May 2, 1892.] 



KNOWLEDGE 



87 



also in its support periodical disturbances of proper motion, 

 brought to vievi by a careful discussion of all the observa- 

 tions of the star, from 1753 to the present time, and 

 indicating, in his opinion, a combination of elliptical 

 travelling with a progressive advance. But the average 

 proper motion of Algol is so very small — less than two 

 seconds of arc a century — that variations or irregularities 

 in it can at present be regarded only as an interesting 

 possibihty. They would give, if confirmed, 2-7" for the 

 longest diameter of the ellipse into which the wide orbit 

 traced out by Algol round its unseen primary is projected 

 upon the sky. And since this little span represents an 

 actual expanse of 38 earth-to-sun distances, or " astro- 

 nomical units," it implies a parallax for the star of 0'07", 

 corresponding to a distance of nearly 47 light years — a 

 statement that is in many ways worth thinking about. 

 Although claiming only qualified credence, it nevertheless 

 conveys the upshot of assuredly the most promising attempt 

 yet made to determine, by indirect means, the parallax of 

 a star. In itself, too, it seems probable enough. Assum- 

 ing its accuracy, we gain the information that Algol emits 

 63 times as much light as the sun, which, in its place, 

 would show with little more than the brightness of a seventh 

 magnitude star. The famous variable, moreover, accord- 

 ing to Dr. Vogel, is just one million of miles in diameter, | 

 so that it presents only once and a third the solar 

 radiating surface ■, yet it is, as a light-giver, G3 times more 

 effective. The remarkable conclusion follows, that Algol 

 is intrinsically 17 times more brilUant than the sun. The 

 emissions from its photosphere are, per unit of area. 47 

 times more powerful. And should its parallax eventually — 

 as seems not unlikely — prove to be smaller than 0'7", 

 this disparity will be still further enhanced. 



By means, accordingly, of investigations of this nature, 

 more fully and securely carried out, the question as to 

 comparative stellar brilliancy may finally obtain a suffi- 

 ciently satisfactory answer. It is a very important one. 

 The process by which photospheric light is manufactured 

 is stUl largely enigmatical ; but the ideas commonlj- enter- 

 tained about it are not easily compatible with the existence 

 of considerable differences m the shining faculty of photo- 

 spheric shells presumably identical in point of chemical 

 composition. Reliable e^-idence of such differences has not 

 hitherto been available. That light-power in stars bore 

 no fixed proportion to mass was patent in numberless 

 examples ; but the density, consequently the dimensions 

 of the emitting bodies remaining unknown, it could not 

 be determined whether distension of substance, or innate 

 strength of incandescence, was more concerned in pro- 

 ducing a great sum-total of light relative to quantity of 

 matter. The indications, however, now derived from Algol 

 are overwhelmingly in favour of the latter alternative. 



The primary member of its system, even if illuminated 

 solely by the borrowed rays of its brilliant neighbour, may 

 not. Dr. Chandler thinks, be out of reach of telescopic 

 discovery. But his hopes, in this case, appear somewhat 

 chimerical. It is not difficult to show that, under the 

 circumstances supposed, a body of planetary constitution 

 could not possibly be disclosed by any optical means at 

 present available. Its position-angle relative to Algol is 

 just now, we are told, 32°; while its distance from the 

 same star is in the inverse ratio of its mass. This is 

 considered by our author to be indeterminate ; but it is 

 not so, unless we reject Dr. Vogel's value for the combined 

 mass of the close pair forming the variable. Assuming its 

 approximate correctness, and that Algol and its immediate 

 attendant accordmgly contain two-thirds the solar quantity 

 of matter, and admitting further that they revolve together, 

 at a distance of nineteen astronomical units, in a period of 



131 years, round their common centre of gravity with 

 another body, it follows that the mass of that body is 

 about equal to that of the sun, and that it circulates at 

 twelve units of distance from the gravitational centre of 

 the system. It should be found, this being so, if found at 

 all, at an apparent interval of rather less than 2" from 

 .\lgol. The real gap of space separating them — the radius, 

 that is to saj-, of Algol's relative orbit — would be measured 

 by thirty-one radii of the earth's orbit ; and the efl'ective- 

 ness for visual purposes of a still problematical body, 

 shining by reflected light alone, can hence be estimated. 

 If of the same density with Algol, it presents a disc of 

 fivefold area, which, endowed with -Jupiter's high reflective 

 power, or an albedo of -62, would possess a total lustre 

 •liosbci tli^.t of the original source of its radiance. This is 

 equivalent to saying that it should be fainter by sixteen 

 stellar magnitudes. Yet the suppositions introduced 

 above are perhaps unduly favourable to conspicuousness. 

 Evidently, however, an eighteenth magnitude star, in the 

 close \-icinity of one of the second, is far below discern- 

 ment with any telescopic or photographic powers likely to 

 be in use for a considerable time, if ever ; so that visual 

 confirmation of Dr. Chandler's theory can only be looked 

 for if the unknown mass it has brought ideally into 

 existence be in some degree self-luminous. 



That theory, as he remarks, " has a much wider cos- 

 mological meaning than the mere explanation of the 

 phenomena " of a single star. Most " eclipse-variables " 

 exhibit irregularities of the same type with those of Algol, 

 and which will doubtless prove amenable to a similar 

 explanation. Moreover, an incalculable number of stars 

 which, from our point of view, escape eclipse, unquestion- 

 ably belong to systems organized on the same general 

 plan. One such, indeed, is already known in a. Virginis, a 

 first-fruit of discovery in this particular branch ; and 

 Procyon, perhaps, is one of many others essentially resem- 

 bhng it, although inaccessible to spectrographic research, 

 because revolving in planes nearly perpendicular to the 

 line of sight. Thus, the intimate association of dark and 

 bright bodies of the same order of mass would appear to 

 be no exception m the universal order. And this scarcely 

 allows us any longer to regard a sun-like condition as 

 representing simply and solely a stage in the condensation 

 of a primitively nebulous mass. Some further conditions 

 are plainly needed to produce the brilliant and concen- 

 trated evolution of light characteristic of " suns." 



Dr. Chandler concludes his valuable paper with an 

 appeal for micrometrical measures of Algol stars, adapted 

 to detect and determine possible systematic disturbances 

 of their proper motions. Measures of the kind might, in 

 his opinion, lead to highly significant results which would 

 probably, in the case of Y Cygni, be reached with parti- 

 cular promptitude. For the cyclical fluctuation of this 

 star is completed in about 600 single periods, or two and 

 a half years, and has an amplitude of no less than four 

 hom-s. Hence, the orbit described, on the light aberration 

 theory, in that short period, must, even if its plane be 

 coincident with the line of vision, approach that of Uranus 

 in size, and the star's movement in it should accordingly 

 be betrayed by ^•ibrations exceeding many times in extent 

 those due to its annual parallax. " If the research gave 

 favourable results in this instance," our author continues, 

 " it could then be extended to A. Tauri, which appears to 

 be also a promising candidate." It is to be hoped that 

 the suggestion will not remain unheeded. Owners of 

 heliometers could hardly turn them to better account than 

 by appljdng this simple criterion to an hypothesis which, 

 if approved as true, opens yet one more road through the 

 daily widening field of sidereal discovery. 



