Double Stars

( Originally Published Early 1900's )

WE have hitherto been considering the stars as isolated points of light dotted hither and thither all over the heavens and as if they had no connection one with an-other. These suppositions are only true in a qualified sense, for the telescope reveals the fact that no inconsiderable number of the stars which we regard as simple points of light are in reality 2 (or in some cases several) single stars which are so close together as to appear to the naked eye to be one.

The proximity of one star to another might in any given case only be an effect of perspective and not an actual fact. For instance, a man standing on the top of a straight road which led up a hill might see 2 men approaching him, seemingly walking shoulder to shoulder, as if they were 2 friends engrossed in conversation, whereas in reality they might be isolated individuals walking up the hill, each on his own account, perhaps 50 yards apart. On the other hand, if the man at the top saw the other 2 men cross from one side of the road to the other simultaneously, and that as one turned his head askew, apparently to look at some distant object, the other did the same thing, he might justly infer that the two were really friends and were really walking side by side.

The foregoing illustrations will define with perfect accuracy the difference between what is called an " optical " double-star (that is, 2 stars which seem to be linked together because of the effect of perspective) and a "binary " double-star ; that is, 2 stars which not only seem to be linked together but truly are so. These last-named are often spoken of as " physical doubles," or 2 stars physically connected. To determine in any given case whether a pair of stars belong to the one class or the other is a mat-ter involving both delicate observations and laborious calculations. More than a century and a quarter ago Michell suspected that there might be a physical connection subsisting between certain stars by considering the probable chance of producing a purely accidental combination if a batch of stars were, so to speak, promiscuously thrown haphazard into space. He found that the chances of bringing together stars such as the Pleiades, of their brightness and at their distance, was 500,000 to t, of 1500 stars visible. The improbability became much greater if the inquiry was based upon the case of stars of the 2nd and 3rd magnitudes and within a few seconds of arc of one another. Yet in point of fact we have several examples of this kind, such as a Centauri and a Geminorum.

But probability does not suffice to establish the truth of a fact. One draws a much more conclusive argument from a consideration of the actual proper motions of the stars where such can be detected. If the stars were accidentally brought together, as they are generally of different magnitudes, their proper motions, both real and apparent, would also differ ; consequently with the lapse of time they ought to separate from one another. Yet it happens that many of these stars, though exhibiting considerable actual motion, preserve very much the same distance from one another during an extremely long interval of time. Such are the two stars composing a Centauri, a Geminorum, y Virginis, Ursae Majoris, and a great number of others, pairs of unequal size. a Centauri, the two constituent stars of which were separable with difficulty in a telescope too years ago, has such a considerable proper motion that the two stars ought now to have be-come separated by an interval of 6 minutes if the proper motion of the one were not shared in by the other. This, perhaps, would not always be an unfailing criterion, be-cause it might so happen that the proper motions only exhibited small differences, notwithstanding the extent and reality of the difference. What, after all, would in any given case plainly decide the question would be the positive fact (where it could be established) that one star turned around the other in a closed orbit in seeming accordance with the recognised principles of the law of gravitation. This great discovery has indeed been made, and we owe it to Sir William Herschel. When that remarkable man had sufficiently perfected his instruments, so that he could penetrate into the depths of space in a way never before attempted by any of his predecessors, he set himself the task of seeking to discover stellar parallax, or the actual distances of the stars from the earth. He selected for his purpose certain large stars which were accompanied by small companions at a distance of only a few seconds of arc. He measured these distances with great care by means of an instrument of his own invention called a " micrometer," which also enabled him to deter-mine the angle made by a line passing through two stars with the meridian. He called this angle the "angle of position" of the two stars, regarding the larger of them as the determining centre of the arc on which the measurement was founded. If there had been any annual parallax that is to say, any apparent displacement of the stars with respect to the celestial background, as a result of viewing the stars from opposite points of the earth's orbit at 6-monthly intervals—that parallax would have been discoverable because there would have been disclosed a variation in the distance or angle, comparing one time with another separated by the interval of 6 months. How-ever, after numerous and painstaking researches, carried out with every attention to detail, Herschel could not satisfy himself that he had obtained any proofs of change, and he gave up the work for a time in despair. Having afterwards improved his instrumental means, he resumed his labours, hoping for better results. Great was his surprise to find that some of the stars which he had formerly seen double had become single, the junior member having disappeared, whilst others had evidently changed both their angular position and their distance. Though all hope of discovering an annual parallax seemed to have vanished, at least he had obtained traces of a parallax of another sort, due either to a general movement of the whole system or to some special movement appertaining to particular stars. Michell's old idea seems to have recurred to Herschel's mind and to have stimulated him to further effort, and after several additional years of painstaking and laborious work, at length in 1802 he found himself in a position to announce to the scientific world his grand discovery that there existed systems formed by pairs of stars revolving about each other in regular elliptic orbits. He coined the word " binary " and gave it to these stars, to distinguish them from mere optical double stars, which do not exhibit any mutual periodic changes of place.

The interval that elapsed between Sir W. Herschel's abandonment of his first re-searches and his renewal of work was about 25 years. This is a period quite sufficient to enable the motion of many binary stars to become evident to the senses, and accordingly no fewer than about 50 stars were noticed ,by Herschel to have undergone change during the time that his operations were suspended. True that his stars had, for the most part, only had time to traverse a portion of their orbits, but more than 90 years having elapsed since Herschel's announcement of 1802, it follows that a certain number of binary stars have not only gone entirely round in their orbits once, but some of them have done so almost twice, and the form and dimensions of their orbits are now fairly well understood. To cut a long story short, it may be stated that fully 200 pairs of stars are now recognised to be in motion round one another in obedience to laws probably identical with what are known as the laws of gravitation, though for obvious reasons their orbits have not all been investigated with equal completeness and accuracy. The following are the names and particulars of a few of the binary stars with periods of less than too years, the nature of whose movements has been ascertained with fair certainty :

NAME OF STAR. Period. Date of Last Passage.

Years.

Comae Berenices 25 1870
Herculis 34 1864
Coronae 41 1891
Herculis 45 1880
Sirius 49 1893
Cancri 59 1868
Ursae Majoris 60 1875
Centauri 77 1875
Coronae?. 85 1840
Ophiuchi 94 1808

Sir W. Herschel's original observations had reference only to pairs of stars, but the further attention which has been given to this subject of late years has resulted in the discovery of the fact that in certain cases there exist systems of stars in triplets, each member of which system has a relation to the other two, which justifies their being called not simply triple stars but " ternary " stars. C Cancri is an object of this type.

It must be added, by way of caution, that though movement on the part of a pair of stars during a course of years is prima facie a proof of physical connection involving motion in a closed orbit. yet this must not be regarded as a rule of universal application. A mere angular displacement may, in a given case, be the effect of individual proper motion on the part of one or both of the stars of a pair, and not the effect of a central force. From this it will follow that sometimes the positions successively occupied by the principal star will not exhibit the line of a sensible curve. Flamsteed's 61st star in Cygnus seems to be one of this character ; it is moving, but its motion is in a straight line.

Before passing away from the subject of double stars, a word should be said about the remarkable circumstances of 2 stars which are well known by reason of their great intrinsic brilliancy Sirius and Procyon. Both these stars are subject to peculiar disturbances of place, which long excited the surprise and curiosity of astronomers. It was suggested that these disturbances were due to the presence of some invisible satellite, and in the case of Sirius this surmise has proved well founded. In 1862 Alvan Clark, a well-known American optician, found near Sirius a minute companion, the existence of which has enabled astronomers to explain some, though perhaps not all, of the irregularities found to exist in the positions of the primary star at different times. Arising out of this is the further conclusion that this faint attendant, which has only 1/12000 of the light of Sirius, possesses a mass more than 1/4 of Sirius. In other words, unless it does thus really approximate in mass to Sirius itself, it is not capable of accomplishing the observed disturbances.

That disturbances are traceable in the movements of Sirius is no new idea, for the great German astronomer, Bessel of Konigsberg, as far back as 1844, not only noticed their existence but suggested the presence of an invisible perturbing body, belonging to the system of Sirius, as an explanation of the fact that the proper motion of Sirius takes place not in a regular line, but in an irregular sinuous line. Accordingly, he suggested that this very bright star possessed a dark satellite. Other astronomers worked at the idea, and may be said to have paved the way for the actual discovery of the satellite by Clark.

A very interesting question often presents itself to students of astronomy, who meditate on what they have seen after they have examined double stars. The question may be put in this form : We on the earth are placed on a certain moving body called a planet, which is one of a number of planets circulating round the sun as their chief ruler or centre. Is this state of things unique? Or, on the other hand, do other suns exist ? Or, to be more precise, do other bodies exist in the universe which are centres of life and motion analogous to our sun ? No one who has seen a bright double star, with its one or more companions, and still more, no one who has seen the many bright stars with companions which are to be found scattered up and down the heavens, can doubt that the answer to the above main question must undoubtedly be in the affirmative. In other words, that there are in the universe many suns, each with its own cortége of planets, and not one sun only. Much beyond this, however, we cannot go. One thing is not a matter of speculation. Whereas our planets revolve round the sun in orbits, which though not truly circular, are yet not very eccentric ; that is, do not depart much from the circular form, yet in the cases of the binary stars, the orbits of all that are known depart very much indeed from the circle. Secchi has well pointed out that if we consider for a moment what is involved in the existence of luminous systems of stars, we may well be struck with the inferences which necessarily follow. In the case of a system the form of whose orbit is very eccentric (such as a Centauri), any attendant planets must be warmed sometimes by 2 suns very near, sometimes by one sun very near, and by another very far off. Who can calculate the transformations of life which go on under such circumstances without remembering the wisdom of Him who often with small apparent means is able to bring about an infinite variety of results ? Add to this the fact that double stars very often exhibit different and complementary colours. The imagination of even a poet would be incapable of describing to us the phases of a day illuminated by, say, a red sun, and of a night illuminated by, say, a green sun ; or of a day in which 2 suns of different colours competed with one another, whilst the night was ushered in by a golden twilight and the next morning was preceded by a blue dawn. But I do not wish in this chapter to drift into star colours, for that is a subject of sufficient importance to deserve a chapter to itself.