A HALF-HOUR WITH ANDROMEDA, CYGNUS, ETC.

Our last half-hour with the double stars, &c., must be a short one, as we have already nearly filled the space allotted to these objects. The observations now to be made are supposed to take place during the fourth quarter of the year,—at ten o'clock on October 23rd; or at nine on November 7th; or at eight on November 22nd; or at seven on December 6th; or at hours intermediate to these on intermediate days.

We look first, as in former cases, for the Great Bear, now lying low down towards the north. Towards the north-east, a few degrees easterly, are the twin-stars Castor and Pollux, in a vertical position, Castor uppermost. Above these, a little towards the right, we see the brilliant Capella; and between Capella and the zenith is seen the festoon of Perseus. Cassiopeia lies near the zenith, towards the north, and the Milky Way extends from the eastern horizon across the zenith to the western horizon. Low down in the east is Orion, half risen above horizon. Turning to the south, we see high up above the horizon the square of Pegasus. Low down towards the south-south-west is Fomalhaut, pointed to by β and α Pegasi. Towards the west, about half-way between the zenith and the horizon, is the noble cross in Cygnus; below which, towards the left, we see Altair, and his companions β and γ Aquilæ: while towards the right we see the brilliant Vega.

During this half-hour we shall not confine ourselves to any particular region of the heavens, but sweep the most conveniently situated constellations.

Plate V.

First, however, we should recommend the observer to try and get a good view of the great nebula in Andromeda, which is not conveniently situated for observation, but is so high that after a little trouble the observer may expect a more distinct view than in the previous quarter. He will see β Andromedæ towards the south-east, about 18° from the zenith, μ and ν nearly in a line towards the zenith, and the nebula about half-way between β and the zenith.

With a similar object it will be well to take another view of the great cluster in Perseus, about 18° from the zenith towards the east-north-east (see the pointers γ and δ Cassiopeiæ in Map 4, Frontispiece), the cluster being between δ Cassiopeiæ and α Persei.

Not very far off is the wonderful variable Algol, now due east, and about 58° above the horizon. The variability of this celebrated object was doubtless discovered in very ancient times, since the name Al-gol, or "the Demon" seems to point to a knowledge of the peculiarity of this "slowly winking eye." To Goodricke, however, is due the rediscovery of Algol's variability. The period of variation is 2d. 20h. 48m.; during 2h. 14m. Algol appears of the second magnitude; the remaining 6¾ hours are occupied by the gradual decline of the star to the fourth magnitude, and its equally gradual return to the second. It will be found easy to watch the variations of this singular object, though, of course, many of the minima are attained in the daytime. The following may help the observer:—

On October 8th, 1867, at about half-past eleven in the evening, I noticed that Algol had reached its minimum of brilliancy. Hence the next minimum was attained at about a quarter-past eight on the evening of October 11th; the next at about five on the evening of October 14th, and so on. Now, if this process be carried on, it will be found that the next evening minimum occurred at about 10h. (circiter) on the evening of October 31st, the next at about 11h. 30m. on the evening of November 20th. Thus at whatever hour any minimum occurs, another occurs six weeks and a day later, at about the same hour. This would be exact enough if the period of variation were exactly 2d. 20m. 48s., but the period is nearly a minute greater, and as there are fifteen periods in six weeks and a day, it results that there is a difference of about 13m. in the time at which the successive recurrences at nearly the same hour take place. Hence we are able to draw up the two following Tables, which will suffice to give all the minima conveniently observable during the next two years. Starting from a minimum at about 11h. 45m. on November 20th, 1867, and noticing that the next 43-day period (with the 13m. added) gives us an observation at midnight on January 2nd, 1868, and that successive periods would make the hour later yet, we take the minimum next after that of January 2nd, viz. that of January 5th, 1868, 8h. 48m., and taking 43-day periods (with 13m. added to each), we get the series—

h.m.h.m.
Jan. 5, 1868,845 P.M.Mar. 10, ——,1025 ——
Feb. 17, ——,858 ——Mar. 13, ——,743 ——[7]
Mar. 31, ——,911 ——Apr. 25, ——,756 ——
May 13, ——,924 ——June 7, ——,89 ——
June 25, ——,937 ——July 20, ——,822 ——
Aug. 7, ——,950 ——Sept. 1, ——,835 ——
Sept. 19, ——,103 ——Oct. 14, ——,848 ——
Nov. 1, ——,1016 ——Nov. 26, ——,91 ——
Dec. 14, ——,1029 ——Jan. 8, 1870,914 ——
Jan. 26, 1869,1042 ——Feb. 20, ——,927 ——

From the minimum at about 10 P.M. on October 31st, 1867, we get in like manner the series—

h.m.h.m.
Dec. 13, 1867,1013 P.M.Jan. 6, 1869,858 ——
Jan. 25, 1868,1026 ——Feb. 18, ——,911 ——
Mar. 8, ——,1039 ——Apr. 2, ——,924 ——
Apr. 20, ——,1052 ——May 15, ——,937 ——
June 2, ——,115 ——June 27, ——,950 ——
June 5, ——,753 ——[8]Aug. 9, ——,103 ——
July 18, ——,86 ——Sept. 21, ——,1016 ——
Aug. 30, ——,819 ——Nov. 3, ——,1029 ——
Oct. 12, ——,832 ——Dec. 16, ——,1042 ——
Nov. 24, ——,845 ——Jan. 28, 1870,1055 ——

From one or other of these tables every observable minimum can be obtained. Thus, suppose the observer wants to look for a minimum during the last fortnight in August, 1868. The first table gives him no information, the latter gives him a minimum at 8h. 19m. P.M. on August 30; hence of course there is a minimum at 11h. 31m. P.M. on August 27; and there are no other conveniently observable minima during the fortnight in question.

The cause of the remarkable variation in this star's brilliancy has been assigned by some astronomers to the presence of an opaque secondary, which transits Algol at regular intervals; others have adopted the view that Algol is a luminous secondary, revolving around an opaque primary. Of these views the former seems the most natural and satisfactory. It points to a secondary whose mass bears a far greater proportion to that of the primary, than the mass even of Jupiter bears to the sun; the shortness of the period is also remarkable. It may be noticed that observation points to a gradual diminution in the period of Algol's variation, and the diminution seems to be proceeding more and more rapidly. Hence (assuming the existence of a dark secondary) we must suppose that either it travels in a resisting medium which is gradually destroying its motion, or that there are other dependent orbs whose attractions affect the period of this secondary. In the latter case the decrease in the period will attain a limit and be followed by an increase.

However, interesting as the subject may be, it is a digression from telescopic work, to which we now return.

Within the confines of the second map in Plate [4] is seen the fine star γ Andromedæ. At the hour of our observations it lies high up towards E.S.E. It is seen as a double star with very moderate telescopic power, the distance between the components being upwards of 10"; their magnitudes 3 and 5½, their colours orange and green. Perhaps there is no more interesting double visible with low powers. The smaller star is again double in first-class telescopes, the components being yellow and blue according to some observers, but according to others, both green.

Below γ Andromedæ lie the stars β and γ Triangulorum, γ a fine naked-eye triple (the companions being δ and η Triangulorum), a fine object with a very low power. To the right is α Triangulorum, certainly less brilliant than β. Below α are the three stars α, β, and γ Arietis, the first an unequal and difficult double, the companion being purple, and only just visible (under favourable circumstances) with a good 3-inch telescope; the last an easy double, interesting as being the first ever discovered (by Hook, in 1664), the colours of components white and grey.

Immediately below α Arietis is the star α Ceti, towards the right of which (a little lower) is Mira, a wonderful variable. This star has a period of 331⅓ days; during a fortnight it appears as a star of the 2nd magnitude,—on each side of this fortnight there is a period of three months during one of which the star is increasing, while during the other it is diminishing in brightness: during the remaining five months of the period the star is invisible to the naked eye. There are many peculiarities and changes in the variation of this star, into which space will not permit me to enter.

Immediately above Mira is the star α Piscium at the knot of the Fishes' connecting band. This is a fine double, the distance between the components being about 3½", their magnitudes 5 and 6, their colours pale green and blue (see Plate [5]).

Close to γ Aquarii (see Frontispiece, Map 4), above and to the left of it, is the interesting double ζ Aquarii; the distance between the components is about 3½", their magnitudes 4 and 4½, both whitish yellow. The period of this binary seems to be about 750 years.

Turning next towards the south-west we see the second-magnitude star ε Pegasi, some 40° above the horizon. This star is a wide but not easy double, the secondary being only of the ninth magnitude; its colour is lilac, that of the primary being yellow.

Towards the right of ε Pegasi and lower down are seen the three fourth-magnitude stars which mark the constellation Equuleus. Of these the lowest is α, to the right of which lies ε Equulei, a fifth-magnitude star, really triple, but seen as a double star with ordinary telescopes (Plate [5]). The distance between the components is nearly 11", their colours white and blue, their magnitudes 5½ and 7½. The primary is a very close double, which appears, however, to be opening out rather rapidly.

Immediately below Equuleus are the stars α1 and α^2 Capricorni, seen as a naked-eye double to the right of and above β. Both α^1 and α^2 are yellow; α^2 is of the 3rd, α^1 of the 4th magnitude; in a good telescope five stars are seen, the other three being blue, ash-coloured, and lilac. The star β Capricorni is also a wide double, the components yellow and blue, with many telescopic companions.

To the right of Equuleus, towards the west-south-west is the constellation Delphinus. The upper left-hand star of the rhombus of stars forming the head of the Delphinus is the star γ Delphini, a rather easy double (see Plate [5]), the components being nearly 12" apart, their magnitudes 4 and 7, their colours golden yellow and flushed grey.

Turn we next to the charming double Albireo, on the beak of Cygnus, about 36° above the horizon towards the west. The components are 34½" apart, their magnitudes 3 and 6, their colours orange-yellow, and blue. It has been supposed (perhaps on insufficient evidence) that this star is merely an optical double. It must always be remembered that a certain proportion of stars (amongst those separated by so considerable a distance) must be optically combined only.

The star χ Cygni is a wide double (variable) star. The components are separated by nearly 26", their magnitudes 5 and 9, their colours yellow and light blue. χ may be found by noticing that there is a cluster of small stars in the middle of the triangle formed by the stars γ, δ, and β Cygni (see Map 4, Frontispiece), and that χ is the nearest star of the cluster to β. The star φ Cygni, which is just above and very close to β (Albireo), does not belong to the cluster. χ is about half as far again from φ as φ from Albireo. But as χ descends to the 11th magnitude at its minimum the observer must not always expect to find it very easily. It has been known to be invisible at the epoch when it should have been most conspicuous. The period of this variable is 406 days.

The star 61 Cygni is an interesting one. So far as observation has yet extended, it would seem to be the nearest to us of all stars visible in the northern hemisphere. It is a fine double, the components nearly equal (5½ and 6), both yellow, and nearly 19" apart. The period of this binary appears to be about 540 years. To find 61 Cygni note that ε and δ Cygni form the diameter of a semicircle divided into two quadrants by α Cygni (Arided). On this semicircle, on either side of α, lie the stars ν and α Cygni, ν towards ε. Now a line from α to ν produced passes very near to 61 Cygni at a distance from ν somewhat greater than half the distance of ν from α.

The star μ Cygni lies in a corner of the constellation, rather farther from ζ than ζ from ε Cygni. A line from ε to ζ produced meets κ Pegasi, a fourth-magnitude star; and μ Cygni, a fifth-magnitude star, lies close above κ Pegasi. The distance between the components is about 5½", their magnitudes 5 and 6, their colours white and pale blue.

The star ψ Cygni may next be looked for, but for this a good map of Cygnus will be wanted, as ψ is not pointed to by any well-marked stars. A line from α, parallel to the line joining γ and δ, and about one-third longer than that line, would about mark the position of ψ Cygni. The distance between the components of this double is about 3½", their magnitudes 5½ and 8, their colours white and lilac.

Lastly, the observer may turn to the stars γ1 and γ2 Draconis towards the north-west about 40° above the horizon (they are included in the second map of Plate [2]). They form a wide double, having equal (fifth-magnitude) components, both grey. (See Plate [5].)

CHAPTER VI.