A HALF-HOUR WITH ORION, LEPUS TAURUS, ETC.
Any of the half-hours here assigned to the constellation-seasons may be taken first, and the rest in seasonal or cyclic order. The following introductory remarks are applicable to each:—
If we stand on an open space, on any clear night, we see above us the celestial dome spangled with stars, apparently fixed in position. But after a little time it becomes clear that these orbs are slowly shifting their position. Those near the eastern horizon are rising, those near the western setting. Careful and continuous observation would show that the stars are all moving in the same way, precisely, as they would if they were fixed to the concave surface of a vast hollow sphere, and this sphere rotated about an axis. This axis, in our latitude, is inclined about 51½° to the horizon. Of course only one end of this imaginary axis can be above our horizon. This end lies very near a star which it will be well for us to become acquainted with at the beginning of our operations. It lies almost exactly towards the north, and is raised from 50° to 53° (according to the season and hour) above the horizon. There is an easy method of finding it.
We must first find the Greater Bear. It will be seen from Plate [1], that on a spring evening the seven conspicuous stars of this constellation are to be looked for towards the north-east, about half way between the horizon and the point overhead (or zenith), the length of the set of stars being vertical. On a summer's evening the Great Bear is nearly overhead. On an autumn evening he is towards the north-west, the length of the set of seven being somewhat inclined to the horizon. Finally, on a winter's evening, he is low down towards the north, the length of the set of seven stars being nearly in a horizontal direction.
Having found the seven stars, we make use of the pointers α and β (shown in Plate [1]) to indicate the place of the Pole-star, whose distance from the pointer α is rather more than three times the distance of α from β.
Now stand facing the Pole-star. Then all the stars are travelling round that star in a direction contrary to that in which the hands of a watch move. Thus the stars below the pole are moving towards the right, those above the pole towards the left, those to the right of the pole upwards, those to the left of the pole downwards.
Next face the south. Then all the stars on our left, that is, towards the east, are rising slantingly towards the south; those due south are moving horizontally to the right, that is, towards the west; and those on our right are passing slantingly downwards towards the west.
It is important to familiarise ourselves with these motions, because it is through them that objects pass out of the field of view of the telescope, and by moving the tube in a proper direction we can easily pick up an object that has thus passed away, whereas if we are not familiar with the varying motions in different parts of the celestial sphere, we may fail in the attempt to immediately recover an object, and waste time in the search for it.
The consideration of the celestial motions shows how advantageous it is, when using an alt-azimuth, to observe objects as nearly as possible due south. Of course in many cases this is impracticable, because a phenomenon we wish to watch may occur when an object is not situated near the meridian. But in examining double stars there is in general no reason for selecting objects inconveniently situated. We can wait till they come round to the meridian, and then observe them more comfortably. Besides, most objects are higher, and therefore better seen, when due south.
Northern objects, and especially those within the circle of perpetual apparition, often culminate (that is, cross the meridian, or north and south line) at too great a height for comfortable vision. In this case we should observe them towards the east or west, and remember that in the first case they are rising, and in the latter they are setting, and that in both cases they have also a motion from left to right.
If we allow an object to pass right across the field of view (the telescope being fixed), the apparent direction of its motion is the exact reverse of the true direction of the star's motion. This will serve as a guide in shifting the alt-azimuth after a star has passed out of the field of view.
The following technical terms must be explained. That part of the field of view towards which the star appears to move is called the preceding part of the field, the opposite being termed the following part. The motion for all stars, except those lying in an oval space extending from the zenith to the pole of the heavens, is more or less from right to left (in the inverted field). Now, if we suppose a star to move along a diameter of the field so as to divide the field into two semicircles, then in all cases in which this motion takes places from right to left, that semicircle which contains the lowest point (apparently) of the field is the northern half, the other is the southern half. Over the oval space just mentioned the reverse holds.
Thus the field is divided into four quadrants, and these are termed north following (n.f.) and south following (s.f.); north preceding (n.p.), and south preceding (s.p.). The student can have no difficulty in interpreting these terms, since he knows which is the following and which the preceding semicircle, which the northern and which the southern. In the figures of plates [3] and [5], the letters n.f., n.p., &c., are affixed to the proper quadrants. It is to be remembered that the quadrants thus indicated are measured either way from the point and feather of the diametral arrows.
Next, of the apparent annual motion of the stars. This takes place in exactly the same manner as the daily motion. If we view the sky at eight o'clock on any day, and again at the same hour one month later, we shall find that at the latter observation (as compared with the former) the heavens appear to have rotated by the twelfth part of a complete circumference, and the appearance presented is precisely the same as we should have observed had we waited for two hours (the twelfth part of a day) on the day of the first observation.
Our survey of the heavens is supposed to be commenced during the first quarter of the year, at ten o'clock on the 20th of January, or at nine on the 5th of February, or at eight on the 19th of February, or at seven on the 6th of March, or at hours intermediate to these on intermediate days.
We look first for the Great Bear towards the north-east, as already described, and thence find the Pole-star; turning towards which we see, towards the right and downwards, the two guardians of the pole (β and γ Ursæ Minoris). Immediately under the Pole-star is the Dragon's Head, a conspicuous diamond of stars. Just on the horizon is Vega, scintillating brilliantly. Overhead is the brilliant Capella, near which the Milky Way is seen passing down to the horizon on either side towards the quarters S.S.E. and N.N.W.
For the present our business is with the southern heavens, however.
Facing the south, we see a brilliant array of stars, Sirius unmistakeably overshining the rest. Orion is shining in full glory, his leading brilliant, Betelgeuse[2] being almost exactly on the meridian, and also almost exactly half way between the horizon and the zenith. In Plate [2] is given a map of this constellation and its neighbourhood.
Let us first turn the tube on Sirius. It is easy to get him in the field without the aid of a finder. The search will serve to illustrate a method often useful when a telescope has no finder. Having taking out the eye-piece—a low-power one, suppose—direct the tube nearly towards Sirius. On looking through it, a glare of light will be seen within the tube. Now, if the tube be slightly moved about, the light will be seen to wax and wane, according as the tube is more or less accurately directed. Following these indications, it will be found easy to direct the tube, so that the object-glass shall appear full of light. When this is done, insert the eye-piece, and the star will be seen in the field.
But the telescope is out of focus, therefore we must turn the small focussing screw. Observe the charming chromatic changes—green, and red, and blue light, purer than the hues of the rainbow, scintillating and coruscating with wonderful brilliancy. As we get the focus, the excursions of these light flashes diminish until—if the weather is favourable—the star is seen, still scintillating, and much brighter than to the naked eye, but reduced to a small disc of light, surrounded (in the case of so bright a star as Sirius) with a slight glare. If after obtaining the focus the focussing rack work be still turned, we see a coruscating image as before. In the case of a very brilliant star these coruscations are so charming that we may be excused for calling the observer's attention to them. The subject is not without interest and difficulty as an optical one. But the astronomer's object is to get rid of all these flames and sprays of coloured light, so that he has very little sympathy with the admiration which Wordsworth is said to have expressed for out-of-focus views of the stars.
We pass to more legitimate observations, noticing in passing that Sirius is a double star, the companion being of the tenth magnitude, and distant about ten seconds from the primary. But our beginner is not likely to see the companion, which is a very difficult object, vowing to the overpowering brilliancy of the primary.
Orion affords the observer a splendid field of research. It is a constellation rich in double and multiple stars, clusters, and nebulæ. We will begin with an easy object.
The star δ (Plate [3]), or Mintaka, the uppermost of the three stars forming the belt, is a wide double. The primary is of the second magnitude, the secondary of the seventh, both being white.
The star α (Betelgeuse) is an interesting object, on account of its colour and brilliance, and as one of the most remarkable variables in the heavens. It was first observed to be variable by Sir John Herschel in 1836. At this period its variations were "most marked and striking." This continued until 1840, when the changes became "much less conspicuous. In January, 1849, they had recommenced, and on December 5th, 1852, Mr. Fletcher observed α Orionis brighter than Capella, and actually the largest star in the northern hemisphere." That a star so conspicuous, and presumably so large, should present such remarkable variations, is a circumstance which adds an additional interest to the results which have rewarded the spectrum-analysis of this star by Mr. Huggins and Professor Miller. It appears that there is decisive evidence of the presence in this luminary of many elements known to exist in our own sun; amongst others are found sodium, magnesium, calcium, iron, and bismuth. Hydrogen appears to be absent, or, more correctly, there are no lines in the star's spectrum corresponding to those of hydrogen in the solar spectrum. Secchi considers that there is evidence of an actual change in the spectrum of the star, an opinion in which Mr. Huggins does not coincide. In the telescope Betelgeuse appears as "a rich and brilliant gem," says Lassell, "a rich topaz, in hue and brilliancy differing from any that I have seen."
Turn next to β (Rigel), the brightest star below the belt. This is a very noted double, and will severely test our observer's telescope, if small. The components are well separated (see Plate [3]), compared with many easier doubles; the secondary is also of the seventh magnitude, so that neither as respects closeness nor smallness of the secondary, is Rigel a difficult object. It is the combination of the two features which makes it a test-object. Kitchener says a 1¾-inch object-glass should show Rigel double; in earlier editions of his work he gave 2¾-inches as the necessary aperture. Smyth mentions Rigel as a test for a 4-inch aperture, with powers of from 80 to 120. A 3-inch aperture, however, will certainly show the companion. Rigel is an orange star, the companion blue.
Turn next to λ the northernmost of the set of three stars in the head of Orion. This is a triple star, though an aperture of 3 inches will show it only as a double. The components are 5" apart, the colours pale white and violet. With the full powers of a 3½-inch glass a faint companion may be seen above λ.
The star ζ, the lowest in the belt, may be tried with a 3½-inch glass. It is a close double, the components being nearly equal, and about 2½" apart (see Plate [3]).
For a change we will now try our telescope on a nebula, selecting the great nebula in the Sword. The place of this object is indicated in Plate [2]. There can be no difficulty in finding it since it is clearly visible to the naked eye on a moonless night—the only sort of night on which an observer would care to look at nebulæ. A low power should be employed.
The nebula is shown in Plate [3] as I have seen it with a 3-inch aperture. We see nothing of those complex streams of light which are portrayed in the drawings of Herschel, Bond, and Lassell, but enough to excite our interest and wonder. What is this marvellous light-cloud? One could almost imagine that there was a strange prophetic meaning in the words which have been translated "Canst thou loose the bands of Orion?" Telescope after telescope had been turned on this wonderful object with the hope of resolving its light into stars. But it proved intractable to Herschel's great reflector, to Lassell's 2-feet reflector, to Lord Rosse's 3-feet reflector, and even partially to the great 6-feet reflector. Then we hear of its supposed resolution into stars, Lord Rosse himself writing to Professor Nichol, in 1846, "I may safely say there can be little, if any, doubt as to the resolvability of the nebula;—all about the trapezium is a mass of stars, the rest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability strongly marked."
It was decided, therefore, that assuredly the great nebula is a congeries of stars, and not a mass of nebulous matter as had been surmised by Sir W. Herschel. And therefore astronomers were not a little surprised when it was proved by Mr. Huggins' spectrum-analysis that the nebula consists of gaseous matter. How widely extended this gaseous universe may be we cannot say. The general opinion is that the nebulæ are removed far beyond the fixed stars. If this were so, the dimensions of the Orion nebula would be indeed enormous, far larger probably than those of the whole system whereof our sun is a member. I believe this view is founded on insufficient evidence, but this would not be the place to discuss the subject. I shall merely point out that the nebula occurs in a region rich in stars, and if it is not, like the great nebula in Argo, clustered around a remarkable star, it is found associated in a manner which I cannot look upon as accidental with a set of small-magnitude stars, and notably with the trapezium which surrounds that very remarkable black gap within the nebula. The fact that the nebula shares the proper motion of the trapezium appears inexplicable if the nebula is really far out in space beyond the trapezium. A very small proper motion of the trapezium (alone) would long since have destroyed the remarkable agreement in the position of the dark gap and the trapezium which has been noticed for so many years.
But whether belonging to our system or far beyond it, the great nebula must have enormous dimensions. A vast gaseous system it is, sustained by what arrangements or forces we cannot tell, nor can we know what purposes it subserves. Mr. Huggins' discovery that comets have gaseous nuclei, (so far as the two he has yet examined show) may suggest the speculation that in the Orion nebula we see a vast system of comets travelling in extensive orbits around nuclear stars, and so slowly as to exhibit for long intervals of time an unchanged figure. "But of such speculations" we may say with Sir J. Herschel "there is no end."
To return to our telescopic observations:—The trapezium affords a useful test for the light-gathering power of the telescope. Large instruments exhibit nine stars. But our observer may be well satisfied with his instrument and his eye-sight if he can see five with a 3½-inch aperture.[3] A good 3-inch glass shows four distinctly. But with smaller apertures only three are visible.
The whole neighbourhood of the great nebula will well repay research. The observer may sweep over it carefully on any dark night with profit. Above the nebula is the star-cluster 362 H. The star ι (double as shown in Plate [3]) below the nebula is involved in a strong nebulosity. And in searching over this region we meet with delicate double, triple, and multiple stars, which make the survey interesting with almost any power that may be applied.
Above the nebula is the star σ, a multiple star. To an observer with a good 3½-inch glass σ appears as an octuple star. It is well seen, however, as a fine multiple star with a smaller aperture. Some of the stars of this group appear to be variable.
The star ρ Orionis is an unequal, easy double, the components being separated by nearly seven seconds. The primary is orange, the smaller star smalt-blue (see Plate [3]).
The middle star of the belt (ε) has a distant blue companion. This star, like ι, is nebulous. In fact, the whole region within the triangle formed by stars γ, κ and β is full of nebulous double and multiple stars, whose aggregation in this region I do not consider wholly accidental.
We have not explored half the wealth of Orion, but leave much for future observation. We must turn, however, to other constellations.
Below Orion is Lepus, the Hare, a small constellation containing some remarkable doubles. Among these we may note ξ, a white star with a scarlet companion; γ, a yellow and garnet double; and ι, a double star, white and pale violet, with a distant red companion. The star κ Leporis is a rather close double, white with a small green companion. The intensely red star R Leporis (a variable) will be found in the position indicated in the map.
Still keeping within the boundary of our map, we may next turn to the fine cluster 2 H (vii.) in Monoceros. This cluster is visible to the naked eye, and will be easily found. The nebula 2 H (iv.) is a remarkable one with a powerful telescope.
The star 11 Monocerotis is a fine triple star described by the elder Herschel as one of the finest sights in the heavens. Our observer, however, will see it as a double (see Plate [3]). δ Monocerotis is an easy double, yellow and lavender.
We may now leave the region covered by the map and take a survey of the heavens for some objects well seen at this season.
Towards the south-east, high up above the horizon, we see the twin-stars Castor and Pollux. The upper is Castor, the finest double star visible in the northern heavens. The components are nearly equal and rather more than 5" apart (see Plate [3]). Both are white according to the best observers, but the smaller is thought by some to be slightly greenish.
Pollux is a coarse but fine triple star (in large instruments multiple). The components orange, grey, and lilac.
There are many other fine objects in Gemini, but we pass to Cancer.
The fine cluster Præsepe in Cancer may easily be found as it is distinctly visible to the naked eye in the position shown in Plate [1], Map I. In the telescope it is seen as shown in Plate [3].
The star ι Cancri is a wide double, the colours orange and blue.
Procyon, the first-magnitude star between Præsepe and Sirius, is finely coloured—yellow with a distant orange companion, which appears to be variable.
Below the Twins, almost in a line with them, is the star α Hydræ, called Al Fard, or "the Solitary One." It is a 2nd magnitude variable. I mention it, however, not on its own account, but as a guide to the fine double ε Hydræ. This star is the middle one of a group of three, lying between Pollux and Al Fard rather nearer the latter. The components of ε Hydræ are separated by about 3½" (see Plate [3]). The primary is of the fourth, the companion of the eighth magnitude; the former is yellow, the latter a ruddy purple. The period of ε Hydræ is about 450 years.
The constellation Leo Minor, now due east and about midway between the horizon and the zenith, is well worth sweeping over. It contains several fine fields.
Let us next turn to the western heavens. Here there are some noteworthy objects.
To begin with, there are the Pleiades, showing to the naked eye only six or seven stars. In the telescope the Pleiades appear as shown in Plate [3].
The Hyades also show some fine fields with low powers.
Aldebaran, the principal star of the Hyades, as also of the constellation Taurus, is a noted red star. It is chiefly remarkable for the close spectroscopic analysis to which it has been subjected by Messrs. Huggins and Miller. Unlike Betelgeuse, the spectrum of Aldebaran exhibits the lines corresponding to hydrogen, and no less than eight metals—sodium, magnesium, calcium, iron, bismuth, tellurium, antimony, and mercury, are proved to exist in the constitution of this brilliant red star.
On the right of Aldebaran, in the position indicated in Plate [1], Map I., are the stars ζ and β Tauri. If with a low power the observer sweep from ζ towards β, he will soon find—not far from ζ (at a distance of about one-sixth of the distance separating β from ζ), the celebrated Crab nebula, known as 1 M. This was the first nebula discovered by Messier, and its discovery led to the formation of his catalogue of 103 nebulæ. In a small telescope this object appears as a nebulous light of oval form, no traces being seen of the wisps and sprays of light presented in Lord Rosse's well known picture of the nebula.
Here I shall conclude the labours of our first half-hour among the stars, noticing that the examination of Plate [1] will show what other constellations besides those here considered are well situated for observation at this season. It will be remarked that many constellations well seen in the third half-hour (Chapter [IV.]) are favourably seen in the first also, and vice versâ. For instance, the constellation Ursa Major well-placed towards the north-east in the first quarter of the year, is equally well-placed towards the north-west in the third, and similarly of the constellation Cassiopeia. The same relation connects the second and fourth quarters of the year.
