STAR 309 stars of each constellation, in the order of their brightness, the letters of the Greek alpha- bet. Since his day cataloguers of stars have introduced several new methods, more or less incongruous. For instance, Flamsteed num- bered the stars in each constellation according to their right ascension in his time ; Piazzi numbered stars in hours of right ascension, the first in each hour being called 1, the next 2, and so on ; W. Struve numbered all the stars he dealt with (in forming a catalogue of double stars), from Oh. Om. Os. onward, till the com- plete circuit of the sphere had been made in right ascension; variable stars have received the letters R, S, T, &c., for each constellation, in order of discovery, the letters A, B, 0, &c., from the other end of the alphabet, having been already employed in continuation of Bay- er's system ; and still other methods have been introduced, to the confusion of learners. Moreover, the regions occupied by the differ- ent constellations have not been definitely as- signed; some astronomers include the new constellations added by Bode and others, while many only allow the constellations of Ptolemy, Hevelius, and Halley (in the southern hemi- sphere) to appear in the maps, omitting gen- erally the constellations Antinous, Cerberus, and Scutum Sobieskii from Hevelius's list, and Robur Carolinum from Halley's. Similar con- fusion exists as respects the method of indi- cating the brightness of stars. Astronomers agree in dividing the stars visible to the naked eye into six orders of brightness called magni- tudes, from the first magnitude or brightness to the sixth, the faintest which ordinary eye- sight can perceive in dark and clear nights without telescopic aid; but for the fainter or telescopic stars four different methods of classification have been employed by Sir J. Herschel and Admiral Smyth in England, and by W. Struve and Argelander on the continent. The relation between the magnitudes of these different systems is indicated in the following : Herschel. 6-4 7-0 7-4 7-8 9-5 10-1 10-4 11-3 11-7 12-5 13-3 14-5 15-9 Smyth. 6 6-5 7 7-5 8 8-5 9 9-5 10 11 12 13 14 15 16 Struve. 5-7 6-3 6-5 6-9 7-4 7'9 8-8 8-9 9-3 10-0 10-4 10-7 10-9 10-9 10-9 Argelander. 5-9 6-4 6-8 7-5 8-0 8-6 9-0 9-4 9-4 10-0 10-6 11-2 11-8 12-4 18-0 It will be perceived that while the systems of Sir J. Herschel and Smyth are nearly enough alike to be practically interchangeable, the systems of Struve and Argelander are unlike for the fainter orders, and both differ markedly from the English system of indicating mag- nitudes. Unfortunately no system has been adopted uniformly by astronomers, or even by the astronomers of any given nation. Per- haps Argelander's is on the whole the best. Herschel's and Smyth's systems err in re- quiring that nicer distinctions should be drawn among very faint stars than ordinary observers can be expected to recognize. Struve's sys- tem appears to err in the opposite direction, by allowing too many stars to be included in the different orders of very faint stars. The word u magnitude " as used in connection with stars refers only to apparent brightness; for the true magnitudes or volumes of stars are unknown. To determine a star's real magni- tude, its distance must be determined and also its apparent diameter. But it is only in a few instances that the annual parallax of a star has been determined; and not a single star, however highly magnified, shows a true disk. Hence it is impossible to determine the volume of any star. In the few cases where the dis- tance has been determined, it becomes possible to infer from the star's apparent brightness the total quantity of light emitted by it ; and if we assume that equal portions of the star's surface and of our sun's emit equal amounts of light, we can compare the surface of such a star with our sun's surface, and so deduce its diameter and volume; but the assumption is not by any means safe. Very few stars have a measurable annual parallax. The following table includes all hitherto dealt with : STARS. Magni- tude. Parallax. Latest measures. a Centauri 61 Cygni 1 6 0-976" 0-348 0-91" 0-55 Lalande 21258 8 0-26 Oeltzen, 17415-6 9 0-25 LjTffi 1 0-16 Sirius 1 150 0-27 70 Ophiuchi . 5 0-16 1 Ursae Majoris 3 0-13 1 0'13 Polaris 2 0-067 0-11 Capella 1 0'05 1 0-12 Of the above measures, we owe the earliest, that of 61 Cygni, to Bessel ; but it will be per- ceived that later measures differ appreciably from his. Henderson gave the earliest mea- sures of a Centauri and Sirius, the corrected es- timate for Sirius having been obtained by Mr. Cleveland Abbe, formerly of the observatory of Pulkova, now of Washington ; most of the remaining measures are due to the labors of Kruger and Peters. When we observe that only a Centauri has given consistent results, we may well doubt whether as yet astrono- mers possess instruments competent to mea- sure small parts of a second of arc. The dis- tance of this star corresponds to the space traversed by light in about 3J years, the dis- tances of the other stars being greater accord- ing as the parallax is less ; so that, for instance, if the parallax of Capella in the above list were strictly exact, Capella would be 19 times fur- ther away than a Centauri, and light would not reach us from it in less than 63 years. It would be a fair inference that the light of
