September, 1903.] 



KNOWLEDGE. 



199 



not wholly clear of obstacles and incongruities. Yet they 

 may probably be described as of a complicating, rather 

 than of a contradictory kind. The theory of tidal friction 

 is not an universal solvent of tlie difficulties encountered 

 in the study of double stars. That the mode of action it 

 deals with had a contributory share towards regulating 

 their mechanical arrangements may, nevertheless, be 

 regarded as certain, while the energy, and perhaps even 

 the manner of its operation, varied extensively from system 

 to system. What precisely it effected in each lies beyond 

 our reach to detennine. For the data available regarding 

 the viscosity, density, and axial movements of Hedgeling 

 star-pairs must always be too scanty and insecure to 

 l)rovide a basis for rigorous computations. The mystery 

 of the fore-time can never be entirely dissipated. Enough 

 if we can look at it through a glass which darkens, without 

 distorting, the objects presented in its field of view. 



STELLAR SATELLITES. 



By J. E. Gore, f.k.a.s. 



The term satellite is usually applied to the moons which 

 revolve round the planets of the solar system, like our 

 own moon and the moons of Jupiter and Saturn. But the 

 term is also sometimes used with reference to the faint 

 companions of bright stars. In most of the known binary, 

 ur revolving double stars, the com]>onent stars which form 

 these stellar systems are usually of nearly ecjual brightness, 

 or at least do not differ very much in relative brilliancy. 

 These may be called pairs of suns, or " twin suns." There 

 are, however, some notable exceptions to this rule. Among 

 those binaries tor which orbits have been computed the 

 following are the most remarkable: — Procyon, magnitudes 

 05 and 13 (or 12| magnitudes difference in brightness) ; 

 Sirius, magnitudes r62 and 10 (about 11^ magnitudes 

 difference) ; 8 Cygni, 3 and 8 ; 'M Herculis, 6 and 1 1 ; 

 85 Pegasi, 6 and 10 ; and t) Cassiopeiae, 4 and 70. Of 

 those double stars which are known to be binary, but in 

 which the motion hitherto has not been sufficient to enable 

 an orbit to be computed, the following may be mentioned: — 

 Ursa? Majoris, 3 and 14; a, Ursae Majoris, 2 and 11; 

 /3 Leporis, 3 and 11 ; i Ursse Majoris, 3 and 10 ; 

 1) Gcminorum, 3 and 10 ; 34 Pegasi, (J and 12 ; and 

 26 Draeonis, 5tV and 11. 



According to an orbit computed by Dr. See, and a 

 parallax of 0"'38 found by Sir David Gill, the mean 

 distance of the companion of Sirius from the bright star 

 is about 21 times the earth's distance from the sun, or a 

 little more than the distance of Uranus from the sun. The 

 mass of the system is 347 times the sun's mass, the bright 

 star being 236 times, and the satellite 111 that of the 

 sun. With the above distance I find that the satellite, as 

 seen from Sirius, would shine with about the brightness 

 of full moonlight. As the mass of the satellite is about 

 the same as tliat of our sun, its inherent light must be 

 very small. If the sun were placed at the same distance 

 from the earth its light would still be more than 1300 

 times that of the full moon. 



The bright star Procyou forms a very similar system to 

 that of Sirius. A close companion was strongly suspected 

 by Otto Struve in 1873, and this was discovered in 

 November, 1896, by Schaeberle with the great 36-incli 

 telescope of the Lick Observatory. It is about the 13th 

 magnitude. Dr. See finds a period of about 40 years, or 

 about the same as that found by Auwers in 1861, from a 

 consideration of some irregularities in the proper motion 

 of Procyon. Dr. See finds the mean distance to be 21 times 

 the sun's distance from the earth, and the mass of the 



satellite equal to that of the sun, or the same as in the 

 Sirian system. The proper motion of Procyon is about 

 the same as that of Sirius — 1-3 seconds of arc per annum 

 —and its parallax about 0"-32, only slightly less. The 

 similarity of the two systems of Sirius and Procyon, in 

 almost every particular, is very curious. The spectrum of 

 Procyon is, however, of the second or solar type, and its 

 mass about double that of Sirius. As Sirius is considerably 

 brighter than Procyon (about two magnitudes), we have 

 here another proof that stars with the solar type of 

 spectrum have a larger mass in proportion to their 

 brilliancy than stars of the Sirian type, or, in other words, 

 of two stars having the same mass, but one with a Sirian 

 and the other with a solar spectrum, the Sirian star would 

 be much brighter than the solar one. 



The 6th magnitude star 85 Pegasi has a " satellite " of 

 the lOtli magnitude. It is a binary star, and Dr. See finds 

 a period of 24 years, with a mean distance of 0"89. 

 Burnham finds 257 years, and 0''-78. The proper motion 

 of the system is about 1"'3, and a small parallax of 0" 054 

 was found by Brunnow. From these data I find that the 

 mean distance is about 15 times the earth's distance from. 

 the sun. The orbits referred to above give the mass of 

 the system from 4 to 8 times the sun's mass. The satellite 

 as seen from the primary star would shine as a small sun, 

 so that it must be considered rather as a companion sun 

 than a satellite. The accuracy of the small parallax is of 

 course somewhat doubtful, but if nearly correct the large 

 proper motion would indicate a velocity of about 70 miles 

 a second. 



In addition to the above there are some stars which 

 have faint companions " or satellites," the measures of 

 which do not (Jircdhj show orbital motion, but which are 

 known to bo physically connected, from the fact that the 

 bright star and its faint attendant have the same " common 

 proper motion." In other words they are moving together 

 through space with the same velocity and in the same 

 direction, and ai-c therefore most probably near enough 

 to be liiikiMl together by the laws of gravitation. In such 

 cases the " satellite " probably revolves round its primary, 

 but owing to the great distance from its central sun, the 

 period of revolution would be very long, and the angular 

 motion would not be perceptible for many years at the 

 great distance at wliicli the system usually lies fmrn the 

 earth. Let us consider some of these stellar systems. 

 From the apparently great apparent distance which 

 separates these satellites from the i)rimary star, they seem 

 to be constituted on a much vaster scale than those binary 

 stars in which the motion is so rapid that an orbit can be 

 computed. We will consider those stars for which a 

 measurable parallax has been found, and for which, 

 therefore, the distance from the earth is approximately 

 known. 



The bright reddish star, Aldebaran, has a faint satellite 

 of about the 11th magnitude, at a distance of about 

 1 1 7 seconds of arc, which was originally discovered by Sir 

 William Herschel. In the year 1888, "this faint star was 

 found to be a close double star by the famous American 

 astronomer Burnham, with the 36-inch telescope of the 

 Lick Observatory. He also found a closer and fainter 

 companion — about the 14th magnitude— while iising the 

 18i-inch telescope of the Chicago Observatory in the year 

 1877. The distance of this faint satellite from the bright 

 star is about 31". Measures in subsequent years have 

 shown that the distant double companion is not moving 

 with Aldebaran. which has a proper motion of about 0"-19 

 per annum, but, curious to say, Burnham's faint com- 

 panion has, notwithstanding its comparatively great 

 distance from Aldebaran, exarilii the same projjer motion 

 as the bright star, and is therefore most probably 



