Half-hours with the Telescope / Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction.

by Richard A. Proctor

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.