A HALF-HOUR WITH LYRA, HERCULES, CORVUS, CRATER, ETC.
The observations now to be commenced are supposed to take place during the second quarter of the year,—at ten o'clock on the 20th of April, or at nine on the 5th of May, or at eight on the 21st of May, or at seven on the 5th of June, or at hours intermediate to these on intermediate days.
We again look first for the Great Bear, now near the zenith, and thence find the Pole-star. Turning towards the north, we see Cassiopeia between the Pole-star and the horizon. Towards the north-west is the brilliant Capella, and towards the north-east the equally brilliant Vega, beneath which, and somewhat northerly, is the cross in Cygnus. The Milky Way passes from the eastern horizon towards the north (low down), and so round to the western horizon.
In selecting a region for special observation, we shall adopt a different plan from that used in the preceding "half-hour." The region on the equator and towards the south is indeed particularly interesting, since it includes the nebular region in Virgo. Within this space nebulæ are clustered more closely than over any corresponding space in the heavens, save only the greater Magellanic cloud. But to the observer with telescopes of moderate power these nebulæ present few features of special interest; and there are regions of the sky now well situated for observation, which, at most other epochs are either low down towards the horizon or inconveniently near to the zenith. We shall therefore select one of these, the region included in the second map of Plate [2], and the neighbouring part of the celestial sphere.
At any of the hours above named, the constellation Hercules lies towards the east. A quadrant taken from the zenith to the eastern horizon passes close to the last star (η) of the Great Bear's tail, through β, a star in Bootes' head, near β Herculis, between the two "Alphas" which mark the heads of Hercules and Ophiuchus, and so past β Ophiuchi, a third-magnitude star near the horizon. And here we may turn aside for a moment to notice the remarkable vertical row of six conspicuous stars towards the east-south-east; these are, counting them in order from the horizon, ζ, ε, and δ Ophiuchi, ε, α, and δ Serpentis.
Let the telescope first be directed towards Vega. This orb presents a brilliant appearance in the telescope. Its colour is a bluish-white. In an ordinary telescope Vega appears as a single star, but with a large object-glass two distant small companions are seen. A nine-inch glass shows also two small companions within a few seconds of Vega. In the great Harvard refractor Vega is seen with no less than thirty-five companions. I imagine that all these stars, and others which can be seen in neighbouring fields, indicate the association of Vega with the neighbouring stream of the Milky Way.
Let our observer now direct his telescope to the star ε Lyræ. Or rather, let him first closely examine this star with the naked eye. The star is easily identified, since it lies to the left of Vega, forming with ζ a small equilateral triangle. A careful scrutiny suffices to indicate a peculiarity in this star. If our observer possesses very good eye-sight, he will distinctly recognise it as a "naked-eye double"; but more probably he will only notice that it appears lengthened in a north and south direction.[4] In the finder the star is easily divided. Applying a low power to the telescope itself, we see ε Lyræ as a wide double, the line joining the components lying nearly north and south. The southernmost component (the upper in the figure) is called ε1, the other ε2. Seen as a double, both components appear white.
Now, if the observer's telescope is sufficiently powerful, each of the components may be seen to be itself double. First try ε1, the northern pair. The line joining the components is directed as shown in Plate [3]. The distance between them is 3"·2, their magnitudes 5 and 6½, and their colours yellow and ruddy. If the observer succeeds in seeing ε1 fairly divided, he will probably not fail in detecting the duplicity of ε2, though this is a rather closer pair, the distance between the components being only 2"·6. The magnitudes are 5 and 5½, both being white. Between ε1 and ε2 are three faint stars, possibly forming with the quadruple a single system.
Let us next turn to the third star of the equilateral triangle mentioned above—viz. to the star ζ Lyræ. This is a splendid but easy double. It is figured in Plate [3], but it must be noticed that the figure of ζ and the other nine small figures are not drawn on the same scale as ε Lyræ. The actual distance between the components of ζ Lyra is 44", or about one-fourth of the distance separating ε1 from ε2. The components of ζ are very nearly equal in magnitude, in colour topaz and green, the topaz component being estimated as of the fifth magnitude, the green component intermediate between the fifth and sixth magnitudes.
We may now turn to a star not figured in the map, but readily found. It will be noticed that the stars ε, α, β, and γ form, with two small stars towards the left, a somewhat regular hexagonal figure—a hexagon, in fact, having three equal long sides and three nearly equal short sides alternating with the others. The star η Lyræ forms the angle next to ε. It is a wide and unequal double, as figured in Plate [3]. The larger component is azure blue; the smaller is violet, and, being only of the ninth magnitude, is somewhat difficult to catch with apertures under 3 inches.
The star δ2 Lyræ is orange, δ1 blue. In the same field with these are seen many other stars.
The stars γ1 and γ2 may also be seen in a single field, the distance between them being about half the moon's mean diameter. The former is quadruple, the components being yellow, bluish, pale blue, and blue.
Turn next to the stars β and γ Lyræ, the former a multiple, the latter an unequal double star. It is not, however, in these respects that these stars are chiefly interesting, but for their variability. The variability of γ has not indeed been fully established, though it is certain that, having once been less bright, γ is now considerably brighter than β. The change, however, may be due to the variation of β alone. This star is one of the most remarkable variables known. Its period is 12d. 21h. 53m. 10s. In this time it passes from a maximum brilliancy—that of a star of the 3·4 magnitude—to a minimum lustre equal to that of a star of the 4·3 magnitude, thence to the same maximum brilliancy as before, thence to another minimum of lustre—that of a star of the 4·5 magnitude—and so to its maximum lustre again, when the cycle of changes recommences. These remarkable changes seem to point to the existence of two unequal dark satellites, whose dimensions bear a much greater proportion to those of the bright components of β Lyræ than the dimensions of the members of the Solar System bear to those of the sun. In this case, at any rate, the conjecture hazarded about Algol, that the star revolves around a dark central orb, would be insufficient to account for the observed variation.
Nearly midway between β and γ lies the wonderful ring-nebula 57 M, of which an imperfect idea will be conveyed by the last figure of Plate [3]. This nebula was discovered in 1772, by Darquier, at Toulouse. It is seen as a ring of light with very moderate telescopic power. In a good 3½-inch telescope the nebula exhibits a mottled appearance and a sparkling light. Larger instruments exhibit a faint light within the ring; and in Lord Rosse's great Telescope "wisps of stars" are seen within, and faint streaks of light stream from the outer border of the ring. This nebula has been subjected to spectrum-analysis by Mr. Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulæ—of which only seven have been detected—seem to belong to the same class as the planetary nebulæ, all of which exhibit the line-spectrum indicative of gaseity. The brightest of the three lines seen in the spectrum of the ring-nebula in Lyra presents a rather peculiar appearance, "since it consists," says Mr. Huggins, "of two bright dots, corresponding to sections of the ring, and between these there is not darkness, but an excessively faint line joining them. This observation makes it probable that the faint nebulous matter occupying the central portion is similar in constitution to that of the ring."
The constellation Hercules also contains many very interesting objects. Let us first inspect a nebula presenting a remarkable contrast with that just described. I refer to the nebula 13 M, known as Halley's nebula (Plate [3]). This nebula is visible to the naked eye, and in a good telescope it is a most wonderful object: "perhaps no one ever saw it for the first time without uttering a shout of wonder." It requires a very powerful telescope completely to resolve this fine nebula, but the outlying streamers may be resolved with a good 3-inch telescope. Sir W. Herschel considered that the number of the stars composing this wonderful object was at least 14,000. The accepted views respecting nebulæ would place this and other clusters far beyond the limits of our sidereal system, and would make the component stars not very unequal (on the average) to our own sun. It seems to me far more probable, on the contrary, that the cluster belongs to our own system, and that its components are very much smaller than the average of separate stars. Perhaps the whole mass of the cluster does not exceed that of an average first-magnitude star.
The nebulæ 92 M and 50 H may be found, after a little searching, from the positions indicated in the map. They are both well worthy of study, the former being a very bright globular cluster, the latter a bright and large round nebula. The spectra of these, as of the great cluster, resemble the solar spectrum, being continuous, though, of course, very much fainter.
The star δ Herculis (seen at the bottom of the map) is a wide and easy double—a beautiful object. The components, situated as shown in Plate [3], are of the fourth and eighth magnitude, and coloured respectively greenish-white and grape-red.
The star κ Herculis is not shown in the map, but may be very readily found, lying between the two gammas, γ Herculis and γ Serpentis (see Frontispiece, Map 2), rather nearer the latter. It is a wide double, the components of fifth and seventh magnitude, the larger yellowish-white, the smaller ruddy yellow.[5]
Ras Algethi, or α Herculis, is also beyond the limits of the map, but may be easily found by means of Map 2, Frontispiece. It is, properly speaking, a multiple star. Considered as a double, the arrangement of the components is that shown in Plate [3]. The larger is of magnitude 3½, the smaller of magnitude 5½; the former orange, the latter emerald. The companion stars are small, and require a good telescope to be well seen. Ras Algethi is a variable, changing from magnitude 3 to magnitude 3½ in a period of 66⅓ days.
The star ρ Herculis is a closer double. The components are 3"·7 apart, and situated as shown in Plate [3]. The larger is of magnitude 4, the smaller 5½; the former bluish-white, the latter pale emerald.
There are other objects within the range of our map which are well worthy of study. Such are μ Draconis, a beautiful miniature of Castor; γ1 and γ2 Draconis, a wide double, the distance between the components being nearly 62" (both grey); and γ1 and γ2 Coronæ, a naked-eye double, the components being 6' apart, and each double with a good 3-inch telescope.
We turn, however, to another region of the sky. Low down, towards the south is seen the small constellation Corvus, recognised by its irregular quadrilateral of stars. Of the two upper stars, the left-hand one is Algorab, a wide double, the components placed as in Plate [3], 23"·5 apart, the larger of magnitude 3, the smaller 8½, the colours pale yellow and purple.
There is a red star in this neighbourhood which is well worth looking for. To the right of Corvus is the constellation Crater, easily recognised as forming a tolerably well-marked small group. The star Alkes, or α Crateris, must first be found. It is far from being the brightest star in the constellation, and may be assumed to have diminished considerably in brilliancy since it was entitled α by Bayer. It will easily be found, however, by means of the observer's star maps. If now the telescope be directed to Alkes, there will be found, following him at a distance of 42·5 s, and about one minute southerly, a small red star, R. Crateris. Like most red stars, this one is a variable. A somewhat smaller blue star may be seen in the same field.
There is another red star which may be found pretty easily at this season. First find the stars η and ο Leonis, the former forming with Regulus (now lying towards the south-west, and almost exactly midway between the zenith and the horizon) the handle of the Sickle in Leo, the other farther off from Regulus towards the right, but lower down. Now sweep from ο towards η with a low power.[6] There will be found a sixth-magnitude star about one-fourth of the way from ο to η. South, following this, will be found a group of four stars, of which one is crimson. This is the star R Leonis. Like R Crateris and R Leporis it is variable.
Next, let the observer turn towards the south again. Above Corvus, in the position shown in the Frontispiece, there are to be seen five stars, forming a sort of wide V with somewhat bowed legs. At the angle is the star γ Virginis, a noted double. In 1756 the components were 6½ seconds apart. They gradually approached till, in 1836, they could not be separated by the largest telescopes. Since then they have been separating, and they are now 4½ seconds apart, situated as shown in Plate [3]. They are nearly equal in magnitude (4), and both pale yellow.
The star γ Leonis is a closer and more beautiful double. It will be found above Regulus, and is the brightest star on the blade of the Sickle. The components are separated by about 3⅕ seconds, the larger of the second, the smaller of the fourth magnitude; the former yellow-orange, the latter greenish-yellow.
Lastly, the star ι Leonis may be tried. It will be a pretty severe test for our observer's telescope, the components being only 2"·4 apart, and the smaller scarcely exceeding the eighth magnitude. The brighter (fourth magnitude) is pale yellow, the other light blue.