48 



KNOWLEDGE 



[March 1, 1892. 



THE MOVEMENTS OF THE STARS. 



By Miss A. M. Clekke. 



PEOPEE motions may be described as the individual, 

 apparent, angular displacements of the stars. They 

 are small residual irregularities, becoming per- 

 sistently manifest year by year, or century by 

 century, after all the usual systematic corrections 

 have been applied, through which star-places of various 

 dates are rendered strictly comparable. And because they 

 are indi^-idual, they can only be ascertained by observation. 

 No process of reasoning is available by which, from the 

 known movements of nine hundred and ninety-nine stars, 

 the unknown movement of a thousandth star can be calcu- 

 lated. There may be a logic by which the feat could be 

 accomplished, but we are absolutely ignorant of its rules. 

 Proper motions are then experiential data, defying formulaic 

 expression, which have to be tabulated one by one — each 

 as a fact apart. Some thousands of them are now, with 

 very fair accuracy, disposable by astronomers ; and their 

 registration has been no trifling achievement. Yet they 

 are only the unshaped stones out of which the edifice of 

 knowledge regarding sidereal structure has to be built : or 

 say rather, the pile of clay collected antecedently to the 

 moulding and baking of bricks for that high purpose. 



It is true that mere empirical acquaintance with proper 

 motions serves all the purposes of practical astronomy, 

 since it confers the power of predicting, for an indefinite 

 time to come, the places on the sphere of the stars they 

 affect. But the physical astronomer has other ends in 

 view. For him the " sphere " has no more reality than 

 the crystalline barriers fencing off one from the other the 

 successive heavens of the ancients ; he regards it as an 

 ideal canvas, upon which the signs and wonders of the sky 

 are pictured. His true concern is with the ocean of space, 

 and the voyages amid its depths of the fiery craft everywhere 

 furrowing them. 



The direction and speed, however, of these voyages are 

 only in part and imperfectly indicated by telescopically 

 measured proper motions. No determinate results, in the 

 physical sense, can be elicited fi-om them until they have 

 first been combined with further items of information of 

 three several kinds. For they include, to begin with, a 

 common perspective element due to the translation of the 

 solar system. This must be eliminated as a preliminary 

 to assigning the " peculiar " movement due to each star 

 as a body traversing space on its own account. Secondly, 

 proper motions are angular amounts only. To render them 

 linear, we should be acquainted with the distances from 

 ourselves of the objects displ9,ced by them. Thirdly, 

 proper motions are the projections upon an imaginary plane 

 of lines of travel forming unknown angles with that plane. 

 Only that portion of stellar movement which lies across the 

 visual ray is represented in them ; they tell nothing about 

 the component along the visual ray. 



None of these three requisites for deducing individual 

 real motions from proper motions is indeed entirely wanting 

 even now ; and all are likely to be more completely 

 provided in the future. A thoroughly satisfactory deter- 

 mination of the sun's course and velocity will evidently 

 before long be accomplished through the accumulation of 

 precisely known stellar radial movements ; and by the 

 same means, the missing component of proper motions 

 first cleared of the effects of our own interstellar journey, 

 can already be supplied in some cases, and will before long, 

 be supplied in many more. The list of measured star- 

 parallaxes, too, lengthens continually ; yet here it must be 

 admitted that the prospect ol improvement is less assured 

 than elsewhere. For relative parallaxes only are given by 



the methods at present in use, and doubts of the gravest 

 kind as to the validity of conclusions from relative to 

 absolute parallax are beginning to force themselves upon 

 astronomical attention. It is, nevertheless, consolatory to 

 reflect that misgivings as to the genuineness of the results 

 so far attained in this important branch of research are 

 strongest where sensibly fixed objects are in question. 

 Stars with appreciable proper motions detach themselves 

 from their background, and so afford a satisfactory criterion 

 for the choice of trustworthy reference-stars. Hence the 

 parallaxes of swift stars can rarely be wholly illusory, and 

 are probably, for the most part, but very slightly under- 

 estimated. 



In the meantime, what conclusions can be derived from 

 the facts actually before us ? Do they provide groimd for 

 even a rational surmise as to the dynamical relations of 

 the stars '? At the threshold of the enquiry wo are startled 

 by the phenomena of what are called "runaway stars." 

 Of these, Groombridge 1830, with its thwartwise velocity 

 of 232 miles a second, is the classical example ; but it is 

 outdone by Arcturus with 375, and by p Cassiopeia with 

 305 miles a second ; and several stars besides ^ — if the 

 small parallaxes attributed to them can be depended 

 upon — shoot through space at rates varying from 70 to 

 upwards of 100 miles a second. Twelve, moreover, out of 

 52 stars with numerically valued tangential velocities, 

 progress at a speed exceeding 50 mUes a second. Stars, 

 then, of the '• flying" description, are no great rarities. 

 Among them are to be found both single and compoimd 

 objects, enormous bodies like Arcturus, and orbs on the 

 modest solar scale, such as Groombridge 1830; and they 

 show spectra of sundry varieties. Some are probably 

 highly luminous in proportion to their mass, others give 

 little light, while exercising a strong mutual attraction. 

 Of the latter sort, at least, are the semi-obscure revolving 

 pair carried with it by o' Eridani in its particularly well 

 authenticated advance of 71 miles a second. The swiftest 

 stars cannot, accordingly, be regarded as forming a class 

 by themselves ; their velocities, although unaccountable, 

 and, by calculable gravitational power, uncontrollable, are 

 evidently systematic, that is, belong to the settled order of 

 sidereal arrangements, and have to be dealt with in any 

 attempt to grapple with the problem of sidereal mechanism. 



No radial velocities at all comparable with the high 

 tangential velocities of late pretty freely disclosed, have yet 

 been detected. Aldebaran, with 30 miles a second of 

 recession fi-om the sun, bears the palm among 47 stars 

 spectrographically determined at Potsdam, and their 

 average rate comes out no more than 10'6 English miles. 

 But the average value of the visible component for 

 52 stars with ostensible parallaxes,* is 42 miles, the effects 

 of the solar motion being of course impartially mixed up 

 with both kinds of result. The tangential rate, however, 

 since it is measured along great circles of the sphere, sums 

 up motion in the two co-ordinates of right ascension and 

 declination, the separate value of each of which, amoimtiug 

 to just 30 miles a second, is the quantity properly to be con- 

 trasted with the figure derived from the Potsdam obser- 

 vations. 



A striking disparity, none the less, still remains, and 

 suggests curious reflections ; although no great stress can 

 be laid upon them until it be seen whether the anomaly 

 giving rise to them be abolished or accentuated by further 

 research. Possibly it may be due to the character of the 

 objects examined for two different purposes. The Potsdam 



* One of them is Arcturus, T\-liich can scarcely be said to liare 

 even au '* ostensible " parallax, since its probable error considerably 

 exceeds its nominal amount. The star should rather be called 

 indefinitely remote. 



