A HALF-HOUR WITH BOOTES, SCORPIO, OPHIUCHUS, ETC.
We now commence a series of observations suited to the third quarter of the year, and to the following hours:—Ten o'clock on the 22nd of July; nine on the 8th of August; eight on the 23rd of August; seven on the 8th of October; and intermediate hours on days intermediate to these.
We look first for the Great Bear towards the north-west, and thence find the Pole-star. Turning towards the north we see Capella and β Aurigæ low down and slightly towards the left of the exact north point. The Milky Way crosses the horizon towards the north-north-east and passes to the opposite point of the compass, attaining its highest point above the horizon towards east-south-east. This part of the Milky Way is well worth observing, being marked by singular variations of brilliancy. Near Arided (the principal star of Cygnus, and now lying due east—some twenty-five degrees from the zenith) there is seen a straight dark rift, and near this space is another larger cavity, which has been termed the northern Coal-sack. The space between γ, δ, and β Cygni is covered by a large oval mass, exceedingly rich and brilliant. The neighbouring branch, extending from ε Cygni, is far less conspicuous here, but near Sagitta becomes brighter than the other, which in this neighbourhood suddenly loses its brilliancy and fading gradually beyond this point becomes invisible near β Ophiuchi. The continuous stream becomes patchy—in parts very brilliant—where it crosses Aquila and Clypeus. In this neighbourhood the other stream reappears, passing over a region very rich in stars. We see now the greatest extent of the Milky Way, towards this part of its length, ever visible in our latitudes—just as in spring we see its greatest extent towards Monoceros and Argo.
I may note here in passing that Sir John Herschel's delineation of the northern portion of the Milky Way, though a great improvement on the views given in former works, seems to require revision, and especially as respects the very remarkable patches and streaks which characterise the portion extending over Cepheus and Cygnus. It seems to me, also, that the evidence on which it has been urged that the stars composing the Milky Way are (on an average) comparable in magnitude to our own sun, or to stars of the leading magnitudes, is imperfect. I believe, for instance, that the brilliant oval of milky light in Cygnus comes from stars intimately associated with the leading stars in that constellation, and not far removed in space (proportionately) beyond them. Of course, if this be the case, the stars, whose combined light forms the patch of milky light, must be far smaller than the leading brilliants of Cygnus. However, this is not the place to enter on speculations of this sort; I return therefore to the business we have more immediately in hand.
Towards the east is the square of Pegasus low down towards the horizon. Towards the south is Scorpio, distinguished by the red and brilliant Antares, and by a train of conspicuous stars. Towards the west is Bootes, his leading brilliant—the ruddy Arcturus—lying somewhat nearer the horizon than the zenith, and slightly south of west. Bootes as a constellation is easily found if we remember that he is delineated as chasing away the Greater Bear. Thus at present he is seen in a slightly inclined position, his head (marked by the third-magnitude star β) lying due west, some thirty degrees from the zenith. It has always appeared to me, by the way, that Bootes originally had nobler proportions than astronomers now assign to him. It is known that Canes Venatici now occupy the place of an upraised arm of Bootes, and I imagine that Corona Borealis, though undoubtedly a very ancient constellation, occupies the place of his other arm. Giving to the constellation the extent thus implied, it exhibits (better than most constellations) the character assigned to it. One can readily picture to oneself the figure of a Herdsman with upraised arms driving Ursa Major before him. This view is confirmed, I think, by the fact that the Arabs called this constellation the Vociferator.
Bootes contains many beautiful objects. Partly on this account, and partly because this is a constellation with which the observer should be specially familiar, a map of it is given in Plate [4].
Arcturus has a distant pale lilac companion, and is in other respects a remarkable and interesting object. It is of a ruddy yellow colour. Schmidt, indeed, considers that the star has changed colour of late years, and that whereas it was once very red it is now a yellow star. This opinion does not seem well grounded, however. The star may have been more ruddy once than now, though no other observer has noticed such a peculiarity; but it is certainly not a pure yellow star at present (at any rate as seen in our latitude). Owing probably to the difference of colour between Vega, Capella and Arcturus, photometricians have not been perfectly agreed as to the relative brilliancy of these objects. Some consider Vega the most brilliant star in the northern heavens, while others assign the superiority to Capella. The majority, however, consider Arcturus the leading northern brilliant, and in the whole heavens place three only before him, viz., Sirius, Canopus, and α Centauri. Arcturus is remarkable in other respects. His proper motion is very considerable, so great in fact that since the time of Ptolemy the southerly motion (alone) of Arcturus has carried him over a space nearly half as great again as the moon's apparent diameter. One might expect that so brilliant a star, apparently travelling at a rate so great compared with the average proper motions of the stars, must be comparatively near to us. This, however, has not been found to be the case. Arcturus is, indeed, one of the stars whose distance it has been found possible to estimate roughly. But he is found to be some three times as far from us as the small star 61 Cygni, and more than seven times as far from us as α Centauri.
The star δ Bootis is a wide and unequal double, the smaller component being only of the ninth magnitude.
Above Alkaid the last star in the tail of the Greater Bear, there will be noticed three small stars. These are θ, ι, and κ Bootis, and are usually placed in star-maps near the upraised hand of the Herdsman. The two which lie next to Alkaid, ι and κ, are interesting doubles. The former is a wide double (see Plate [5]), the magnitudes of components 4 and 8, their colours yellow and white. The larger star of this pair is itself double. The star κ Bootis is not so wide a double (see Plate [5]), the magnitudes of the components 5 and 8, their colours white and faint blue—a beautiful object.
The star ξ Bootis is an exceedingly interesting object. It is double, the colours of the components being orange-yellow and ruddy purple, their magnitudes 3½ and 6½. When this star was first observed by Herschel in 1780 the position of the components was quite different from that presented in Plate [5]. They were also much closer, being separated by a distance of less than 3½ seconds. Since that time the smaller component has traversed nearly a full quadrant, its distance from its primary first increasing, till in 1831 the stars were nearly 7½ seconds apart, and thence slowly diminishing, so that at present the stars are less than 5 seconds apart. The period usually assigned to the revolution of this binary system is 117 years, and the period of peri-astral passage is said to be 1779. It appears to me, however, that the period should be about 108 years, the epoch of last peri-astral passage 1777 and of next peri-astral passage, therefore, 1885. The angular motion of the secondary round the primary is now rapidly increasing, and the distance between the components is rapidly diminishing, so that in a few years a powerful telescope will be required to separate the pair.
Not far from ξ is π Bootis, represented in Plate [5] as a somewhat closer double, but in reality—now at any rate—a slightly wider pair, since the distance between the components of ξ has greatly diminished of late. Both the components of π are white, and their magnitudes are 3½ and 6.
We shall next turn to an exceedingly beautiful and delicate object, the double star ε Bootis, known also as Mirac and, on account of its extreme beauty, called Pulcherrima by Admiral Smyth. The components of this beautiful double are of the third and seventh magnitude, the primary orange, the secondary sea-green. The distance separating the components is about 3 seconds, perhaps more; it appears to have been slowly increasing during the past ten or twelve years. Smyth assigns to this system a period of revolution of 980 years, but there can be little doubt that the true period is largely in excess of this estimate. Observers in southern latitudes consider that the colours of the components are yellow and blue, not orange and green as most of our northern observers have estimated them.
A little beyond the lower left-hand corner of the map is the star δ Serpentis, in the position shown in the Frontispiece, Map 3. This is the star which at the hour and season depicted in Map 2 formed the uppermost of a vertical row of stars, which has now assumed an almost horizontal position. The components of δ Serpentis are about 3½ seconds apart, their magnitudes 3 and 5, both white.
The stars θ1 and θ2 Serpentis form a wide double, the distance between the components being 21½ seconds. They are nearly equal in magnitude, the primary being 4½, the secondary 5. Both are yellow, the primary being of a paler yellow colour than the smaller star. But the observer may not know where to look for θ Serpentis, since it falls in a part of the constellation quite separated from that part in which δ Serpentis lies. In fact θ lies on the extreme easterly verge of the eastern half of the constellation. It is to be looked for at about the same elevation as the brilliant Altair, and (as to azimuth) about midway between Altair and the south.
The stars α1 and α2 Libræ form a wide double, perhaps just separable by the naked eye in very favourable weather. The larger component is of the third, the smaller of the sixth magnitude, the former yellow the latter light grey.
The star β Libræ is a beautiful light-green star to the naked eye; in the telescope a wide double, pale emerald and light blue.
In Scorpio there are several very beautiful objects:—
The star Antares or Cor Scorpionis is one of the most beautiful of the red stars. It has been termed the Sirius of red stars, a term better merited perhaps by Aldebaran, save for this that, in our latitude, Antares is, like Sirius, always seen as a brilliant "scintillator" (because always low down), whereas Aldebaran rises high above the horizon. Antares is a double star, its companion being a minute green star. In southern latitudes the companion of Antares may be seen with a good 4-inch, but in our latitudes a larger opening is wanted. Mr. Dawes once saw the companion of Antares shining alone for seven seconds, the primary being hidden by the moon. He found that the colour of the secondary is not merely the effect of contrast, but that this small star is really a green sun.
The star β Scorpionis is a fine double, the components 13"·1 apart, their magnitudes 2 and 5½, colours white and lilac. It has been supposed that this pair is only an optical double, but a long time must elapse before a decisive opinion can be pronounced on such a point.
The star σ Scorpionis is a wider but much more difficult double, the smaller component being below the 9th magnitude. The colour of the primary (4) is white, that of the secondary maroon.
The star ξ Scorpionis is a neat double, the components 7"·2 apart, their magnitudes 4½ and 7½, their colours white and grey. This star is really triple, a fifth-magnitude star lying close to the primary.
In Ophiuchus, a constellation covering a wide space immediately above Scorpio, there are several fine doubles. Among others—
39 Ophiuchi, distance between components 12"·1, their magnitudes 5½ and 7½, their colours orange and blue.
The star 70 Ophiuchi, a fourth-magnitude star on the right shoulder of Ophiuchus, is a noted double. The distance between the components about 5½", their magnitudes 4½ and 7, the colours yellow and red. The pair form a system whose period of revolution is about 95 years.
36 Ophiuchi (variable), distance 5"·2, magnitudes 4½ and 6½, colours red and yellow.
ρ Opiuchi, distance 4", colours yellow and blue, magnitudes 5 and 7.
Between α and β Scorpionis the fine nebula 80 M may be looked for. (Or more closely thus:—below β is the wide Double ω1 and ω2 Scorpionis; about as far to the right of Antares is the star σ Scorpionis, and immediately above this is the fifth-magnitude star 19.) The nebula we seek lies between 19 and ω, nearer to 19 (about two-fifths of the way towards ω). This nebula is described by Sir W. Herschel as "the richest and most condensed mass of stars which the firmament offers to the contemplation of astronomers."
There are two other objects conveniently situated for observation, which the observer may now turn to. The first is the great cluster in the sword-hand of Perseus (see Plate [4]), now lying about 28° above the horizon between N.E. and N.N.E. The stars γ and δ Cassiopeiæ (see Map 3 of Frontispiece) point towards this cluster, which is rather farther from δ than δ from γ, and a little south of the produced line from these stars. The cluster is well seen with the naked eye, even in nearly full moonlight. In a telescope of moderate power this cluster is a magnificent object, and no telescope has yet revealed its full glory. The view in Plate [5] gives but the faintest conception of the glories of χ Persei. Sir W. Herschel tried in vain to gauge the depths of this cluster with his most powerful telescope. He spoke of the most distant parts as sending light to us which must have started 4000 or 5000 years ago. But it appears improbable that the cluster has in reality so enormous a longitudinal extension compared with its transverse section as this view would imply. On the contrary, I think we may gather from the appearance of this cluster, that stars are far less uniform in size than has been commonly supposed, and that the mere irresolvability of a cluster is no proof of excessive distance. It is unlikely that the faintest component of the cluster is farther off than the brightest (a seventh-magnitude star) in the proportion of more than about 20 to 19, while the ordinary estimate of star magnitudes, applied by Herschel, gave a proportion of 20 or 30 to 1 at least. I can no more believe that the components of this cluster are stars greatly varying in distance, but accidentally seen in nearly the same direction, (or that they form an enormously long system turned by accident directly towards the earth), than I could look on the association of several thousand persons in the form of a procession as a fortuitous arrangement.
Next there is the great nebula in Andromeda—known as "the transcendantly beautiful queen of the nebulæ." It will not be difficult to find this object. The stars ε and δ Cassiopeiæ (Map 3, Frontispiece) point to the star β Andromedæ. Almost in a vertical line above this star are two fourth-magnitude stars μ and γ, and close above ν, a little to the right, is the object we seek—visible to the naked eye as a faint misty spot. To tell the truth, the transcendantly beautiful queen of the nebulæ is rather a disappointing object in an ordinary telescope. There is seen a long oval or lenticular spot of light, very bright near the centre, especially with low powers. But there is a want of the interest attaching to the strange figure of the Great Orion nebula. The Andromeda nebula has been partially resolved by Lord Rosse's great reflector, and (it is said) more satisfactorily by the great refractor of Harvard College. In the spectroscope, Mr. Huggins informs us, the spectrum is peculiar. Continuous from the blue to the orange, the light there "appears to cease very abruptly;" there is no indication of gaseity.
Lastly, the observer may turn to the pair Mizar and Alcor, the former the middle star in the Great Bear's tail, the latter 15' off. It seems quite clear, by the way, that Alcor has increased in brilliancy of late, since among the Arabians it was considered an evidence of very good eyesight to detect Alcor, whereas this star may now be easily seen even in nearly full moonlight. Mizar is a double star, and a fourth star is seen in the same field of view with the others (see Plate [5]). The distance between Mizar and its companion is 14"·4; the magnitude of Mizar 3, of the companion 5; their colours white and pale green, respectively.