous cases, pairs of stars appear to be remarkably close to one another. For instance, in the constellation Grus, which is near the zenith in the early evening now, the main line of stars contains four pairs. The question at once arises whether such grouping can be the effect of chance distribution. In any single pail', of course, however close the two stars appear, they may well be at widely different distances from us and may have no real physical connection with one another. But what is quite likely in a single case may be most improbable in a large number of similar ones. Close pairs seem to be much more numerous than is consistent with probability unless many of- the associations, are real and not merely apparent. As soon as any optical aid is employed, the case for the existence of some physical connection between the units of a pair is strengthened enormously. Most, people are familiar with our nearest bright neighbour, Alpha Centauri, and know that, whilst ifc appears single to the naked eye, it is resolved by even a small telescope, into a pair of brilliant points of light. Similarly when we turn the telescope towards the Southern Cross we find that Alpha and Gamma are each divided into three, but with their units quite differently arranged, and that the fainter Kappa is found to be a beautiful cluster. A five-inch telescope shows over sixty stars in this compact group. Similar results are found, whatever direction we turn to. The higher the telescopic power which wo employ, the greater is the number of stellar points which break up each into two or more. The number of double stars known at present is so great that, even if we had no confirmatory evidence, we might well conclude that the great majority ,of such pairs must he in reality physically connected systems. But this leads to a series of more difficult problems. What iA the nature of the bond that holds these stars together? When and how was- the partnership formed? In this branch of study, as in so many others, Sir William Herschel led the way. He didn't know the vast field he was opening up. At first he thought it probable that in some of the close pairs which he discovered, in which one star was far brighter than the other, that the latter must be far more distant. This seemed likely to afford a means of gauging the depths of the Galaxy. ' The annual motion of the earth round the sun should make the nearer star appear to move relatively to the more distant one. By determining the annual relative motion Herschel hoped to get a clue as to the distance of the nearer star. The result was surprising. The earth's motion failed to produce any 'visible change in the apparent positions of the stars. But this led to an important discovery that was soon followed>by another.- Herschol concluded from his observations that even the nearer stars must be far more distant than had hitherto been suspected. The universe was greater than anyone had imagined. But although the stars failed to respond to the earth's motions, they showed, when examined after an interval, some change with regard to one another. Ultimately he concluded that the constituents of a pair were revolving round one another, or rather round their common centre of gravity, like the earth and the moon. Gravitation was at work in the Galaxy as in the solar system. This second discovery led naturally to a close and careful study of binary stars being undertaken by many worker's. It has been found that such pairs are astonishingly numerous. Already abbot twenty thousand double stars have been catalogued. Of all the stars down to the ninth magnitude, one in every eighteen has been found to be an optical double. Many-show distinct evidence of revolution, and the elements of the actual orbits' of about one hundred • and ti£ij' have been determined.

In 188£> Pickering was examining Mizar in the.Great Bear, Mizar and the fainter star Alcor form a naked eye double, which- used to .be considered a good, test for.keen sight. Alcor ha* probably become brighter in recent times, for the double star is now considered an easy naked eye object. Witn a two-inch telescope, Mizar, whifch then appears widely separated frttm • Aleor, is ■ seen to be itself a double star. Pickering, when' examining the spectrum of the brighter cptnponent of it, found that the dark liSgSSin.'ah'e /spec.truiii^aippeared to be. double"''at • regular "intervals of time. He concluded that this particular component of the telescopic double star, which forms in turn one member of the naked eye double, was not a single body, but consisted of two stars revolving round one another in twenty and a half days. They are too close to one another to be detected separately in the telescope, but the spectroscopic evidence' of the composite character is quite conclusive. . This discovery opened up another great branch of astronomical study. Spectroscopic binaries proved to bo even more Abundant than optical ones. Campbell : states that at least one star in every five that are examined'is found to be a spectfoscopic binary. There is a curious unbridged gap between the two classes of binaries. The telescope can resolve only those pairs whose constituents are sufficiently far apart. The spectroscope, on the other hand, can detect the composite character of those pairs only whose constituents are very close together and consequently move sufficiently fast to show the separated lines. There may be innumerable stellar pairs with the constituents too close together for the telescope* to separate, and yet much too far" apart" for the spectroscope to detect. With improved instru-. ments and methods of observation, it may ultimately be found that solitary stars are rather the exception than the rule. Much work has already been done in the way of determining the distances, masses, orbits, and periods of binary stars, and knowledge- with regard to' all ( these details is growing fast. Amongst the bestknown systems are. those of Alpha Centauri, Castor, and Polaris. Alpha'Centauri consists of two suns, one of which is remarkably like ouv own sun in size, mass, ' liud ' stugu of. life; whilst, the other is not very different Irora it.

These two, suns are. circling in orbits round their common centre of.gravity in about 81 years. The 'two visible components of Castor have a period of about 300 . years, but each .is itself a spectroscopic binary, and the periods of these are .only three days and nine days respectively. The triple system of Polaris consists of a spectrosvopic binary with a period of nearly four days, revolving round v dark star in over twenty years, whilst the whole system appears to be revolving about another star in about twenty thousand years. Several astronomers are meeting with surprising success, in' determining similar particulars regarding great numbfirs of other remarkable stellar systems. The results should supply most valuable data to bo used in the. solution of the outstanding problems as to the origin of these forms. Is there a common element in the life history of all, or has one system developed in one way, another in another? Some think that in the formation of double stars condensation, took place round separate nuclei in a vast rotating nebulous mass; some hold that fission, due to rapid rotation subsequent to condensation, is the normal process of birth.; a few see in stellar encounter indications of a power capable of producing all the observed forms and endowing them with the requisite energy. The problem is of considerable interest, as it may throw light on the greater mystery of the life history of the universe.