DOUBLE STARS.] 



ASTRONOMY. 



953 



from the sun, but that the smaller body, in many 

 instances, described an orbit round the larger star. It 

 should not be forgotten, however, and it tends much to 

 the credit of philosophical conjecture, that this remark- 

 able law was previously surmised by the celebrated 

 Lambert, who considered it possible that there were 

 some groups in which the stars might make complete 

 revolutions round a common centre of gravity in a com- 

 paratively short period of time. Mitchell conjectured 

 the same law to apply to the more simple case of a 

 double star ; and, in 1784, he supposes that in a few years 

 this question would be resolved by the stars whose posi- 

 tions and distances were ascertained by Herschel. The 

 great discovery was first published by Herschel in 1803, 

 who had then perceived a decided change in the positions 

 and distances of several stars, as Ursse Majoris, Castor, 

 { Bootis, 70 Ophiuchi, J Cancri, J Herculis, <bc. , <fec. 

 The labours of the elder Herschel were resumed in more 

 modern times by his celebrated son, who, in conjunction 

 with Sir J. South, reobserved all the stars in the northern 

 hemisphere, and who has also observed independently, 

 and measured all those discovered by him in the southern 

 hemisphere. Struve, at Dorpat and St. Petersburg, 

 also made the subject his constant care since the year 

 1814, and, with the powerful means at his command, has 

 observed and reobserved their positions with the greatest 

 possible accuracy ; indeed, the publication of his great 

 work. MemtuneMicrometrictetitellarumDuplicium, forms 

 an era in this subject. This work contains observations 

 of three thousand one hundred and twelve double stars, 

 nearly three-fourths of wliich were discovered by him- 

 self. 



The combinations of the components of double stars 

 take every variety, both in regard to magnitude, dis- 

 tance, and, we might add, colour. The components of 

 Castor are each of the third magnitude, and of the same 

 ashy white colour, and close together, as are likewise 

 those of 7 Arietis in the same respects ; whilst the stars 

 Polaris, a in Herculis, 7 in Delphini, are very different 

 either in magnitude, distance, or colour. Out of the 

 3,000 stars detected by Struve, only a small portion have 

 as yet been determined to be physically connected, or to 

 form binary systems, as it has been termed ; and thu 

 number* whose orbits and periods have been even ap- 

 proximately determined, are fewer still. The time which 

 has elapsed since the discovery, has been too inconsider- 

 able to determine their true periods ; some of which, as 

 that of < Lyne, cannot be less than 2,000 years ; and 

 in others, as in 61 Cygni, or 7 Leonis, the period is seve- 

 ral hundred years. Among those whose orbits have been 

 determined with more or less exactness, we may mention 

 the following : 



M. Savary was the first who determined that the revo- 

 lution of a star round its primary was performed in the 

 same manner as that of a planet round the sun, and con- 

 formably to the laws of gravitation ; and the same prin- 

 ciples applied to other stars, have shown that the first two 

 laws of Kepler, founded upon the motions of Mars, ex- 

 tend to the revolution of sun around sun. In determin- 

 ing the orbit, supposing it to be an ellipse, the same 

 elements have to be deduced as in a planetary orbit, with 

 the exception that the period of revolution and the semi- 

 major axis are here two distinct elements, and that the 

 latter is not expressed in linear measure, but in an arc of 

 a great circle. From four observations of the angles of 



VOL. I. 



position and the distances, knowing the intervals and 

 taking the Keplerian laws as the basis of calculation, the 

 seven elements of period, semi-major axis, eccentricity, 

 nodes, inclination, position of peri-astre, and epoch of 

 the peri-astre, can be determined. In the first place, 

 however, the apparent ellipse must be determined before 

 the real one can be arrived at. The first is that which 

 the star describes around its primary on the plane of the 

 sky ; but the orbit may be inclined at any angle to this 

 plane ; and it is only when the plane of the orbit is per- 

 pendicular to the line of sight, that the true and apparent 

 ellipses are identical. The projection of the circular or 

 elliptic orbit on the plane of the heavens, remains always 

 an ellipse, but the projection of the focus and the major 

 and minor axes will take different positions ; and the 

 proportions of the latter to one another, and consequently 

 the eccentricity of the orbit, will be different. In conse- 

 quence of this difference between the apparent and true 

 orbits, we sometimes see the one star projected on, or 

 occulted by, its companion. Such was the case with y 

 Virginis, which, from 1834 to 1836, appeared as a well- 

 defined single star. The plane of the orbit of the com- 

 panion of 44 Bootis, is almost perpendicular to the plane 

 of the heavens ; and between 1802 and 1819 an almost 

 central occultation must have taken place. The com- 

 panion of J Herculis has twice undergone an eclipse, in 

 consequence of the great inclination of the orbit to the 

 plane of vision once in 1802, and again in 1831. 



In addition to the double stars, triple and quadruple 

 stars are sometimes met with ; and if the fortuitous com- 

 bination of the former is so little to be expected, that of 

 the latter is much less probable. Among the more con- 

 spicuous of the triple stars, that of J Cancri holds the 

 most prominent position ; the three stars, all of which are 

 nearly of the same magnitude, being physically connected. 

 The close double star has made upwards of a revolution 

 in a retrograde direction since 1782, and the more distant 

 one has moved fifty degrees in the same time. In the 

 list of quadruple stars, the most remarkable is that of 

 < and 5 Lyre, which can be detected as double, by a keen 

 eye, without the aid of any instrument ; but each of 

 which, when examined with a power of two hundred, 

 appear as a double star, the four components being nearly 

 of the same magnitude and colour. This system likewise 

 seems to be physically connected. Among the multiple 

 stars, that of Oriouis, situated in the centre of the great 

 nebula, is the most apparent, six stars being situated in 

 a circle of twenty-three seconds in diameter. 



The colours seen in the components of many double 

 stars have been described by some as an optical illusion, 

 on the ground that it is always the complementary colours 

 which are thus perceived, as, in a similar manner, a white 

 spot seen on a red ground will appear green. It is, howr 

 ever, impossible to view the components of a Herculis or 

 7 Andromeda, without coming to the conclusion that 

 they have a proper colour of their own, and that tho blue 

 and orange so vividly distinct is something more than the 

 effect of contrast. In those stars which show .high and . 

 brilliant colours, the larger star is always sof; a golden/ 

 orange, the smaller greenish or bluish. Iri many,, how- 

 ever, the components are of the same colour ; but it se 

 dom happens that the two colours a e of a bluish or 

 greenish tint; most frequently they are white and yellow. 



TEMPORARY AND VARIABLE STARS. Those stars which 

 vary in brilliancy from time to time, and/ others which 

 suddenly seeui to start into existence and.: disappear,, as. 

 precipitately as they have become visible, must be re- 

 garded as the most curious objects in the heavens, lot, 

 they prove manifestations of life and motion in the im- 

 measurably distant regions of space ; they form one of. 

 the subjects, too, in which even a lover of science who 

 does not possess a telescope, can, by simple observation of, 

 the relative brightness of the various stars, recorded on 

 favourable opportunities, confer considerable benefit on 

 this branch of sidereal astronomy. A telescope, in thi 

 case, is of little value ; the field of view is much too small 

 to include the stars proper for comparison with the object 

 whose light is suspected to be variable, and the photo- 

 meter hag hitherto been of very little lerTice. The moit 



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