THE SOUTHERN CONSTELLATIONS.

Evening Star, Issue 4111, 1 May 1876, Page 4

THE SOUTHERN CONSTELLATIONS.

111. Centaubus (Continued),

DOUBLE STABS. 5. Alpha Centauri, If. A. 14hrs ! *ec. 60° 2 O' S.; magnitudes 1,2 ; position angle and distance at epoch 1876.21—p05. 43. b°, dist. 5.52"; colo. s - yellow, deeper yellow. ’ This magnificent double star, the fiuest object of the kind in the heavens, is a binary star of comparatively short period—Mr Powell makes its period oboyt 76J years, and the mean distance between the two component stars somewhat more than 20 seconds of arc. The parallax of the star is slightly less than one second. That is to say, it an astronomer on Alpha Centanri could see our Earth and the Sun when they are most favorably placed for observation, he would find the apparent distance between them to be nearly one second of arc, or about one-eighteen-hundredth part of the Moon's diameter as it appears to us. In order that a' line of a given length may subtend an angle of 1 second it must bo seen from a distance 206,265 ‘imesits own length. Taking the Sun’s mean distance its 92,000,000 miles, and multiplying this by 206,265, we get a instance of about nineteen billions of miles, or as the parallax is less than a second, we may soy that Alpha Centaur! is about twenty billions of miles from us. The mean distance between the stars adopted by Mr Powell would correspond to about twenty-two times the Earth’s distance from the Sun, or considerably more than the mean distance of Uranus from the Sun. Mr Proctor infers from Powell’s elements of the orbit of this star that the mass of the two component stars is about twice that of our own Sun. The velocity of light affords ns means of forming some faint idea of the enormous distance which intervenes between us and this “ the nearest fixed star." The light from an object placed at a distance from the Earth equal to the circumference of the Earth would come to us in less than one-eighth of a second. Light from the Moon arrives at the Earth in less than 1$ sec.; the Sun’s light reaches us in less than eight minutes, but the light of Alpha Centanri takes about three years to travel to our part of the universe. This star has a very large proper motion of considerably more than 3 seconds perannum, that is, it changes its position relative to other stars by this amount. It was this, together with the great brightness of the star, that made Professor Henderson think it not improbable that it .was nearer -to the Earth than other stars, and induced him to commence those labors which ultimately revealed to us the grand secret of the star's distance from us. Alpha Centanri stands third on the list of bright stars, Sirius coming first, and then Canopus. Sir J. Herschel found, however, that the hitter star is three times as bnght as Alpha Contain:!, and Sirius four times as bright. We have noticed a peculiarity in the colors of this star which ought perhaps to be mentioned. When the weather is very clear, the two stars * eem to be of exactly the same tint, a “straw yellow,” verging ooTyellowish. white. When, however, there is more or less haze the large star becomes golden yellow, and the smaller one brownish yellow. This is probably owing to the same cause that makes n “telescope of moderate size show the colors of stdrs better than a much larger one does: in all probability great brightness renders shades of colors less distinct, by producing such an overwhelming effect on the retina as to render it less capable of being affected by mere tints. The spectrum of Alpha Centanri is extremely like that of the sun. The principal dark lines in the spectrum are narrow, and apparently in the same position as the principal solar lines. The res- mblanco between the two spectra, indeed, is so striking that no one who saw them both in our instrument conld fail to notice the'similarity. More dispersive power, and the paeans of accurately measuring the position of

"theAe lines,' ‘might - show that this only apparent, but it would .surprise us greatly to learn that this is the case. We do not doubt that in physical constitution Alpha Centanri very closely resembles our own Sun. If is probably less developed than the Sun; for, as Mr Proctor has pointed out, its light is brighter than its mass would lead us to expect it to be, judging from the light of our Sun as compared with hi a mass. While the mass of the star is to the mass of the Sun as 2 : 1; the light of the star _is to the light of the Sun as 3:1, Now if it be true, as physicists have good grounds for believing, that the Sun is and has been very slowly but surely losing his heat, just as our earth has most certainly lost an enormous amount of hers, there mu*t have been a time when the Sun and his system were less developed, but far hotter and brighter than they are now—that is to say, there was, quite possibly, a time when the light from our Sun bore the same relation to his weight as the light from Alpha Centanri bears to its weight. Does not then the analogy of our own system lead us to infer that, as we said, matters are not in so advanced a stage in the planets of this neighboring system as they are with ns? This star has been rapidly closing of late. Three years ago the components were far apart, so much so that a power of 35 on a 3-in. telescope would separate them capitally. At present they are best seen with a power of about 100. The large size of tbe spurious disc of the laige star renders a pie tty high power necessary for neatly dividing the two stars. This star can be wed seen in the day-time in fine weather. On one occasion it was distinctly visible (both components) with anaperture reduced to one inch and a half. Unfortunately it is almost impossible to find a star in the day-time without the aid of an equatorial stand. 6. Gamma Centanri. 11. A. 12hrs. 35min. ; Dec. 48° 18' S. ; mags. 4.4., pos. 356° or 176°; dist. 2T" ; col. white white. Draw a line from Alpha Crucis through Gamma Crucis to the first largo star; this is the object named. It is difficult; owing to the size of the stars they are not quite separated by the equatorial; the position may Jje a degree or more wrong; the distance is not for out. Fine binary—ln Herschel’s time very close, f sec. Browning’s 8j inch reflector separate's these stars quite’eafily with a power of 212. 7. Q. Centanri, E.A. 13hrs. 34min. Dec 53° sb' S. Mags. G. 7. ; pos. 165°; dist. 6"; cols., white, lilac. This is to the south of Epsilon Ccntauri; it is a very neat double star.

8. Y Uentauri, RA., 14hrs. 14min., Dec., 57° 53' S , mags. 6.8. p >s. 162.5°. Dist, 9.3". Col. canary yellow, verdigris green. Just visible to the naked eye, to the N. of Alpha and Beta Ceutauri, and forming an obtuse angled isosceles triangle with them. Colors very fine. There is a smaller companion star.

9. B.A C 3907 —K.A. llhrs22min.,Dec. 42°0 S., mags. pos 167°, dist 13"; cols, white, steel grey.

A line from Beta through Gamma Crucis carried on for a considerable distance will pass through this star, which is very visible to the naked eye.

10, B. 4629H.A.13hrs40min.,Dec. 61° 30' S., mags. B.Bi, pos. 36", dist. 12", col. red and green. Yery fine colors.

South preceding Beta Centanri; hard to find,

11. B. 4763-R A., 14h om, Dec., 53° 6' P. mags. 8.9, pos. 22.5°, dist. 21.9", cols, fine red, indigo blue. 27 min. followingiEpsilon Centanri, 18min. S. This star is probably a,binary ; it is rather too small to measure with the equatorial, but both the position and the distance have almost certainly changed since Herchel’s time. It forms a parallelogram with Alpha, Beta, and Epsilon Centanri.

12. H. 4649—R A., 14h Im, Dec., 59° 8' S., mags. 9i.9^j; pos. 65°, more or less (or 245°); dist. 10" (estb: ated). Both stars are scarlet.

Sir J. Herschel suspected that this was a rapid binary. It is too small to measure properly with our instrument, but there can be no doubt that its position and distance have changed but little, if at all, during the last forty years. It is N. following Beta Ccntauri,

Note. —Considerable trouble has been taken with the measures that are given to the first or second decimal place; in other cases the measures are “rough.”