Moving Stars

( Originally Published Early 1900's )

THE term " fixed stars " is a familiar one, and in a certain sense it is the expression of a truth, but modern science has shown that the term, as applied to the stars, needs to be employed under reserve, for a great many stars are not "fixed." I am not, of course, alluding to their apparent annual or diurnal movements : we have considered that matter in a previous chapter, and I hope the reader understands by this time (at any rate generally) what these apparent movements are and how they arise. What we have now to deal with is actual proper motion, and with this a considerable number of the stars are endued.

It must be understood, of course, that though the ancients divided the stars into two classes those which were stationary, and those which moved they knew nothing of the stars which form the subject of this chapter being moving stars. The objects to which the ancients applied the designation of "wandering stars " were what we now call Planets, or Comets. Indeed, the very word " planet " itself is derived from a Greek word meaning "a wanderer." What we have now to consider are the movements of certain stars, which movements are, as a rule, very small in amount, and proceed very slowly. Sir John Herschel's statement of the case can hardly be improved on. He says : Motions which require whole centuries to accumulate before they produce changes of arrangement such as the naked eye can detect, though quite sufficient to destroy that idea of mathematical fixity which precludes speculation, are yet too trifling, as far as practical applications go, to induce a change of language and lead us to speak of the stars in common parlance as otherwise than fixed. Small as they are, however, astronomers once assured of their reality have not been wanting in attempts to explain and reduce them to general laws."

What the expression the "proper motion" of a star means or involves may perhaps be best understood by some such illustration as the following : A man standing in Trafalgar Square and looking down Whitehall may at a given moment see in the direction of the Houses of Parliament an omnibus, a cab, and a van. After an interval of 2 minutes he may see the same vehicles, but their order may be first the van, then the cab, and lastly the omnibus. This may imply either (1) that the van has remained stationary, the omnibus and the cab having moved for-wards, the omnibus travelling at a more rapid pace than the cab ; or (2) that all three have moved somewhat, but each at a different pace ; or (3) that the van has backed towards Trafalgar Square, only the omnibus and the cab going forward. If such a condition of things were conceived to be transferred to the heavens, our ideal omnibus, cab, and van being transformed into stars, we should have an analogue of the problem which the astronomer has to solve in detecting and valuing the proper motions of 3 neighbouring stars, or, it may be, of only 2, or perhaps even of only i, of such stars. Be it remembered, too, that in the illustration I have given it may well happen that the Trafalgar Square spectator, from his position astride of one of Landseer's lions, though he may be quite sure that the omnibus and the cab have both moved forwards, may yet be totally incapable of determining whether their movement amounts to 10 yards or 50 yards, because he is viewing the whole proceeding " end on," or, in astronomical language, the 3. vehicles are nearly in his " line of sight." Things would, however, present quite a different aspect to a second spectator standing, say, in front of the Horse Guards. His would be a " broadside " view of the several vehicles ; and whether they had all moved, or, if only some of them, then which of them, and how much had each moved, would be points upon which he could pronounce an opinion promptly and (let us hope) accurately.

The above simile in each and all its stages and aspects may be taken to be a counterpart of the problem presented to an astronomer called upon to investigate stellar proper motions. And what Fontenelle once said in respect of the star known as Altair in the constellation Aquila is in keeping with the illustration which I have borrowed from what may be seen any day at Trafalgar Square. Said Fontenelle : " There is a star in the Eagle which, if all things continue their present course, will, after the lapse of a great number of ages, have to the west another star which at present appears to the east of it." * Fontenelle's remark is just such a remark as my ideal spectator at the Horse Guards might make because of his enjoying a "broadside" view of the changes in the positions of the vehicles going down Whitehall. But the original spectator at the Nelson Monument has also his circumstances reproduced in the heavens ; for, even though in the case of any given star no indications, or but slight indications, of lateral change of place can be detected, yet such star may nevertheless be endued with a rapid motion of either approach or recession which can be found out by a secondary method. Thanks to the spectroscope and the ingenuity of modern astronomers, motions of approach to or recession from the earth have been discovered in the case of certain stars, notwithstanding that those stars, being seen " end on " (alias, in the line of sight), seem on mere visual observation to be practically stationary.

But I am anticipating too much. The fact that certain of the stars are endued with a proper motion of their own was first ascertained in 1718 by the English astronomer Halley. By comparing the positions of Sirius, Arcturus, and Aldebaran, as laid down in the most ancient catalogues, with the positions determined by himself in 1717, he found, after making every allowance for the effects of precession and the variation in the obliquity of the ecliptic, that these stars seemed to have got out of place to the extent in each case of more than i°, a displacement too considerable to be ascribable to errors of observation or errors of copying. In the case of Aldebaran it was further found that that star had undergone at Athens in 509 A. D. an occultation by the Moon which could not have taken place if the star had occupied 1400 years ago the same or nearly the same place that it occupies at the present time. The utmost that Halley could do was to surmise that the Stars in question were affected by proper motion, because in those days no long continued series of observations of places taken by exact instruments were in existence. Such observations, however, soon began to accumulate as the 18th century rolled on, and accordingly in .138 James Cassini was able to say with some confidence that Arcturus had undergone a displacement of 5' in a century and a half, whilst the neighbouring star n Bootis had been exempt from such displacement. Inasmuch, however, as precise and exact instrumental observations of star places can only be said to date from 176o (being the epoch of Bradley's Catalogue of important stars) and as that was only a century and a quarter ago it is evidently clear that the study of stellar proper motions must be regarded as a branch of the science which is still in its in-fancy, especially seeing that in the case of the star having the largest known proper motion (183o Groombridge Ursae Majoris), the amount is only 7", and that only in the case of about a dozen stars does the amount exceed 3". It was to a fact such as this that Sir j. Herschel alluded in the paragraph quoted on a previous page, when he spoke of " motions which require whole centuries to accumulate before they produce changes of arrangement such as the naked eye can detect."

Year by year is adding to the number of the observations, which by their exactitude enable us to detect proofs of proper motion when those observations are placed in juxtaposition with observations of the same stars made in the earlier part of the present century, say between 1800 and 183o. The materials already available seem to point to the fact that the proper motions of the brighter stars are, as a rule, greater than those of the fainter stars. The average proper motions of the 1st magnitude stars known to possess proper motions has been set down at 1/4" annually, whilst the average displacement of the 6th magnitude stars known to be affected amounts to no more than 2Y.

This law, if law it may be properly called, is subject to exceptions, for there are some small stars, such as 1830 Groombridge Ursae Majoris, 9352 Lac. Piscis Australis, 61 Cygni, 21185 Lalande Ursae Majoris, and 21258 LaIande, which have very considerable proper motions.

The reader who has followed attentively the Trafalgar Square illustration will have no difficulty in understanding the statement that a knowledge of a star's proper motion conveys very little information as to the said star's real motion reckoned in miles per second. When we say that a star's proper motion amounts to 4" a year (which is about 6 1/2' in a century), the record is simply that the star's apparent lateral displacement is so much in such a length of time along a line assumed to run at right angles to the observer's line of sight. But the true direction may not be at right angles as aforesaid ; it may be in a path which the observer may only see foreshortened. Or, in an extreme case, if the motion takes place directly in the line of sight so that the star is moving straight towards us, or from us, it may be in rapid motion and yet visually seem to have no motion at all ; that is, to be undergoing no change of apparent place which can be detected by comparing observations taken at different times.

Whilst it cannot be said that we know much about the actual motions of many of the stars, yet we do know something. The spectroscope furnishes us with some clue, the basis of which is a principle of physics first enunciated by Doppler in 1842. This principle may be thus defined : " When the distance between us and a body which is emitting regular vibrations either of sound or light is decreasing, then the number of pulsations received by us in each second is increased, and the length of the waves is correspondingly diminished." In the case sup-posed the musical pitch rises, and in the same way the refrangibility of a wave of light which depends upon its wave length is increased so that it will fall nearer the violet end of the spectrum. A practical illustration of this principle may often be had by a person standing on the platform of a railway station through which a fast train is passing at a high rate of speed, whistling continuously as it passes. It will be noticed in such a case that the pitch of the whistle continuously varies as the train approaches the spectator, whilst it goes on continually varying, but in the opposite direction, after it has passed him and the distance gradually augments. Whatever was the pitch, say, at 200 yards, before the engine came up to the platform, the pitch will be the same at 200 yards in the opposite direction after the engine has passed the platform.

Huggins, in 1868, thought to apply the foregoing principle by spectroscopic observations on certain stars with a view of seeing whether a particular line in a spectrum underwent after an interval any displacement from its normal position towards either end of the spectrum. If in any given case there was a displacement towards the red end of the spectrum, the conclusion would have to be that the star was receding from the earth : if, on the other hand, the displacement was towards the violet end of the spectrum, the conclusion would have to be that the star was approaching the earth. A general deduction to be drawn from the observations of Huggins and of others who have worked in the same field, seems to be that there are several dozen prominent stars in motion at speeds varying from 2 to 50 miles a second.

When astronomers once came to recognise the fact that certain stars were in motion it naturally followed that there was a desire to ascertain whether any particular consequences, and if so what consequences, were involved in the discovery. Sir W. Herschel in 1783 began by trying to classify the proper motions of the stars, so far as known to him at that epoch. Having done this and finding evident signs that they converged towards a point in the Constellation Hercules, he was led to conclude that the Solar System as a whole was moving towards a point in the Celestial Sphere not far from the star X Herculis. The principle involved has been thus defined by Professor Young : " On the whole, the stars appear to drift bodily in a direction opposite to the sun's real motion. Those in that quarter of the sky which we are approaching open out from each other, and those in the rear close up be-hind us. The motions of the individual stars lie in all possible directions, but when we deal with them by thou-sands, the individual is lost in the general, and the prevailing drift appears."

The effect here stated by Young may be seen, and being seen, may be easily realised, by walking through a field dotted over with units of any kind, such as sheaves of corn put up in shocks, or haycocks, or any similar aggregations of produce. As the pedestrian approaches a row of such things, the row which at a distance seemed almost continuous will be found on nearing it to have its units separated by several feet or yards of distance : as he passes forwards across the field the first and subsequent rows will gradually seem to close up behind him into a more or less compact mass.

Sir W. Herschel's endeavours to find out the "apex of the sun's way (as it is called) have been followed up by other astronomers since, and about 20 different determinations are now available. There is a remarkable general accord between them all. Perhaps on the whole the most trustworthy because it is based upon a very ]arge number of stars is, L. Struve's. He has found the point of convergence to be situated in R. A. 18h. 13m.; and Decl. + 27°. Huggins, by spectroscopic observations of an ingenious character, has confirmed the general conclusions thus stated.

A skilful and careful German astronomer, named MadIer, at that time employed at the Observatory of Dorpat in Russia, put forth, in 1846, an idea that there exists some central point in the universe around which the sun, with its bevy of planets and comets, revolves in the course of millions of years ; and he suggested that such centre is situate in the direction of Alcyone, one of the Pleiades. It is difficult to pronounce dogmatically for or against this idea (which, by the way, was rather a revival of a theory put forth by Wright in 1750 than Mädler's own), but Grant's remarks may be considered to meet the case :

It is manifest that all such speculations are far in advance of practical astronomy, and therefore they must be regarded as premature."