Part 15

Among those interesting objects, the variable stars, are several which may be well observed without optical assistance. Of these may be mentioned Algol, of which all the fluctuations of light may be easily observed with the naked eye; Mira Ceti, which may be well observed when at its brightest; Lambda Tauri, a variable star of the Algol type; Betelgeuse (Alpha Orionis), which is slightly variable; Zeta Geminorum, a fourth magnitude star, which varies about three-quarters of a magnitude in a period of about ten days; R. Hydræ, which is visible to the naked eye at maximum; Beta Lyræ, period about thirteen days; Eta Aquilæ, period about seven days; and Delta Cephei, which varies about one magnitude in a period of a little over five days. Of all these stars useful observations may be made without optical assistance of any sort.

Observations, and even discoveries, of new or “temporary” stars may also be made with the naked eye. This occurred in the case of the “temporary” stars of 1572, 1604, 1670, 1866, and 1870, but, of course, these were bright objects at the time of their discovery. Hind’s “new star” of 1848 in Ophiuchus was, however, only of the fifth magnitude when it appeared, and it might have escaped detection with the naked eye. A star of this magnitude might, however, be easily detected by an observer who is familiar with the principal stars of a constellation.

The Milky Way may, perhaps, be better seen with the naked eye than with any instrument, although an opera-glass brings out well, in some places, its more delicate details. A mere passing glance might lead a casual observer to suppose that the Galaxy stretched as a band of nearly uniform brightness across the heavens. But good eyesight, careful attention, and a clear sky will soon disclose numerous details previously unsuspected; streams and rays of different brightness, intersected by rifts of darkness, and interspersed with spots and channels of comparatively starless spaces. An excellent drawing of the Milky Way—the result of five years’ observations with the naked eye alone—has recently been completed by Dr. Otto Boeddicker at Lord Rosse’s observatory in Ireland. This beautiful picture is exquisitely drawn, and shows a wonderful amount of detail. A writer in the Saturday Review of November 30, 1889, says: “His maps are in many respects a completely new disclosure. Features barely suspected before come out in them as evident and persistent; every previous representation appears, by comparison, structureless.” This shows what can be done with the naked eye in the study of this wonderful zone.

Among the nebulæ and clusters there are not many objects visible to the naked eye. A hazy appearance about the middle star in Orion’s “sword” indicates the presence of the “great Nebula,” one of the finest objects in the heavens. The “great Nebula in Andromeda,” aptly termed “the Queen of the Nebulæ,” is distinctly visible to the naked eye on a very clear night. It lies near the four and a half magnitude star, Nu Andromedæ (a few degrees north of Beta Andromedæ), and may be well seen in the early evening hours in the month of January, when it is high in the sky. It somewhat resembles a small comet. This nebula was known long before the invention of the telescope, and it was described by one of the earlier astronomers as resembling “a candle shining through horn,” a not inapt description.

Of star clusters visible without optical aid may be mentioned the double cluster Chi Persei, which appears to the eye as a luminous spot in the Milky Way; the cluster known as 35 Messier, a little north of Eta Geminorum, just visible to the naked eye on a very clear night; and there are others in the Southern Hemisphere, notably the globular cluster known as Omega in the Centaur, which shines as a hazy star of the fourth magnitude. Among the clusters may perhaps be included the Præsepe, or the “Beehive,” in Cancer, which has a nebulous appearance to the naked eye.

Coming now to the Solar System, the sun and moon, of course, first attract attention. Cases of sun-spots visible to the naked eye are recorded, but, of course, spots of such enormous size are of rare occurrence. Of lunar detail little can be seen without a telescope of some sort, but the larger markings are sufficiently distinct to good eyesight to convince the observer that they do not alter perceptibly, thus showing clearly that the moon always turns the same side to the earth.

Of the planets, nothing of their appearance in the telescope can, of course, be seen with the naked eye, but it is easy to identify the brighter planets. Mercury, owing to its proximity to the sun, is rarely visible in Europe and North America, but when favorably situated, it may sometimes be detected near the sun shortly after sunset or a little before sunrise. Notwithstanding the difficulty of seeing it, it was well known to the ancients, an observation of the planet dating back to 264 B. C. It is easier, however, to see in more southern latitudes, and I have frequently observed it as bright as a star of the first magnitude in the clear air of the Punjab sky. I have also seen it on several occasions in Ireland, and the Rev. S. S. Johnson, F.R.A.S., tells me he has seen it with the naked eye no less than one hundred times in the south of England. The brilliant planet Venus can hardly be mistaken when seen in the morning or evening sky. When at its brightest it considerably exceeds Jupiter and Mars, and far surpasses Sirius, the brightest star in the heavens.

If a very bright planet is seen rising at sunset, it can not be Venus, which is never seen beyond a limited distance from the sun. The observer may, therefore, conclude with certainty that the planet is either Jupiter or Mars. The latter, which occasionally rivals Jupiter in brilliancy, may be easily distinguished from the “giant planet” by its distinctly reddish color. Saturn shines with a yellowish light, and is never so bright as Mars or Jupiter when at their brightest. The planet Uranus is just visible to the naked eye, and may be found without optical assistance when its position is accurately known.

Some observers think that they can see the crescent of Venus with the naked eye when the planet is in that phase, but this seems very doubtful. Cases have been recorded of one or two satellites of Jupiter having been seen with the unaided eyesight, but few are gifted with such keen vision.

Occultations of bright stars may be well seen with the naked eye, especially when they pass behind the moon’s dark limb, and as the disappearance of a star is practically instantaneous, really valuable observations may be made without a telescope, by merely noting the exact time at which the star vanishes.

Most of the comets discovered by astronomers are small and faint, and only visible in good telescopes. At intervals, however, a brilliant visitor appears on the scene, and its path among the stars may be watched from night to night with the naked eye. Before the invention of the telescope, bright comets were watched in this way, and their course recorded so carefully that it has been found possible to calculate their orbits with some approach to accuracy. In these days of large telescopes and instruments of almost mathematical precision, such a method of observation is, of course, superseded; but we may still watch the movements of a bright comet with interest, and note its apparent path across the sky with pleasure and profit. Shooting stars and fire-balls may be best observed with the naked eye, and the excellent work done in this way by Mr. W. F. Denning, F.R.A.S., should encourage others to take up this interesting branch of astronomy.

Another object which may be well seen with the naked eye—indeed, it may best be observed in this way—is the Zodiacal Light. This is a lenticular or cone-shaped beam of light, which makes its appearance at certain times of the year, above the eastern horizon before the dawn, and above the western horizon after sunset, when the sky is clear and the moon absent. In the tropics it is much more easily seen, the twilight being shorter, and I have often observed it in India shining with great brilliancy.

From the above sketch my readers will see how much may be learned of astronomy without optical assistance of any kind, and I hope that those who do not possess a telescope will use their eyes instead, and thus gain some knowledge of the wonders and beauties of the starry heavens. The knowledge thus gained will stimulate their curiosity and will give them keener interest in reading books which describe the still greater wonders revealed by the telescope.

FOOTNOTES:

Also known as the Dipper and Charles’s Wain.—E. S.

The Arabian names Dubhe (Alpha), Merak (Beta), Phecda (Gamma), Megrez (Delta), Alioth (Epsilon), Mizar (Zeta), and Alkaid (Eta). —E. S.

Bellatrix.

This is the nearest star to the earth.—E. S.

THE MILKY WAY.—RICHARD A. PROCTOR

To those who rightly appreciate its meaning the Milky Way is the most magnificent of all astronomical phenomena. However opinions may vary as to the configuration of the star-streams composing this object, no doubt now exists among astronomers that the Milky Way consists really of suns, some doubtless falling short of our own sun in brilliancy, but many probably surpassing it. Around these suns, we may fairly conceive, there revolve systems of dependent orbs, each supporting its myriads of living creatures. We have afforded to us a noble theme for contemplation in the consideration of the endless diversities of structure, and of arrangement, which must prevail throughout this immensity of systems.

The Galaxy traverses the constellation Cassiopeia. Thence it throws off a branch toward Alpha Persei (Mirfak), prolonged faintly toward the Pleiades. The main stream, here faint, passes on through Auriga, between the feet of Gemini and the Bull’s horns, over Orion’s club to the neck of Monoceros. Thence, growing gradually brighter, the stream passes over the head of Canis Major, in a uniform stream, until it enters the prow of Argo, where it subdivides. One stream continues to Gamma Argus, the other diffuses itself broadly, forming a fan-like expanse of interlacing branches, which terminate abruptly on a line through Lambda and Gamma Argus. Here there is a gap beyond which the Milky Way commences in a similar fan-shaped grouping, converging on the brilliant (and in other respects remarkable) star Eta Argus. Thence, it enters the Cross by a narrow neck, and then directly expands into a broad, bright mass, extending almost to Alpha Centauri. Within this mass is a singular cavity known as the Coal-Sack. At Alpha Centauri the Milky Way again subdivides, a branch running off at an angle of 20°, and losing itself in a narrow streamlet. The main stream increases in breadth, until, “making an abrupt elbow,” it subdivides into one continuous but irregular stream, and a complicated system of interlacing streams covering the region around the tail and following claw of Scorpio. A wide interval separates this part of the Galaxy from the great branch on the northern side, terminating close on Beta Ophiuchi.

The main stream, after exhibiting several very remarkable condensations, passes through Aquila, Sagitta, and Vulpecula to Cygnus. In Cygnus there is a “confused and patchy” region marked by a broad vacancy, not unlike the Coal-Sack. From this region there is thrown off the offset to Beta Ophiuchi, already mentioned; the main stream is continued to Cassiopeia.

There only remains to be noticed “a considerable offset or protuberant appendage,” thrown from the head of Cepheus directly toward the pole. Galileo was the first to prove, though earlier astronomers had entertained the notion, that the Milky Way was composed of a vast number of stars crowded closely together. But no attempt was made to offer a theory of its structure until, in 1754 Thomas Wright, in his Theory of the Universe, propounded views closely according with those entertained later by Sir W. Herschel. Wright, having examined a portion of the Galaxy with a reflecting telescope, only one foot in focal length, came to the conclusion that our sun is in the midst of a vast stratum of stars; that it is when we look along the direction in which this stratum extends that we see the zone of light constituting the Milky Way; and that as the line of sight is inclined at a greater and greater angle to the mean plane of the stratum, the apparent density of the star-grouping gradually diminishes.

But it is to Sir W. Herschel, and the supplementary labors of Sir J. Herschel, that we owe the more definite views now commonly entertained respecting the Via Lactea. The elder Herschel, whose nobly speculative views of nature were accompanied by practical common-sense, and a wonderful power of patient observation, applied to the heavens his celebrated method of gauging. He assumed as a first principle, to be modified by the results of observation, that there is a tolerable uniformity in the distribution of stars through space. Directing his twenty-foot reflector successively toward different parts of the heavens, he counted the number of stars which were visible at any single view. The field of view of this reflector was fifteen minutes in diameter, so that the portion of the sky included in any one view was less than one-fourth of that covered by the moon. He found the number of stars visible in different parts of the heavens in a field of view of this size to be very variable. Sometimes there were but two or three stars in the field; indeed, on one occasion he counted only three stars in four fields. In other parts of the heavens the whole field was crowded with stars. In the richer parts of the Galaxy as many as four hundred or five hundred stars would be visible at once, and on one occasion he saw as many as five hundred and eighty-eight. He calculated that in one-quarter of an hour 116,000 stars traversed the field of his telescope, when the richest part of the Galaxy was under observation. Now, on the assumption above named, the number of stars visible when the telescope was pointed in any given direction was a criterion of the depth of the bed of stars in that direction. Thus, by combining a large number of observations, a conception—rough, indeed, but instructive—might be formed of the figure of that stratum of stars within which our sun is situated.

Sir J. Herschel, during his residence at the Cape of Good Hope, carried out an extensive series of observations of the southern heavens. Applying his father’s methods of gauging with a telescope of equal power, he obtained a result agreeing, in a most remarkable manner, with those obtained by Sir William Herschel. It appeared, however, that the Southern Hemisphere is somewhat richer in stars than the Northern—a result which has been accepted as indicating that our system is probably somewhat nearer the southern than the northern part of the galactic nebula. Moreover, Sir J. Herschel was led to believe that the sidereal system forms a cloven flat ring rather than a disk.

I think no one who has attentively examined the glories of Orion, the richly jeweled Taurus, the singular festoon of stars in Perseus, and the closely set stars of Cassiopeia, but must have felt that the association of splendor along this streak of the heavens is not wholly accidental. The stars here seem to form a system, and a system which one can hardly conceive to be wholly unconnected with the neighboring stream of the Milky Way. But in the southern portion the arrangement is yet more remarkable and significant. From Scorpio, over the feet of the Centaur, over the keel of Argo, to Canis Major, there is a clustering of brilliant stars, which it seems wholly impossible not to connect with the background of nebulous light. It is noteworthy, also, that this stream of stars merges into the stream commencing with the group of Orion, already noticed. Nor is this all. It is impossible not to be struck by the marked absence of bright stars in the region of the heavens between Algol, Crux, and Corvus. One has the impression that the stars have been attracted toward the region of the stream indicated, so as to leave this space comparatively bare.

Now, this last circumstance would appear less remarkable if the paucity of stars here noticed were common also in parts of the heavens far removed from the Milky Way. But this is not the case. Beyond this very region, which we find so bare of stars, we come to a region in which stars are clustered in considerable density, a region including Crater, Corvus, and Virgo, with the conspicuous stars Algores, Alkes, and Spica. But what is very remarkable, while we can trace a connection between the stream of bright stars over the Milky Way and the stream of nebulous light in the background, it is obvious that the two streams are not absolutely coincident in direction.

The stream lies on one side of the Milky Way near Scorpio, crosses it in the neighborhood of Crux, and passes to the other side along Canis Major, Orion, and Taurus. Does the stream return to the Milky Way? It seems to me that there is clear evidence of a separation near Aldebaran, one branch curving through Auriga, Perseus, and Cassiopeia, the other proceeding (more nearly in the direction originally observed) through Aries (throwing out an outlier along the band of Pisces), over the Square of Pegasus, and along the streams which the ancients compared to water from the urn of Aquarius (but which in our modern maps are divided between Aquarius and Grus). The stream-formation here is very marked, as is evident from the phenomenon having attracted the notice of astronomers so long ago. But modern travels have brought within our ken the continuation of the stream over Toucan, Hydrus, and Reticulum (the two latter names being doubtless suggested by the convolutions of the stream in this neighborhood). Here the stream seems to end in a sort of double loop, and it is not a little remarkable that the Nubecula Major lies within one loop, the Nubecula Minor within the other. It is also noteworthy that from the foot of Orion there is another remarkable stream of stars, recognized by the ancients under the name of the River Eridanus, which proceeds in a sinuous course toward this same region of the Nubeculæ.