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SIRIUS AND ITS SYSTEM.
IRIUS, or the Dog Star, is the brightest star in the heavens,
and from its superior brilliancy has been termed “the monarch of the skies.” Measures of its light show that it is about two magnitudes, or over six times brighter than an average first magnitude star, like Altair or Spica, and about equal in lustre to three stars like Vega or Capella. Sir John Herschel found the light of Sirius equal to 324 times the light of a star of the sixth magnitude, about the faintest visible to average eyesight. But it is probably over 600 times brighter than a sixth magnitude star. It has been seen in daylight with a telescope of only half an inch in aperture. Some observers have even seen it with the naked eye in sunshine, and it has been observed to cast a shadow like Venus when at its brightest.
The origin of the name Sirius is somewhat doubtful. It may possibly be derived from the Sanscrit word surya, the sun. Professor Max Müller thinks that the Greek word seirios comes from the Sanscrit svar or suonasirau. Sirius is first mentioned as a star by Hesiod, who connects it with the dog days. These, according to Theon of Alexandria, commenced twenty days before Sirius rose with the sun, and ended twenty days after that date. These socalled dog days commence on July 3, and end on August 11 ; but, owing to the precession of the equinoxes, Sirius does not now rise with the sun-or heliacally, as it is termed-until August 25, or fourteen days after the dog days have ended. The fancied connection of Sirius with the forty days of summer heat has, therefore, no longer any existence, and must-like many such ideas-be consigned to "the myths of an uncritical period.”
Sirius was worshipped by the ancient Egyptians under the names of Sothis (Horus), Anubis, and Thoth, and represented as a man with the head of a dog. Some identify it with the Mazzaroth of Job. It was also supposed to represent Orion's hound, and it may perhaps be identical with the Cerberus of the Greeks.
It seems to be a popular idea that Sirius, now of a brilliant white colour, was a red star in ancient times. But such a remarkable change of hue is not well established. It seems more probable that
the idea of change is due to the mistranslation of a word applied to the star by the ancient writers, a word which probably referred to its brightness rather than its colour. Mr. T. J. J. See has, however, recently collected strong evidence from the classical writers to show that Sirius was really a red star in ancient times. Such a change would, of course, be most interesting and remarkable, indicating, as it would, some wonderful change in the star's chemical constitution.
Like many other stars, Sirius has a considerable "proper motion " across the face of the sky, amounting to about 1'3 seconds of arc per annum. Some irregularities in this proper motion led the astronomers Bessel, Peters, and Safford to the conclusion that the motion of Sirius was disturbed by the attraction of an invisible close companion revolving round it. From the recorded observations Peters computed an orbit for the supposed companion, and found a period of about fifty years. Safford also investigated the problem, and announced in 1861 the probable position of the invisible companion. About four months after the publication of Safford's results, Mr. Alvan Clark, the famous American optician, observing with a telescope of 18.5 inches aperture, detected a small star near Sirius, the position of which agreed closely with that of Safford's hypothetical companion. Here was a case somewhat similar to the discovery of the planet Neptune-the prediction, by mathematical analysis, of the existence of a celestial body previously unknown to astronomers. Numerous observations of this small star have been made since its discovery, and there is now no doubt that it is revolving round its brilliant primary. That the observed irregularities in the proper motion of Sirius are wholly due to the influence of this companion seems, however, to be still an open question. Several orbits have been computed, most of which assign a period of forty-nine or fifty years ; but an orbit recently computed by the present writer gives a period of about 58 years, and Howard finds a period of fifty-seven years. Burnham, however, thinks that fiftythree years is probably nearer the truth. As the companion has now approached Sirius so closely as to be invisible with even the giant telescope of the Lick Observatory, some years must elapse before the exact length of the period can be definitely settled.
The great brilliancy of Sirius has naturally suggested proximity to the earth, and modern measures of its distance have confirmed the accuracy of this idea. The most reliable determinations of its parallax (or the angle subtended by the radius of the earth's orbit at the place of the star) make it about four-tenths of a second of arc, and places it about fourth in order of distance from the earth.' Assuming a parallax of o'39 of a second (about a mean of the results found by Drs. Elkin and Gill), the distance of Sirius would be 528,884 times the sun's distance from the earth, a distance which light would take about 8.} years to traverse.
Knowing the distance of Sirius from the earth, and its annual proper motion, it is easy to calculate its actual velocity in a direction at right angles to the line of sight. This comes out about ten miles a second. The spectroscope shows that Sirius has also a motion in the line of sight, and hence its real velocity through space must be greater than that indicated by its proper motion. In the year 1864 observations by Dr. Huggins showed that Sirius was receding from the earth at the rate of twenty-nine miles a second. Some years afterwards careful measures of the star's spectrum showed that this motion had ceased; subsequent measures showed that the motion was reversed, and recent observations by Dr. Vogel indicate unmistakably that the motion has now been changed into a motion of approach! It seems difficult to understand how this curious change in the direction of the star's motion can be accounted for otherwise than by orbital movement; in the same way that the planet Venus is sometimes approaching the earth and sometimes receding from it, owing to its orbital motion round the sun. The motion may possibly be due to the existence of some invisible close companion.
Placed at the distance of Sirius, the Sun would, I find, be reduced to a star of only the third magnitude, or about four magnitudes fainter than Sirius appears to us. This indicates that Sirius is about forty times brighter than the sun would be in the same position, and would imply that Sirius is a far more massive sun than ours. If we assume the same intrinsic brilliancy of surface and the same density for both bodies, the above result would make the diameter of Sirius 6'32 times the sun's diameter, and its mass no less than 253 times the mass of the sun. As, however, the intrinsic brightness of the surface of Sirius and its density, or specific gravity, may differ widely from those of the sun, these calculations are of course open to much uncertainty. The light of Sirius, analysed by the spectroscope, differs considerably from the solar light, and the strong development of the hydrogen lines in the star's spectrum denotes that Sirius is, in its chemical constitution, not comparable with our sun. It may possibly be very much hotter and
I The th nearest stars are : a Centauri (parallax 0.76 of a second), 61 Cygni (0:45"), and Lalande 21,185, for which Kapteyn found a parallax of 0.434", and Winnecke 0.5". For the star , Herculis a parallax of 0.40" was found by Belopolsky and Wagner ; but this does not seem to have been confirmed by any other astronomer.
therefore smaller in diameter and mass than the figures given above would indicate. Fortunately we can find the mass of a binary or revolving double star by another and more certain method. Knowing the orbit of the star and its distance from the earth, we can calculate the combined mass of the components in terms of the sun's mass. Making the necessary computations for Sirius, I find that the combined mass of Sirius and its companion is a little over three times the mass of the sun, and the mean distance between them twenty-two times the sun's distance from the earth, or a little more than the distance of the planet Uranus from the sun. This result--recently confirmed by Dr. Auwer's calculations--would imply that Sirius is intrinsically a much brighter sun-surface for surface-than ours, and that “the monarch of the skies is a “giant only in appearance ; the greater brightness of its surface and its comparative proximity to the earth accounting for its great apparent brilliancy.
The companion of Sirius has been estimated as of the tenth magnitude. This would imply that the light of Sirius is about 25,000 times the light of the small star. If, therefore, the two bodies were of the same density and intrinsic brightness, the mass of Sirius would be about four million times as great as the mass of the companion. But Dr. Auwers concludes, from his researches on the proper motion of Sirius, that the companion is about one-half the mass of the primary, and equal in mass to our sun ! It must, therefore, be nearly a dark body. It has been suggested that the companion may possibly shine by reflected light from Sirius in the same way that the planets of the solar system shine by reflected light from the sun. Some calculations which I have recently made show, however, that this hypothesis is wholly untenable. Assuming, with Auwers, that the mass and diameter of the companion are equal to those of the sun, I find that the companion would, if illuminated solely by reflected light from Sirius, shine as a star of only 16. magnitude. A star of this magnitude-about the faintest visible in the great Lick telescope-placed close to a brilliant star like Sirius would, even when most favourably situated, be utterly invisible in our largest telescopes. If its mass is much less than one-half that of Sirius-as its faintness would seem to suggest—it is possibly a comparatively small body, and the reflected light from its primary would be proportionately less. It seems clear, therefore, that the companion must shine with some inherent light of its own, otherwise it could not possibly be so bright as the tenth magnitude. It is probably a sun of small luminosity revolving round Sirius in the same way that Journal of the British Astronomical Association, March 1891.
the companions to other binary stars revolve round their primary. The disparity in brightness is, however, remarkable, no other binary star showing so great a difference in the brilliancy of the components.
As I have said above, the sun, if placed at the distance of Sirius, would shine as a star of the third magnitude. There is, therefore, a difference of seven stellar magnitudes between the light of the sun and that of the Sirian satellite. This implies that the light emitted by the sun is 631 times greater than that radiated by the companion of Sirius. If of the same intrinsic brightness of surface, the latter would, therefore, have a diameter about th of the sun's diameter, or
But if of the same mass as the sun, its density with this small diameter would be enormous—in fact, vastly greater than we can imagine possible for any body, large or small. Indeed, if we suppose its diameter to be one-half that of the sun, its density would be 11'52 (1'44 x 8), or about equal in density to lead, and it seems very improbable that a self-luminous body could have so high a density as this. We must conclude, therefore, that the satellite of Sirius is a comparatively large body having a small intrinsic brilliancy of surface-possibly a cooling body verging towards the utter extinction of its light. If this be so, it will probably, in the course of ages, disappear altogether from telescopic vision, and its continued existence will only be known by its influence on the motion of Sirius.
If there are any planets revolving round Sirius they will probably remain for ever unknown to us. A planet comparable with Jupiter in size would be utterly invisible in the giant telescope of the Lick Observatory, or even with an instrument very much larger. I am disposed, however, to think that these binary stars may perhaps form exceptions to the general rule of stellar systems, and that single stars, like our sun, more probably form the centres of planetary systems like our own. Or possibly the reverse of this may be true, the single stars forming the exceptions and binary stars the rule. In either case we may conclude, I think, judging from the analogy of our sun, that single stars are more likely to have planets revolving round them.
J. ELLARD GORE.