Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/889

The constellations added by Hevelius are the following (those marked * being little used):—

*1. Antinous, 2. Mons Menelai, 3. Canes Venatici, 4. Camelopardalis, *5. Cerberus, 6. Coma Berenices, 7. Lacerta, 8. Lynx, *9. Scutum Sobieskii, 10. Sextans, 11. Triangulum Australe, 12. Leo Minor, Antinous. Mount Menelaus. The Greyhounds. The Giraffe. Cerberus. Berenice s Hair. Tho Lizard. The Lynx. Sobieski s Shield. The Sextant. The Southern Triangle. The Little Lion.

The constellations added by Halley in the southern hemisphere are,—

1. Columba Noachi, Noah s Dove. *2. Robur Carolinum, The Royal Oak. 3. Grus, The Crane. 4. Phcenix, The Phcenix. 5. Pavo, The Peacock. G. Apus, The Bird of Paradise. 7 . Musca, The Fly. 8. Chameleon, The Chameleon.

In considering the fixed stars, the first point to which our attention is naturally directed is the determination of their real dimensions, and a necessary, though not sufficient, preliminary is the determination of their distances from us. An obvious consequence of the annual motion of the earth is the existence of an annual parallax of the stars ; but on account of the enormous distances of these bodies, this effect of the earth s motion is so small, that it cannot be easily measured ; and there are even now very few cases in which, with the utmost refinements of methods and instruments, a measurable parallax has been detected. We find that, compared with the distances of the fixed stars, the diameter of the earth s orbit is merely a point ; for in most cases, the most careful observation of the same star, at intervals of six months, indicates no variation what ever in the star s position, after the proper corrections have been made for the small effects produced by dif ferent known causes. The limits of the errors of modern observations cannot well be supposed to exceed 1". It follows, therefore, that, seen from the distance of the fixed stars, the diameter of the ecliptic, which exceeds ISO millions of miles, subtends an angle of less than I". Had the annual parallax exceeded this small quantity, it could scarcely have escaped the multiplied efforts that have been made to detect it ; yet the distance of a star having a parallax of 1" is -. x radius of earth s orbit, sin. i that is, about 19 billions of miles.

The first star actually shown to have an annual parallax was the star 61 Cygni, a binary system of two sixth-magni tude stars, having a large proper motion. The parallax was detected and measured by Bessel, using the Konigs- berg heliometer. He compared the position of the point midway between the pair with that of two very small and presumably more distant stars. From the observations made between October 1837 and March 1840, Bessel deduced as the parallax 0" 3483, corresponding to a distance of about 600,000 radii of the earth s orbit. This result was confirmed shortly after by Peters at Poulkowa, who, from observations of the zenith distances, deduced the parallax //> 349. It will presently be seen that more recent measures do not accord well with these results. In the meantime, though the result was published later, Mr Henderson, at the Cape Town Observatory, had been observing a Centauri, one of the brightest of the southern stars, with the object of determining if it has a sensible parallax. Notwithstanding the inferiority of the instrument he employed (a mural circle imperfectly graduated), Mi- Henderson succeeded in recognising a parallax, which he estimated at 1" 16. Maclear, who was his successor as astronomer royal at the Cape, determined the parallax to be 0" 9128, and subsequent observations assign to the parallax the value 0" 9187. Doubtless Vl 9 fairly repre sents the most probable value. It is less than a tenth of the solar parallax ; in other words, the orbit of the earth as seen from a Centauri subtends less than a tenth of the arc which the earth s disk subtends as seen from the sun. The distance corresponding to this parallax is in round numbers about 20 billions of miles, a distance which light traverses in about 3 1/4 years.

Other stars have since so far yielded to the attacks made upon them by astronomers as to show signs of having measurable parallax. But it must be admitted that many of the results hitherto obtained are open to considerable doubt, as the following table serves sufficiently to indicate:—

Magn. Parallax formerly given by Sir J. Hwschel. Latest Measure ments of Parallax. o Centauri, 1 976 (Henderson, cor i 0-Q1 61 Cygni 6 rected by Maclear) 0-348 (Bessel) O-f.fi Lalande, 21,258.. 8 0-260 (Kruo er).. j No new deter Oeltzen, 17,415-6

0-247 (Kruger) mination. a Lyrse 1 0-155 (W. Struve, cor j " Sirius 1 rected by 0. Strave) Q 150 (Henderson, cor i I 0-7 70 Ophiuchi 5 rected by Peters) . . . 0-160 (Kruger) j i Xo new deter i Ursse Majoris ... 3 133 (Peters) mination. Arcturus 1 0-127 (Peters) Polaris. .. 2 067 (Peters) O H Capella 1 0-046 (Peters) j No new deter Procyon 1 mination. 12

If we consider that, with the exception of a Centauri, no star has been observed with accordant results, we may question whether the instruments employed by astronomers are as yet competent to measure accurately small parts of a second of arc. As for the parallax assigned to the five last stars of the above list, it would be absurd to place any reliance on the estimated value, save as indicating that those stars lie at so enormous a distance that their parallax cannot be measured.

As the most powerful telescope does not show the real disk of a star, it is not possible to determine by actual measurement the size of even the largest or the nearest. The amount of light received from a star whose distance is known can, however, be compared with that which our sun would give if removed to the star s distance ; and if we then assume equal intrinsic surface-lustre, we may infer (though not very safely) the surface, and therefore the size, of the star. Thus the distance of a Centauri exceeds the sun s 230,000 times, so that the sun, removed to the star s distance, would shine with only the 52,900,000,000th part of his observed lustre. But a Centauri shines with about the 16,950,000,000th part of the sun s brightness. Hence the star emits three times as much light as the sun, or (on our assumption) has a surface three times, a diameter ,J 3 times, and a volume 3^/3 times (or more than 5 times) as great as his. Sirius dealt with in like manner would appear to have a volume exceeding the sun s about 2700 times. But it is probable that the larger stars shine also with a greater intrinsic lustre, mile for mile of surface, and therefore are not so much larger as we should infer from the above method of reasoning.

That the stars resemble the sun iu general constitution

and condition has been proved by spectroscopic analysis,

the spectra of stars being in general respects like the spectrum of the sun. Nevertheless, characteristic differences