186 



KNOWLEDGfi 



[August 1, 1890. 



object, under proper conditions, and is not likely to ever 

 get beyond the reach of the large refractor." 



Assuming a parallax of 0*40" (which is about the aver- 

 age of recent measuiTs) my elements give the sum of the 

 masses of Sirius and its companion equal to 2-886 times 

 the sun's mass, with a mean distance between them of 

 21-4.5 times the sim's distance from the earth. I find that 

 the plane of the orbit lies at right angles to the general 

 plane of the Milky Way. The diagram, Fig. 4, shows the 

 (ipjiannt orbit as foimd by me. The letter in marks the 

 point of minimum distance (and M that of maximum dis- 

 tance). This diagram shows clearly that the minimum 

 distance between the components of a binary star does not 

 always occur at the periastron, as some have supposed. C 

 is the centre of the (qqmrent ellipse. 



9. y Coroncp Aiixtnilin. — For this remarkable southern 

 binary star several orbits have been computed, with periods 

 ranging from 55 years to nearly 101, but none altogether 

 satisfactory. An orbit by the present writer gi\-es 81-78 

 years, and one recently computed by Rlr. Powell 93-338 

 years. 



10. ^ ' '(Uteri. — A well-known triple star, the close pair 

 revolving in a period of about sixty years. Nearly two 

 revolutions have now been completed since its discovery 



Fig. 9. — Appakekt Orbit of Struve 228. 



by Sir W. Herschel in 1781. All three stars form 

 probably a connected system, but the motion of the third 

 star round the binary pair is very slow and irregular. 

 Prof. Seeliger has recently investigated the motion of this 

 interesting system, and has come to the conclusion that to 

 make the observations agree with theory it is necessary to 

 assume that the third star is in reality a very close double, 

 the components of which revolve round their centre of 

 gravity in about 17-6 years, and both round the common 

 centre of gravity of the components of the close pair. The 

 supposed duplicity of the third star has not, however, yet 

 been detected with the telescope. Burnham, in 1889, 

 using a power of 1,500 failed to see any other component. 



11. $ Ursrr Miijoris. — This very interesting binary 

 was the first pair for which an ortjit was computed — by 

 Savary in 1830. More than a complete revolution has now 

 been performed since its discovery by Sir W. Herschel in 

 1780. The period has, therefore, been well determmed, 

 and seems to be about sixty years. Fig. 5 represents 

 the apparent ellipse, which I have drawn from Dr. Dunt'r's 

 orbit. From this it will be seen that although the com- 

 ponents are not at present near their maximum distance 

 apart, they are yet within the range of small telescopes, 

 the distance being about If seconds, and the magnitudes 

 of the components not very unequal, about four and five. 



12. O. Struve, 234. — This is a very close double star for 

 which I computed an orbit in 1886, and found a period of 

 63-45 years. Owing to the discordancy of the measures, 

 however, this orbit will jirobably require revision when 

 further measures are available. Fig. 6 represents the 

 apparent orbit I found, and shows the position of the 

 companion from 1844 to 1880. It will be seen fi-om the 

 diagram that the maximum distance between the com- 

 ponents is less than half a second of arc, so that the pair 

 is beyond the reach of all but the largest telescopes. 



13. a Crntaun. — This bright southern star, the nearest 

 of all the fixed stars to the earth, is also a remarkable 

 binary. The magnitudes of the components are, according 

 to Gould, 1 and 3i, but as a single star it is brighter 

 than an average star of the 1st magnitude, and about 

 equal in brightness to Arcturus. Although it has been 

 carefully measured for many years the period is still 

 somewhat uncertain. While Downing and Elkin make it 

 76 or 77 years, Powell maintains that a longer period of 

 about 87 years is more probable. Fig. 7 represents the 

 ri'dl ellipse as drawn from Downing's correction of Elkin's 

 orbit. Assuming a period of 77 years, and a parallax of 

 0-75", I find the sum of the masses of the components 

 2-14 times the mass of the sim, and the mean distance 

 between them 23-333 times the sun's distance from the 

 earth, or somewhat greater than the distance between 

 the sun and Uranus. 



Scale 0/ Seconds. 



Fig. 10. — Appakent Orbit op ^ Scobpii. 



14. 70 Opliiiirhi. — A splendid double star; magnitudes 

 about 4 and 6. More than a complete revolution has now 

 been described by the companion since its discovery, by 

 Sir W. Herschel, in 1779. Numerous orbits have been 

 computed, with periods ranging from 73f to 94J years, but 

 none altogether satisfactory. I have recently computed 

 an orbit, and find a period of 87-84 years. This orbit 

 represents all the measures satisfactorily, and cannot, I 

 think, be far from the truth. The diagram. Fig. 8, repre- 

 sents the apparent orbit I find, and shows the position 

 of the comijanion in various years, fi-om 1802 to 1888. 

 The distance between the components is at present 

 slightly increasing as the companion approaches the peri- 

 astron ; after which it will diminish for some years, but 



