Map No. 13.
Passing near to the region covered bymap No. 13, we find the remaining portions of the constellations Sagittarius and Scutum Sobieskii. It will be advisable to finish with the latter first. Glance at the clusters 4426 and 4437. Neither is large, but both are rich in stars. The nebula 4441 is a fine object of its kind. It brightens toward the center, and Herschel thought he had resolved it into stars. The variable R is remarkable for its eccentricities. Sometimes it attains nearly the fourth magnitude, although usually at maximum it is below the fifth, while at minimum it is occasionally of the sixth and atother times of the seventh or eighth magnitude. Its period is irregular.
Turning back to Sagittarius, we resume our search for interesting objects there, and the first that we discover is another star cluster, for the stars are wonderfully gregarious in this quarter of the heavens. The number our cluster bears on themapis 4424, corresponding with M 22 in Messier's catalogue. It is very bright, containing many stars of the tenth and eleventh magnitudes, as well as a swarm of smaller ones. Sir John Herschel regarded the larger stars in this cluster as possessing a reddish tint. Possibly there was some peculiarity in his eye that gave him this impression, for he has described a cluster in the constellation Toucan in the southern hemisphere as containing a globular mass of rose-colored stars inclosed in a spherical shell of white stars. Later observers have confirmed his description of the shape and richness of this cluster in Toucan, but have been unable to perceive the red hue of the interior stars.
The eastern expanse of Sagittarius is a poor region compared with the western end of the constellation, where the wide stream of the Milky Way like a great river enriches its surroundings. The variables T and R are of little interest to us, for they never become bright enough to be seen without the aid of a telescope. In 54 we find, however, an interesting double, which with larger telescopes than any of ours appears as a triple. The two stars that we see are of magnitudes six and seven and a half, distance 45", p. 42°, colors yellow and blue. The third star, perhaps of thirteenth magnitude, is distant 36", p. 245°.
Retainingmap No. 13as our guide, we examine the western part of the constellation Capricornus. Its leader α is a naked-eye double, the two stars being a little morethan 6' apart. Their magnitudes are three and four, and both have a yellowish hue. The western star is α1, and is the fainter of the two. The other is designated as α2. Both are double. The components of α1are of magnitudes four and eight and a half, distance 44", p. 220°. With the Washington twenty-six-inch telescope a third star of magnitude fourteen has been found at a distance of 40", p. 182°. In α2the magnitudes of the components are three and ten and a half, distance 7.4", p. 150°. The smaller star has a companion of the twelfth or thirteenth magnitude, distance 1.2", p. 240°. This, of course, is hopelessly beyond our reach. Yet another star of magnitude nine, distance 154", p. 156, we may see easily.
Dropping down to β, we find it to be a most beautiful and easy double, possessing finely contrasted colors, gold and blue. The larger star is of magnitude three, and the smaller, the blue one, of magnitude six, distance 205", p. 267°. Between them there is a very faint star which larger telescopes than ours divide into two, each of magnitude eleven and a half; separated 3", p. 325°.
Still farther south and nearly in a line drawn from α through β we find a remarkable group of double stars, σ, π, ρ, and ο. The last three form a beautiful little triangle. We begin with σ, the faintest of the four. The magnitudes of its components are six and nine, distance 54", p. 177°. In π the magnitudes are five and nine, distance 3.4", p. 145°; in ρ, magnitudes five and eight, distance 3.8", p. 177° (a third star of magnitude seven and a half is seen at a distance of 4', p. 150°); in ο, magnitudes six and seven, distance 22", p. 240°.
The star cluster 4608 is small, yet, on a moonless night, worth a glance with the five-inch.
We now pass northward to the region covered bymap No. 14, including the remainder of Ophiuchus and Serpens.Beginning with the head of Serpens, in the upper right-hand corner of themap, we find that β, of magnitude three and a half, has a ninth-magnitude companion, distance 30", p. 265°. The larger star is light blue and the smaller one yellowish. The little star ν is double, magnitudes five and nine, distance 50", p. 31°, colors contrasted but uncertain. In δ we find a closer double, magnitudes three and four, distance 3.5", p. 190°. It is a beautiful object for the three-inch. The leader of the constellation, α, of magnitude two and a half, has a faint companion of only the twelfth magnitude, distance 60", p. 350°. The small star is bluish. The variable R has a period about a week short of one year, and at maximum exceeds the sixth magnitude, although sinking at minimum to less than the eleventh. Its color is ruddy.
Map No. 14.
Passing eastward, we turn again into Ophiuchus, and find immediately the very interesting double, λ, whose components are of magnitudes four and six, distance 1", p. 55°. This is a long-period binary, and notwithstanding the closeness of its stars, our four-inch should separate them when the seeing is fine. We shall do better, however, to try with the five-inch. Σ 2166 consists of two stars of magnitudes six and seven and a half, distance 27", p. 280°. Σ 2173 is a double of quite a different order. The magnitudes of its components are both six, the distance in 1899 0.98", p. 331°. It is evidently a binary in rapid motion, as the distance changed from about a quarter of a second in 1881 to more than a second in 1894. The star τ is a fine triple, magnitudes five, six, and nine, distances 1.8", p. 254°, and 100", p. 127°. The close pair is a binary system with a long period of revolution, estimated at about two hundred years. We discover another group of remarkable doubles in 67, 70, and 73. In the first-named star the magnitudes are four and eight, distance55", p. 144°, colors finely contrasted, pale yellow and red.
Much more interesting, however, is 70, a binary whose components have completed a revolution since their discovery by Sir William Herschel, the period being ninety-five years. The magnitudes are four and six, or, according to Hall, five and six, distance in 1894 2.3"; in 1900, 1.45", according to Maw. Hall says the apparent distance when the stars are closest is about 1.7", and when they are widest 6.7". This star is one of those whose parallax has been calculated with a reasonable degree of accuracy. Its distance from us is about 1,260,000 times the distance of the sun, the average distance apart of the two stars is about 2,800,000,000 miles (equal to the distance of Neptune from the sun), and their combined mass is three times that of the sun. Hall has seen in the system of 70 Ophiuchi three stars of the thirteenth magnitude or less, at distances of about 60", 90", and 165" respectively.
The star 73 is also a close double, and beyond our reach. Its magnitudes are six and seven, distance 0.7", p. 245°. It is, no doubt, a binary.
Three star clusters in Ophiuchus remain to be examined. The first of these, No. 4256, is partially resolved into stars by the five-inch. No. 4315 is globular, and has a striking environment of bystanding stars. It is about one quarter as broad as the full moon, and our largest aperture reveals the faint coruscation of its crowded components. No. 4410 is a coarser and more scattered star swarm—a fine sight!
Farther toward the east we encounter a part of Serpens again, which contains just one object worth glancing at, the double θ, whose stars are of magnitudes four and four and a half, distance 21", p. 104°. Color, both yellow, the smaller star having the deeper hue.
Let us next, with the guidance ofmap No. 15, enter the rich star fields of Hercules, and of the head and first coils of Draco. According to Argelander, Hercules contains more stars visible to the naked eye than any other constellation, and he makes the number of them one hundred and fifty-five, nearly two thirds of which are only of the sixth magnitude. But Heis, who saw more naked-eye stars than Argelander, makes Ursa Major precisely equal to Hercules in the number of stars, his enumeration showing two hundred and twenty-seven in each constellation, while, according to him, Draco follows very closely after, with two hundred and twenty stars. Yet, on account of the minuteness of the majority of their stars, neither of these constellations makes by any means as brilliant a display as does Orion, to which Argelander assigns only one hundred and fifteen naked-eye stars, and Heis one hundred and thirty-six.
We begin in Hercules with the star κ, a pretty little double of magnitudes five and a half and seven, distance 31", p. 10°, colors yellow and red. Not far away we find, in γ, a larger star with a fainter companion, the magnitudes in this case being three and a half and nine, distance 38", p. 242°, colors white and faint blue or lilac. One of the most beautiful of double stars is α Herculis. The magnitudes are three and six, distance 4.7", p. 118°, colors orange and green, very distinct. Variability has been ascribed to each of the stars in turn. It is not known that they constitute a binary system, because no certain evidence of motion has been obtained. Another very beautiful and easily separated double is δ, magnitudes three and eight, distance 19", p. 175°, colors pale green and purple.
Sweeping northwestward to ζ, we encounter a celebrated binary, to separate which at present requires thehigher powers of a six-inch glass. The magnitudes are three and six and a half, distance in 1899, 0.6", p. 264°; in 1900, 0.8", p. 239°. The period of revolution is thirty-five years, and two complete revolutions have been observed. The apparent distance changes from 0.6" to 1.6". They were at their extreme distance in 1884.
Map No. 15.
Two pleasing little doubles are Σ 2101, magnitudes six and nine, distance 4", p. 57°, and Σ 2104, magnitudes six and eight, distance 6", p. 20°. At the northern end of the constellation is 42, a double that requires the light-grasping power of our largest glass. Its magnitudes are six and twelve, distance 20", p. 94°. In ρ we discover another distinctly colored double, both stars being greenish or bluish, with a difference of tone. The magnitudes are four and five and a half, distance 3.7", p. 309°. But the double 95 is yet more remarkable for the colors of its stars. Their magnitudes are five and five and a half, distance 6", p. 262°, colors, according to Webb, "light apple-green and cherry-red." But other observers have noted different hues, one calling them both golden yellow. I think Webb's description is more nearly correct. Σ 2215 is a very close double, requiring larger telescopes than those we are working with. Its magnitudes are six and a half and eight, distance 0.7", p. 300°. It is probably a binary. Σ 2289 is also close, but our five-inch will separate it: magnitudes six and seven, distance 1.2", p. 230°.
Turning to μ, we have to deal with a triple, one of whose stars is at present beyond the reach of our instruments. The magnitudes of the two that we see are four and ten, distance 31", p. 243°. The tenth-magnitude star is a binary of short period (probably less than fifty years), the distance of whose components was 2" in 1859, 1" in 1880, 0.34" in 1889, and 0.54" in 1891, when the positionangle was 25°, and rapidly increasing. The distance is still much less than 1".
For a glance at a planetary nebula we may turn with the five-inch to No. 4234. It is very small and faint, only 8" in diameter, and equal in brightness to an eighth-magnitude star. Only close gazing shows that it is not sharply defined like a star, and that it possesses a bluish tint. Its spectrum is gaseous.
The chief attraction of Hercules we have left for the last, the famous star cluster between η and ζ, No. 4230, more commonly known as M 13. On a still evening in the early summer, when the moon is absent and the quiet that the earth enjoys seems an influence descending from the brooding stars, the spectacle of this sun cluster in Hercules, viewed with a telescope of not less than five-inches aperture, captivates the mind of the most uncontemplative observer. With the Lick telescope I have watched it resolve into separate stars to its very center—a scene of marvelous beauty and impressiveness. But smaller instruments reveal only the in-running star streams and the sprinkling of stellar points over the main aggregation, which cause it to sparkle like a cloud of diamond dust transfused with sunbeams. The appearance of flocking together that those uncountable thousands of stars present calls up at once a picture of our lone sun separated from its nearest stellar neighbor by a distance probably a hundred times as great as the entire diameter of the spherical space within which that multitude is congregated. It is true that unless we assume what would seem an unreasonable remoteness for the Hercules cluster, its component stars must be much smaller bodies than the sun; yet even that fact does not diminish the wonder of their swarming. Here the imagination must bear science on its wings, else science can make no progress whatever.It is an easy step from Hercules to Draco. In the conspicuous diamond-shaped figure that serves as a guide-board to the head of the latter, the southernmost star belongs not to Draco but to Hercules. The brightest star in this figure is γ, of magnitude two and a half, with an eleventh-magnitude companion, distant 125", p. 116°. Two stars of magnitude five compose ν, their distance apart being 62", p. 312°. A more interesting double is μ, magnitudes five and five, distance 2.4", p. 158°. Both stars are white, and they present a pretty appearance when the air is steady. They form a binary system of unknown period. Σ 2078 (also called 17 Draconis) is a triple, magnitudes six, six and a half, and six, distances 3.8", p. 116°, and 90", p. 195°. Σ 1984 is an easy double, magnitudes six and a half and eight and a half, distance 6.4", p. 276°. The star η is a very difficult double for even our largest aperture, on account of the faintness of one of its components. The magnitudes are two and a half and ten, distance 4.7", p. 140°. Its near neighbor, Σ 2054, may be a binary. Its magnitudes are six and seven, distance 1", p. 0°. In Σ 2323 we have another triple, magnitudes five, eight and a half, and seven, distances 3.6", p. 360°, and 90", p. 22°, colors white, blue, and reddish. A fine double is ε, magnitudes five and eight, distance 3", p. 5°.
The nebula No. 4373 is of a planetary character, and interesting as occupying the pole of the ecliptic. A few years ago Dr. Holden, with the Lick telescope, discovered that it is unique in its form. It consists of a double spiral, drawn out nearly in the line of sight, like the thread of a screw whose axis lies approximately endwise with respect to the observer. There is a central star, and another fainter star is involved in the outer spiral. The form of this object suggests strange ideas as to its origin. But the details mentioned are far beyond the reach of ourinstruments. We shall only see it as a hazy speck. No. 4415 is another nebula worth glancing at. It is Tuttle's so-called variable nebula.
Map No. 16.
There are three constellations represented onmap No. 16to which we shall pay brief visits. First Aquila demands attention. Its doubles may be summarized as follows: 11, magnitudes five and nine, distance 17.4", p. 252°; π, magnitudes six and seven, distance 1.6", p. 122°; 23, magnitudes six and ten, distance 3.4", p. 12°—requires the five-inch and good seeing; 57, magnitudes five and six, distance 36", p. 170°; Σ 2654, magnitudes six and eight, distance 12", p. 234°; Σ 2644, magnitudes six and seven, distance 3.6", p. 208°.
The star η is an interesting variable between magnitudes three and a half and 4.7; period, seven days, four hours, fourteen minutes. The small red variable R changes from magnitude six to magnitude seven and a half and back again in a period of three hundred and fifty-one days.
Star cluster No. 4440 is a striking object, its stars ranging from the ninth down to the twelfth magnitude.
Just north of Aquila is the little constellation Sagitta, containing several interesting doubles and many fine star fields, which may be discovered by sweeping over it with a low-power eyepiece. The star ζ is double, magnitudes five and nine, distance 8.6", p. 312°. The larger star is itself double, but far too close to be split, except with very large telescopes. In θ we find three components of magnitudes seven, nine, and eight respectively, distances 11.4", p. 327°, and 70", p. 227°. A wide double is ε, magnitudes six and eight, distance 92", p. 81°. Nebula No. 4572 is planetary.
Turning to Delphinus, we find a very beautiful double in γ, magnitudes four and five, distance 11", p. 273°, colorsgolden and emerald. The leader α, which is not as bright as its neighbor β, and which is believed to be irregularly variable, is of magnitude four, and has a companion of nine and a half magnitude at the distance 35", p. 278°. At a similar distance, 35", p. 335°, β has an eleventh-magnitude companion, and the main star is also double, but excessively close, and much beyond our reach. It is believed to be a swiftly moving binary, whose stars are never separated widely enough to be distinguished with common telescopes.
"This Orpheus struck when with his wondrous songHe charmed the woods and drew the rocks along."—Manilius.
"This Orpheus struck when with his wondrous songHe charmed the woods and drew the rocks along."—Manilius.
We resume our celestial explorations with the little constellation Lyra, whose chief star, Vega (α), has a very good claim to be regarded as the most beautiful in the sky. The position of this remarkable star is indicated inmap No. 17. Every eye not insensitive to delicate shades of color perceives at once that Vega is not white, but blue-white. When the telescope is turned upon the star the color brightens splendidly. Indeed, some glasses decidedly exaggerate the blueness of Vega, but the effect is so beautiful that one can easily forgive the optical imperfection which produces it. With our four-inch we look for the well-known companion of Vega, a tenth-magnitude star, also of a blue color deeper than the hue of its great neighbor. The distance is 50", p. 158°. Under the most favorable circumstances it might be glimpsed with the three-inch, but, upon the whole, I should regard it as too severe a test for so small an aperture.
Vega is one of those stars which evidently are not only enormously larger than the sun (one estimate makes the ratio in this case nine hundred to one), but whose physical condition, as far as the spectroscope reveals it, is very different from that of our ruling orb. Like Sirius, Vega displays the lines of hydrogen most conspicuously, and it isprobably a much hotter as well as a much more voluminous body than the sun.
Map No. 17.
Close by, toward the east, two fourth-magnitude stars form a little triangle with Vega. Both are interesting objects for the telescope, and the northern one, ε, has few rivals in this respect. Let us first look at it with an opera glass. The slight magnifying power of such an instrument divides the star into two twinkling points. They are about two and a quarter minutes of arc apart, and exceptionally sharp-sighted persons are able to see them divided with the naked eye. Now take the three-inch telescope and look at them, with a moderate power. Each of the two stars revealed by the opera glass appears double, and a fifth star of the ninth magnitude is seen on one side of an imaginary line joining the two pairs. The northern-most pair is named ε1, the magnitudes being fifth and sixth, distance 3", p. 15°. The other pair is ε2, magnitudes fifth and sixth, distance 2.3", p. 133°. Each pair is apparently a binary; but the period of revolution is unknown. Some have guessed a thousand years for one pair, and two thousand for the other. Another guess gives ε1a period of one thousand years, and ε2a period of eight hundred years. Hall, in his double-star observations, simply says of each, "A slow motion."
Purely by guesswork a period has also been assigned to the two pairs in a supposed revolution around their common center, the time named being about a million years. It is not known, however, that such a motion exists. Manifestly it could not be ascertained within the brief period during which scientific observations of these stars have been made. The importance of the element of time in the study of stellar motions is frequently overlooked, though not, of course, by those who are engaged in such work. The sun, for instance, and many of thestars are known to be moving in what appear to be straight lines in space, but observations extending over thousands of years would probably show that these motions are in curved paths, and perhaps in closed orbits.
If now in turn we take our four-inch glass, we shall see something else in this strange family group of ε Lyræ. Between ε1and ε2, and placed one on each side of the joining line, appear two exceedingly faint specks of light, which Sir John Herschel made famous under the name of thedebillissima. They are of the twelfth or thirteenth magnitude, and possibly variable to a slight degree. If you can not see them at first, turn your eye toward one side of the field of view, and thus, by bringing their images upon a more sensitive part of the retina, you may glimpse them. The sight is not much, yet it will repay you, as every glance into the depths of the universe does.
The other fourth-magnitude star near Vega is ζ, a wide double, magnitudes fourth and sixth, distance 44", p. 150°. Below we find β, another very interesting star, since it is both a multiple and an eccentric variable. It has four companions, three of which we can easily see with our three-inch; the fourth calls for the five-inch; the magnitudes are respectively four, seven or under, eight, eight and a half, and eleven; distances 45", p. 150°; 65", p. 320°; 85", p. 20°; and 46", p. 248°. The primary, β, varies from about magnitude three and a half to magnitude four and a half, the period being twelve days, twenty-one hours, forty-six minutes, and fifty-eight seconds. Two unequal maxima and minima occur within this period. In the spectrum of this star some of the hydrogen lines and the D3line (the latter representing helium, a constituent of the sun and of some of the stars, which, until its recent discovery in a few rare minerals was not known to existon the earth) are bright, but they vary in visibility. Moreover, dark lines due to hydrogen also appear in its spectrum simultaneously with the bright lines of that element. Then, too, the bright lines are sometimes seen double. Professor Pickering's explanation is that β Lyræ probably consists of two stars, which, like the two composing β Aurigæ, are too close to be separated with any telescope now existing, and that the body which gives the bright lines is revolving in a circle in a period of about twelve days and twenty-two hours around the body which gives the dark lines. He has also suggested that the appearances could be accounted for by supposing a body like our sun to be rotating in twelve days and twenty-two hours, and having attached to it an enormous protuberance extending over more than one hundred and eighty degrees of longitude, so that when one end of it was approaching us with the rotation of the star the other end would be receding, and a splitting of the spectral lines at certain periods would be the consequence. "The variation in light," he adds, "may be caused by the visibility of a larger or smaller portion of this protuberance."
Unfortunate star, doomed to carry its parasitical burden of hydrogen and helium, like Sindbad in the clasp of the Old Man of the Sea! Surely, the human imagination is never so wonderful as when it bears an astronomer on its wings. Yet it must be admitted that the facts in this case are well calculated to summon the genius of hypothesis. And the puzzle is hardly simplified by Bélopolsky's observation that the body in β Lyræ giving dark hydrogen lines shows those lines also split at certain times. It has been calculated, from a study of the phenomena noted above, that the bright-line star in β Lyræ is situated at a distance of about fifteen million miles fromthe center of gravity of the curiously complicated system of which it forms a part.
We have not yet exhausted the wonders of Lyra. On a line from β to γ, and about one third of the distance from the former to the latter, is the celebrated Ring Nebula, indicated on themapby the number 4447. We need all the light we can get to see this object well, and so, although the three-inch will show it, we shall use the five-inch. Beginning with a power of one hundred diameters, which exhibits it as a minute elliptical ring, rather misty, very soft and delicate, and yet distinct, we increase the magnification first to two hundred and finally to three hundred, in order to distinguish a little better some of the details of its shape. Upon the whole, however, we find that the lowest power that clearly brings out the ring gives the most satisfactory view. The circumference of the ring is greater than that of the planet Jupiter. Its ellipticity is conspicuous, the length of the longer axis being 78" and that of the shorter 60". Closely following the nebula as it moves through the field of view, our five-inch telescope reveals a faint star of the eleventh or twelfth magnitude, which is suspected of variability. The largest instruments, like the Washington and the Lick glasses, have shown perhaps a dozen other stars apparently connected with the nebula. A beautiful sparkling effect which the nebula presents was once thought to be an indication that it was really composed of a circle of stars, but the spectroscope shows that its constitution is gaseous. Just in the middle of the open ring is a feeble star, a mere spark in the most powerful telescope. But when the Ring Nebula is photographed—and this is seen beautifully in the photographs made with the Crossley reflector on Mount Hamilton by the late Prof. J. E. Keeler—this excessively faint star imprints its imageboldly as a large bright blur, encircled by the nebulous ring, which itself appears to consist of a series of intertwisted spirals.
Not far away we find a difficult double star, 17, whose components are of magnitudes six and ten or eleven, distance 3.7", p. 325°.
From Lyra we pass to Cygnus, which, lying in one of the richest parts of the Milky Way, is a very interesting constellation for the possessor of a telescope. Its general outlines are plainly marked for the naked eye by the figure of a cross more than twenty degrees in length lying along the axis of the Milky Way. The foot of the cross is indicated by the star β, also known as Albireo, one of the most charming of all the double stars. The three-inch amply suffices to reveal the beauty of this object, whose components present as sharp a contrast of light yellow and deep blue as it would be possible to produce artificially with the purest pigments. The magnitudes are three and seven, distance 34.6", p. 55°. No motion has been detected indicating that these stars are connected in orbital revolution, yet no one can look at them without feeling that they are intimately related to one another. It is a sight to which one returns again and again, always with undiminished pleasure. The most inexperienced observer admires its beauty, and after an hour spent with doubtful results in trying to interest a tyro in double stars it is always with a sense of assured success that one turns the telescope to β Cygni.
Following up the beam of the imaginary cross along the current of the Milky Way, every square degree of which is here worth long gazing into, we come to a pair of stars which contend for the name-letter χ. On ourmapthe letter is attached to the southernmost of the two, a variable of long period—four hundred and six days—whosechanges of brilliance lie between magnitudes four and thirteen, but which exhibits much irregularity in its maxima. The other star, not named but easily recognized in themap, is sometimes called 17. It is an attractive double whose colors faintly reproduce those of β. The magnitudes are five and eight, distance 26", p. 73°. Where the two arms of the cross meet is γ, whose remarkablecortégeof small stars running in curved streams should not be missed. Use the lowest magnifying power.
At the extremity of the western arm of the cross is δ, a close double, difficult for telescopes of moderate aperture on account of the difference in the magnitudes of the components. We may succeed in dividing it with the five-inch. The magnitudes are three and eight, distance 1.5", p. 310°. It is regarded as a binary of long and as yet unascertained period.
In ο2we find a star of magnitude four and orange in color, having two blue companions, the first of magnitude seven and a half, distance 107", p. 174°, and the second of magnitude five and a half, distance 358", p. 324°. Farther north is ψ, which presents to us the combination of a white five-and-a-half-magnitude star with a lilac star of magnitude seven and a half. The distance is 3", p. 184°. A very pretty sight.
We now pass to the extremity of the other arm of the cross, near which lies the beautiful little double 49, whose components are of magnitudes six and eight, distance 2.8", p. 50°. The colors are yellow and blue, conspicuous and finely contrasted. A neighboring double of similar hues is 52, in which the magnitudes are four and nine, distance 6", p. 60°. Sweeping a little way northward we come upon an interesting binary, λ, which is unfortunately beyond the dividing power of our largest glass. A good seven-inch or seven-and-a-half-inch should split it underfavorable circumstances. Its magnitudes are six and seven, distance 0.66", p. 74°.
The next step carries us to a very famous object, 61 Cygni, long known as the nearest star in the northern hemisphere of the heavens. It is a double which our three-inch will readily divide, the magnitudes being both six, distance 21", p. 122°. The distance of 61 Cygni, according to Hall's parallax of 0.27", is about 70,000,000,000,000 miles. There is some question whether or not it is a binary, for, while the twin stars are both moving in the same direction in space with comparative rapidity, yet conclusive evidence of orbital motion is lacking. When one has noticed the contrast in apparent size between this comparatively near-by star, which the naked eye only detects with considerable difficulty, and some of its brilliant neighbors whose distance is so great as to be immeasurable with our present means, no better proof will be needed of the fact that the faintness of a star is not necessarily an indication of remoteness.
We may prepare our eyes for a beautiful exhibition of contrasted colors once more in the star μ. This is really a quadruple, although only two of its components are close and conspicuous. The magnitudes are five, six, seven and a half, and twelve; distances 2.4", p. 121°; 208", p. 56°; and 35", p. 264°. The color of the largest star is white and that of its nearest companion blue; the star of magnitude seven and a half is also blue.
The star cluster 4681 is a fine sight with our largest glass. In themapwe find the place marked where the new star of 1876 made its appearance. This was first noticed on November 24, 1876, when it shone with the brilliance of a star of magnitude three and a half. Its spectrum was carefully studied, especially by Vogel, and the very interesting changes that it underwent werenoted. Within a year the star had faded to less than the tenth magnitude, and its spectrum had completely changed in appearance, and had come to bear a close resemblance to that of a planetary nebula. This has been quoted as a possible instance of a celestial collision through whose effects the solid colliding masses were vaporized and expanded into a nebula. At present the star is very faint and can only be seen with the most powerful telescopes. Compare with the case of Nova Aurigæ, previously discussed.
Underneath Cygnus we notice the small constellation Vulpecula. It contains a few objects worthy of attention, the first being the nebula 4532, the "dumb-bell nebula" of Lord Rosse. With the four-inch, and better with the five-inch, we are able to perceive that it consists of two close-lying tufts of misty light. Many stars surround it, and large telescopes show them scattered between the two main masses of the nebula. The Lick photographs show that its structure is spiral. The star 11 points out the place where a new star of the third magnitude appeared in 1670. Σ 2695 is a close double, magnitudes six and eight, distance 0.96", p. 78°.
Map No. 18.
We turn tomap No. 18, and, beginning at the western end of the constellation Aquarius, we find the variable T, which ranges between magnitudes seven and thirteen in a period of about two hundred and three days. Its near neighbor Σ 2729 is a very close double, beyond the separating power of our five-inch, the magnitudes being six and seven, distance 0.6", p. 176°. Σ 2745, also known as 12 Aquarii, is a good double for the three-inch. Its magnitudes are six and eight, distance 2.8", p. 190°. In ζ we discover a beauty. It is a slow binary of magnitudes four and four, distance 3.1", p. 321°. According to some observers both stars have a greenish tinge. The star 41 is awider double, magnitudes six and eight, distance 5", p. 115°, colors yellow and blue. The uncommon stellar contrast of white with light garnet is exhibited by τ, magnitudes six and nine, distance 27", p. 115°. Yellow and blue occur again conspicuously in ψ, magnitudes four and a half and eight and a half, distance 50", p. 310°. Rose and emerald have been recorded as the colors exhibited in Σ 2998, whose magnitudes are five and seven, distance 1.3", p. 346°.
The variables S and R are both red. The former ranges between magnitudes eight and twelve, period two hundred and eighty days, and the latter between magnitudes six and eleven, period about three hundred and ninety days.
The nebula 4628 is Rosse's "Saturn nebula," so called because with his great telescope it presented the appearance of a nebulous model of the planet Saturn. With our five-inch we see it simply as a planetary nebula. We may also glance at another nebula, 4678, which appears circular and is pinned with a little star at the edge.
The small constellation Equuleus contains a surprisingly large number of interesting objects. Σ 2735 is a rather close double, magnitudes six and eight, distance 1.8", p. 287°. Σ 2737 (the first star to the left of Σ 2735, the name having accidentally been omitted from themap) is a beautiful triple, although the two closest stars, of magnitudes six and seven, can not be separated by our instruments. Their distance in 1886 was 0.78", p. 286°, and they had then been closing rapidly since 1884, when the distance was 1.26". The third star, of magnitude eight, is distant 11", p. 75°. Σ 2744 consists of two stars, magnitudes six and seven, distance 1.4", p. 1.67°. It is probably a binary. Σ 2742 is wider double, magnitudes both six, distance 2.6", p. 225°. Another triple, one of whosecomponents is beyond our reach, is γ. Here the magnitudes are fifth, twelfth, and sixth, distances 2", p. 274° and 366". It would also be useless for us to try to separate δ, but it is interesting to remember that this is one of the closest of known double stars, the magnitudes being fourth and fifth, distance 0.4", p. 198°. These data are from Hall's measurements in 1887. The star is, no doubt, a binary. With the five-inch we may detect one and perhaps two of the companion stars in the quadruple β. The magnitudes are five, ten, and two eleven, distances 67", p. 309°; 86", p. 276°; and 6.5", p. 15°. The close pair is comprised in the tenth-magnitude star.
Map No. 19.
Map No. 19introduces us to the constellation Pegasus, which is comparatively barren to the naked eye, and by no means rich in telescopic phenomena. The star ε, of magnitude two and a half, has a blue companion of the eighth magnitude, distance 138", p. 324°; colors yellow and violet. A curious experiment that may be tried with this star is described by Webb, who ascribes the discovery of the phenomenon to Sir John Herschel. When near the meridian the small star in ε appears, in the telescope, underneath the large one. If now the tube of the telescope be slightly swung from side to side the small star will appear to describe a pendulumlike movement with respect to the large one. The explanation suggested is that the comparative faintness of the small star causes its light to affect the retina of the eye less quickly than does that of its brighter companion, and, in consequence, the reversal of its apparent motion with the swinging of the telescope is not perceived so soon.
The third-magnitude star η has a companion of magnitude ten and a half, distance 90", p. 340°. The star β, of the second magnitude, and reddish, is variable to the extent of half a magnitude in an irregular period, and γ,of magnitude two and a half, has an eleventh-magnitude companion, distance 162", p. 285°.
Map No. 20.
Our interest is revived on turning, with the guidance ofmap No. 20, from the comparative poverty of Pegasus to the spacious constellation Cetus. The first double star that we meet in this constellation is 26, whose components are of magnitudes six and nine, distance 16.4", p. 252°; colors, topaz and lilac. Not far away is the closer double 42, composed of a sixth and a seventh magnitude star, distance 1.25", p. 350°. The four-inch is capable of splitting this star, but we shall do better to use the five-inch. In passing we may glance at the tenth-magnitude companion to η, distance 225", p. 304°. Another wide pair is found in ζ, magnitudes three and nine, distance 185", p. 40°.
The next step brings us to the wonderful variable ο, or Mira, whose changes have been watched for three centuries, the first observer of the variability of the star having been David Fabricius in 1596. Not only is the range of variability very great, but the period is remarkably irregular. In the time of Hevelius, Mira was once invisible for four years. When brightest, the star is of about the second magnitude, and when faintest, of the ninth magnitude, but at maximum it seldom exhibits the greatest brilliance that it has on a few occasions shown itself capable of attaining. Ordinarily it begins to fade after reaching the fourth or fifth magnitude. The period averages about three hundred and thirty-one days, but is irregularly variable to the extent of twenty-five days. Its color is red, and its spectrum shows bright lines, which it is believed disappear when the star sinks to a minimum. Among the various theories proposed to account for such changes as these the most probable appears to be that which ascribes them to some cause analogous to thatoperating in the production of sun spots. The outburst of light, however, as pointed out by Scheiner, should be regarded as corresponding to the maximum and not the minimum stage of sun-spot activity. According to this view, the star is to be regarded as possessing an extensive atmosphere of hydrogen, which, during the maximum, is upheaved into enormous prominences, and the brilliance of the light from these prominences suffices to swamp the photospheric light, so that in the spectrum the hydrogen lines appear bright instead of dark.
It is not possible to suppose that Mira can be the center of a system of habitable planets, no matter what we may think of the more constant stars in that regard, because its radiation manifestly increases more than six hundred fold, and then falls off again to an equal extent once in every ten or eleven months. I have met people who can not believe that the Almighty would make a sun and then allow its energies "to go to waste," by not supplying it with a family of worlds. But I imagine that if they had to live within the precincts of Mira Ceti they would cry out for exemption from their own law of stellar utility.
The most beautiful double star in Cetus is γ, magnitudes three and seven, distance 3", p. 288°; hues, straw-color and blue. The leading star α, of magnitude two and a half, has a distant blue companion three magnitudes fainter, and between them are two minute stars, the southernmost of which is a double, magnitudes both eleven, distance 10", p. 225°.
The variable S ranges between magnitudes seven and twelve in a somewhat irregular period of about eleven months, while R ranges between the seventh and the thirteenth magnitudes in a period of one hundred and sixty-seven days.
The constellation Eridanus, represented inmap No. 21, contains a few fine double stars, one of the most interesting of which is 12, a rather close binary. The magnitudes are four and eight, distance 2", p. 327°. We shall take the five-inch for this, and a steady atmosphere and sharp seeing will be necessary on account of the wide difference in the brightness of the component stars. Amateurs frequently fail to make due allowance for the effect of such difference. When the limit of separating power for a telescope of a particular aperture is set at 1" or 2", as the case may be, it is assumed that the stars composing the doubles on which the test is made shall be of nearly the same magnitude, or at least that they shall not differ by more than one or two magnitudes at the most. The stray light surrounding a comparatively bright star tends to conceal a faint companion, although the telescope may perfectly separate them so far as the stellar disks are concerned. Then, too, I have observed in my own experience that a very faint and close double is more difficult than a brighter pair not more widely separated, usually on account of the defect of light, and this is true even when the components of the faint double are of equal magnitude.