Telescopic Work for Starlight Evenings

CHAPTER XVI.
THE STARS.

Sidereal Work.—Greek alphabet.—Learning the Names of the Stars.—The Constellation figures.—Means of Measurement.—Dividing power.—Number of Stars.—Magnitudes.—The Milky Way.—Scintillation of the Stars.—Star-Disks.—Distance of the Stars.—Proper Motion of Stars.—Double Stars and Binary Systems.—Variable Stars.—New or Temporary Stars.—Star Colours.—Groups of Stars.—Further Observations.

The planetary observer has to accept such opportunities as are given him; he must use his telescope at the particular seasons when his objects are well presented. These are limited in number, and months may pass without one of them coming under favourable review. In stellar work no such irregularities can affect the progress of observations. The student of sidereal astronomy has a vast field to explore, and a diversity of objects of infinite extent. They are so various in their lustre, in their grouping, and in their colours, that the observer’s interest is actively retained in his work, and we often find him pursuing it with unflagging diligence through many years. No doubt there would be many others employing their energies in this rich field of labour but for the uninteresting character of star-disks, which are mere points of light, and therefore incapable of displaying any detail. Those who study the Sun, Moon, or planets have a large amount of surface-configuration to examine and delineate, and this is ever undergoing real or apparent changes. But this is wholly wanting in the telescopic images of stars, which exhibit a sameness and lack of detail that is not satisfying to the tastes of every observer. True there are some beautiful contrasts of colour and many striking differences of magnitude in double stars; there are also the varying position and distance of binary systems, the curious and mysterious fluctuations in variable stars, and some other peculiarities of stellar phenomena which must, and ever will, attract all the attention that such important and pleasing features deserve. And these, it must be conceded, form adequate compensation for any other shortcomings. The observer who is led to study the stars by comparisons of colour and magnitude or measures of position, will not only find ample materials for a life-long research, but will meet with many objects affording him special entertainment. And his work, if rightly directed and accurately performed, will certainly add something to our knowledge of a branch in which he will certainly find much delectation.

Greek Alphabet.—The amateur must, at the outset of his career, thoroughly master the Greek alphabet. This will prevent many time-wasting references afterwards, and avoid the doubt and confusion that must otherwise result. The naked-eye stars in each constellation have Greek letters affixed to them on our celestial globes and star-maps.

The letters are applied progressively to the stars (generally according to brightness) in each constellation. The 1st-mag. stars frequently have a duplicate name. Thus α Leonis is also known as Regulus, and α Canis Majoris as Sirius, the Dog-star.

Learning the Names of the Stars.—A knowledge of the stars as they are presented in the nocturnal sky may be regarded as the entrance to the more advanced and difficult branches of the science, and forms the young observer’s introductory lesson. When he has learnt a few of the principal constellations, and can point them out to his friends, he already begins to feel more at home with the subject, and regards it with a different eye to what he did before when the names and configurations of the stars were alike unknown to him. He no longer views the heavens as a mysterious assemblage of confusing objects, for here and there he espies certain well-known groups always preserving the same relative positions to each other. The unconscious gaze he formerly directed to the sky has given way to the intelligent look of recognition with which he now surveys the firmament.

An acquaintance with the leading constellations, and with the names or the letters of the brighter stars in each, becomes very important in some departments of observation, and various methods have been suggested as likely to impress the positions and names on the memory. The beginner must first be content to get familiar with a few of the brighter stars, and make these the base for extending his knowledge. The objects are so numerous that it is impossible his primary attempts can be anything like complete. He must advance step by step in his survey, and feel his way cautiously, setting out from certain conspicuous stars with which he has already become conversant. A lantern and a series of star-maps are the only aids required, and with these he ought to make satisfactory progress. The stars as they are seen in the sky may be compared with those figured in the maps, and their names and the constellations in which they lie may then be identified. It is an excellent plan as conducing to fix the positions indelibly in the memory to construct maps from personal observation, and to compare these afterwards with the published maps for identification of the constituent stars. This plan, if repeated several times, has the effect of impressing the positions of the leading stars forcibly upon the observer’s mind.

It is not intended to give, in this place, any details as to the places or distribution of the stars. Without diagrams, such a description could not be made readily intelligible. To those, however, who are commencing their studies, I would recommend the northern sky as the most suitable region to aid their initiatory efforts. For

The seven bright stars of Ursa Major are familiar to nearly everyone. Two of them, called the Pointers, serve to direct the eye to the Polar star, which, though not a brilliant one, stands out prominently in a region comparatively bare of large stars. It is important to know the Polar star, as it is situated near the centre of the apparent motion of the firmament. When the student has assured himself as to the northern stars he will turn his attention southwards, and recognize the beautiful Orion and the curious groups in Taurus. He will also observe, much further east, the well-known sickle of Leo, and in time become acquainted with the many other constellations that make the winter sky so attractive.

Means of Measurement.—A micrometer becomes an indispensable instrument to those who make sidereal observations of an exact character. Without such means it will be impossible to determine either positions or distances except by mere estimation, and this is not sufficiently precise for double-star work. With a reliable micrometer53 excellent results may be obtained, especially with regard to the varying angles of binary systems. Frequent remeasurement of these is desirable for comparison with the predicted places in cases where the orbits have been computed. In this department of astronomy the names of Herschel, South, Struve, Dawes, Dembowski, Burnham, and others are honourably associated, and it is notable that refracting-telescopes have accomplished nearly the whole of the work. But reflectors are little less capable, though their powers seem to have been but rarely employed in this field. Mr. Tarrant has lately secured a large number of accurate measures with a 10-inch reflector by Calver, and if care is taken to secure correct adjustment of the mirrors, there is no reason why this form of instrument should not be nearly as effective as its rival. Mr. Tarrant advises those who use reflectors in observing double stars “to test the centering of the flat at intervals during the observations, as the slightest shift of the large mirror in its cell will frequently occasion a spurious image which, if it by chance happens to fall where the companion is expected to be seen, will often lead to the conclusion that it has been observed. In addition to this, any wings or the slightest flare around a bright star will generally completely obliterate every trace of the companion, especially if close and of small magnitude, and such defects will in nine cases out of ten be found to be due to defective adjustment. Undoubtedly for very close unequal pairs the refractor possesses great advantages over a reflector of equal aperture; in the case of close double stars the components of which are nearly equal there appears to be little, if any, difference between the two classes of instruments; while for any detail connected with the colour of stars the reflector certainly comes to the fore from its being perfectly achromatic.” These remarks from a practical man will go far to negative the disparaging statements sometimes made with regard to reflectors and stellar work, and ought to encourage other amateurs possessing these instruments to take up this branch in a systematic way.

Dividing Power.—This mainly depends upon the aperture, and it was made the subject of careful investigation and experiment by Dawes, who found that the diameters of the star-disks varied inversely as the aperture of the telescope. Adopting an inch as the standard, he ascertained that this aperture divided stars of the sixth magnitude 4″·56 apart, and on this base he constructed the following table:—

Dallmeyer, the optician, confirmed these values by remarking:—“In all the calculations I have made, I find that 4·33 divided by the aperture gives the separating power. Thus, 4·33 inches separates 1
″.” But a good deal depends upon the character of the seeing and upon other conditions. A large aperture will sometimes fail to reveal a difficult and close comes to a bright star when a smaller aperture will succeed. This is due to the position of the bright diffraction-ring, which in a large instrument may overlap the faint companion and obscure it, while in a small one the ring falls outside and the small star is visible54. Dawes concluded that “tests of separation of double stars are not tests of excellence of figure,” and he gave much valuable information with regard to micrometers and double-star observations generally in the ‘Monthly Notices,’ vol. xxvii. pp. 217-238. This paper will well repay attentive reading.

Number of Stars.—In the northern hemisphere there are about 500055 stars perceptible to the naked eye. This is less than an observer would suppose from a casual glance at the firmament, but hasty ideas are often inaccurate. The scintillation of the stars and the fact that many small stars are momentarily glimpsed but cannot be held steadily have a tendency to occasion an exaggerated estimate of their numbers. Authorities differ as to the total of naked-eye stars. Sir R. S. Ball says “the number of stars which can be seen with the unaided eye in England may be estimated at about 3000.” Gore gives 4000. Backhouse mentions 5600 as visible in the northern hemisphere. Argelander, who has charted 324,188 stars between 2° S. of the equator and the N. pole, gives the following numbers of stars up to the 9th magnitude:—

With every decrease in magnitude there is a great increase in numbers, and if this is extended to still smaller magnitudes down to the 15th or 16th we can readily understand that there exist vast multitudes of fainter stars. Paul Henry, of the Paris Observatory, says there are about 1,500,000 stars of the 11th mag., and Dr. Schönfield, of Bonn, gives 3,250,000 as of the 11½ mag. It is probable that by means of photography and the largest telescopes considerably more than 50 millions of stars may be charted, but this is a mere approximation. Roberts has photographed 16,206 stars within an area of four square degrees in a very rich region of the Galaxy near η Cygni, and Gore computes that were the distribution equal to this over the whole firmament the number of stars would reach 167 millions. He also remarks that in the Paris photographs of the Pleiades, 2326 stars are shown in a space covering about three square degrees, and this gives for the entire sky a total of 33 millions. It is, however, manifest that unusually plentiful spots in the heavens cannot be accepted as affording a criterion of the whole.

Magnitudes.—According to Argelander’s figures, above quoted, each magnitude exhibits a rise of about 300 per cent. But authorities rarely agree as to scale, as the following comparison between Sir J. Herschel and Struve will show:—

Argelander’s magnitudes come between those of Herschel and Struve. Such disagreements are perplexing to observers, and it is fortunate that in regard to the naked-eye stars we are now furnished with a more consistent and accurate series of magnitudes. Photometric determinations of the light of 4260 stars not fainter than the 6th mag., and between the N. pole and 30° S. declination, were made at Harvard College Observatory, and similar measures of 2784 stars between the N. pole and 10° S. declination were effected at the Oxford University Observatory, and the results published in 1885. The two catalogues are in very satisfactory agreement, the accordances within one tenth of a mag. being 31 per cent., within one quarter of a mag. 71 per cent., and within one third of a magnitude 95 per cent. The photometers used in the two independent researches were constructed on very different principles, and the substantial agreement in the results indicates that “a great step has been accomplished towards an accurate knowledge of the relative lustre of the stars” (‘Monthly Notices,’ vol. xlvi. p. 277).

The Milky Way.—On dark nights when the Moon is absent and the air clear, a broad zone of glimmering, filmy material is seen to stretch irregularly across the heavens. It may be likened to a milky river running very unevenly amongst the constellations, and showing many curves and branches along its course. On very favourable occasions the unaided eye glimpses many hundreds of glittering points on this light background. A field-glass reveals some thousands, and shows that it is entirely composed of stars the blended and confused lustre of which occasions that track of whiteness which is so evident to the eye. In a good telescope stars and star-dust exist in countless profusion, and great diversity is apparent in their numbers and manner of grouping. In certain regions the stars are concentrated into swarms, and the sky is aglow with them; while in others there are very few, and dark cavernous openings offer a striking contrast to the silvery sheen of surrounding stars. There are many of these void spaces in Scorpio, and a circular one in Sagittarius R.A. 17^h 56^m, Dec.-27° 51´ has been particularly remarked. These inequalities of grouping may be easily recognized with the naked eye, especially in Cygnus, where bright star-lit regions frequently alternate with dark void spaces. In the southern sky there is a noteworthy instance. Near the brilliant stars of Crux and Centaurus and closely surrounded by the Milky Way there is a large black vacancy very obvious at a glance, and so striking to ordinary observers that it is known as the “Coal-sack,” a name applied to it by the early navigators of the southern seas.

The course of the Milky Way may be described generally as flowing through Auriga, the club of Orion, feet of Gemini, western part of Monoceros, Argo Navis, Crux, feet of Centaurus, Circinus, Ara, where it separates into two branches, the western of which traverses the northern part of the tail of Scorpio, eastern side of Serpens, Taurus Poniatowski, Anser, and Cygnus. The eastern branch crosses the tail of Scorpio, the bow of Sagittarius, Antinous, Aquila, Vulpecula, and then enters Cygnus, where it reunites with the other branch. It thence passes through Cepheus, Cassiopeia, Perseus, and enters Auriga. In breadth it varies greatly, being in some places only 4° or 5°, whereas in others it reaches 20°. It is, of course, best visible when twilight is absent, but it is sometimes very plain, even at midsummer, for at this season some of its more conspicuous sections are favourably placed for observation. It is supposed that fully nine tenths of the total number of stars in the firmament are included within the borders, of the Milky Way.

Some of the ancient philosophers, including Democritus, formed just conceptions as to the real nature of this appearance. Though they lacked instruments wherewith to observe the stars forming it, they yet saw them with the eye of reason. But very vague and incorrect notions prevailed in early times, when superstition was rife, as to many celestial phenomena. Some of the ancient poets and learned men refer to the Galaxy as the path by which heroes ascended to heaven. Thus we read in Ovid:—

Scintillation of the Stars.—The rapid variations of light known as the “twinkling” of the stars received notice from many ancient observers, including Aristotle, Ptolemy, and others, and they severally endeavoured to account for it, but not in a manner altogether satisfactory. At low altitudes bright stars exhibit this twinkling or scintillation in a striking degree, but it is much less perceptible in stars placed at considerable elevations. Sirius, the brightest star in the sky, is a noted twinkler. His excessive lustre and invariably low position are conditions eminently favourable to induce this effect. But the planets seldom exhibit scintillation in a very marked degree. The light of Jupiter and Saturn is steady, even when these planets are close to the horizon. Mercury, however, twinkles most obviously, and Venus and Mars, when low down, are often similarly affected, especially in stormy weather when the air is much disturbed. Hooke, in 1667, concluded that the scintillation was due “to irregular refractions of the light of the stars by differently heated layers of atmosphere.” M. Arago said it arose “from the peculiar properties possessed by the constituent rays of light, of moving with different velocities through the strata of the atmosphere, and of producing what are called interferences.” More recently, M. Montigney has conducted some interesting researches into this subject, and he believes “that not only is twinkling caused, to a great extent, by the deviations of portions of a star’s light altogether away from us by variable layers of atmosphere, but it is also affected, both in frequency and in the colours displayed, by the nature of the light emitted by the individual star.” The planets are little subject to scintillation, as they present disks of sensible size, and thus are enabled to neutralize the effect of atmospheric interferences. It is curious, however, that the steadiness of telescopic images does not appear to be much improved at high altitudes, and that the phenomenon of scintillation still operates powerfully as observed from mountainous stations. In February 1888, Dr. Pernter, of the Vienna Academy of Sciences, found “that the scintillation of Sirius was actually greater at the top of Sonnblick, 10,000 feet high, than it was at the base of the mountain, and he formed the opinion that scintillation has its origin in the upper strata of the atmosphere and not in the lower as usually assumed.” It would appear from this that lofty situations do not possess all the advantages claimed for them in regard to the employment of large telescopes.

Star-Disks.—The stars as observed in telescopes are shorn of the false rays apparent to the naked eye, and they are seen with small spurious disks. That the disks are spurious is evident from the fact that the larger the telescope employed, the smaller the star-disks become. And moreover, when a star is occulted by the Moon, it disappears instantaneously. There is no gradual diminution of lustre; the star vanishes with great suddenness. Bright stars, like Aldebaran or Regulus, have been watched up to the Moon’s limb, and observers have been sometimes startled at the abruptness with which they were blotted out. An appreciable disk could not be withdrawn in this instantaneous manner; it would exhibit a perceptible decadence as the Moon increasingly overlapped it, but no such appearance is observed. On the occasion of the occultation of Jupiter on Aug. 7, 1889, the planet’s diameter was 41″·4, and the disappearance occupied 85 seconds. Now had Aldebaran or Regulus a real disk of only 1″ it would prevent their sudden extinctions, and their disappearances would be spread over perceptible though short intervals of time56. But there is every reason to conclude that the actual disks are to be represented by a small fraction of 1″, so that the largest instrument and the highest powers fail to reveal it. In this connection, Mr. Gore, in his ‘Scenery of the Heavens,’ p. 152, says:—“Let us take the case of α Centauri, which is, as far as is known at present, the nearest fixed star to the Earth. The distance of this star is about 25 billions of miles. From comparisons made between its light and the Moon, it has been found that its intrinsic brilliancy must be about four times that of the Sun. Supposing its greater lustre is due to its greater size—a not improbable supposition—it would subtend, if placed at the Sun’s distance, an angle twice as great, or about 1°, and hence we find that the angle subtended at its distance of 25 billions of miles would be about 1/76th of a second of arc, which the most powerful telescope yet constructed would be incapable of showing as a visible disk.”

Distance of the Stars.—The distances of the outer planets Uranus and Neptune, mentioned in an earlier chapter of this work, are sufficiently large to amaze us; but the distances of the stars may be said to be relatively infinite. For many years the problem of stellar distances defied all attempts to resolve it. At length, in 1838-39, Bessell, Henderson, and Struve obtained results for three stars—viz. 61 Cygni, α Centauri, and α Lyræ,—which practically settled the question. More recent measures of stellar parallax, while correcting the earlier values, have virtually corroborated them; though the figures adopted can only be regarded as approximations, owing to the difficult and delicate nature of the work. The binary star α Centauri appears to be the nearest of all; it has a parallax of 0″·75, and its distance from us is equal to 275,000 times the distance of the Sun. Light traversing space at the rate of 187,000 miles per second would occupy 4-1/3 years in crossing this interval. In the Northern hemisphere 61 Cygni is the nearest star, with a parallax of 0″·44 and a distance of about 470,000 times the Sun’s distance. Light would take more than seven years in reaching us from this star, α Lyræ has a parallax of 0″·15, equal to nearly 22 light-years. α Crucis shows a very small parallax (0″·03), and its distance is excessively remote—equal to about 108 light-years!

Proper Motion of Stars.—A very slight motion affects the places of many of the so-called fixed stars. This must, after the lapse of long intervals of time, materially alter the configuration of the constellations. But the change is a very gradual one, and must operate through many centuries before its effects will become appreciable in a general way. The greatest proper motion yet observed is that in regard to two small stars (one in Ursa Major and the other in Piscis Australis), which amounts to about 7″ annually. Another motion has been recognized, viz. in the line of sight. Dr. Huggins made the initiatory efforts in this research by measuring the displacement of the F line in the spectrum of Sirius. The work has been actively pursued at the observatories of Greenwich and Rugby, and with interesting results. While certain stars exhibit a motion of approach, others display a motion of recession. Thus Vega, Arcturus, and Pollux are approaching us at the rate of about 40 miles per second; while Rigel is receding at the rate of 17 miles per second, Castor at the rate of 19, Regulus 14, Betelgeuse 25, and Aldebaran 31. Sirius, in the years from 1875 to 1878, was receding from us at the rate of 22 miles per second; but this decreased in subsequent years, and in 1884-85 the star was approaching with a motion of about 22 miles per second. In 1886 and 1887 this rate was increased to about 30 miles per second, as observed both at Greenwich and Rugby. This confirms the idea that Sirius is moving in an elliptical orbit. Similar observations, in regard to the variable star Algol, have revealed that changes of velocity are connected with its changes of lustre. Before minimum the star recedes at the rate of 24½ miles per second, while after minimum the star approaches with a speed of 28½ miles per second (‘Monthly Notices,’ vol. 1. p. 241).

Double Stars and Binary Systems.—Telescopic power will often reveal two stars where but one is seen by the naked eye. Sometimes the juxtaposition of such stars is merely accidental; though they are placed nearly in the same line of sight the conjunction is an optical one only, and no connection apparently subsists between them. In other cases, however, pairs are found which have a physical relation, for one is revolving round the other; and these are termed binary stars. Sir W. Herschel was the first to announce them, from definite observations, in 1802. Of double stars more than 10,000 are now known; many of these are telescopic, but the list includes some fine examples of naked-eye stars.

The determination of the angles of position and distance of double stars forms a very important and extensive branch of work in connection with sidereal astronomy. In cases where double stars preserve stationary places relatively to each other, there is clearly no need for frequent re-observation. But in those numerous instances where the two components form a binary system it is desirable to obtain as many measures as possible, so as either to verify the calculated orbit or to furnish the materials for an orbit if one has not been computed before. Dr. Doberck, whose name is well known in these researches, mentioned, in 1882, that ample data for purposes of computation had not been secured for more than thirty or forty binaries out of between five and six hundred such systems that were probably known to exist. Sir W. Herschel, in 1803, estimated the period of revolution of α Geminorum as 342 yrs. 2 mths. and of γ Virginis as 1200 yrs. Orbits57 do not appear, however, to have been computed until 1827, when Savery of Paris showed that the companion of ξ Ursæ Majoris was revolving in an ellipse with a period of 58-1/4 years. The accomplished Encke also turned his attention to this work, and adopted a more elaborate method; and many others have pursued the subject with very interesting and valuable results. On pp. 302-305 is a selected list of some of the most noteworthy double and binary stars, arranged according to the distance between the components.

In compiling the above list, I have used some of the latest measures available, as most of these doubles are binary systems, and therefore in motion, so that a few years effect a perceptible difference in the angles of position and distance of the components. Some of the pairs are closing up, others are opening, and thus it happens that a binary star, divided with great difficulty to-day, may become an easy object some years hence, and vice versâ. In fact, as telescopic tests they are constantly varying.

Before leaving this part of the subject it may be interesting to refer individually to a few brilliant examples of double stars.

α Canis Majoris (Sirius). A red star according to ancient records, but it is now intensely white. In 1844 Bessel inferred from certain little irregularities in the proper motion of this star that it consisted of a binary system with a period of about half a century58. Peters confirmed this idea in 1851, and it was observationally verified eleven years afterwards. On Jan. 31, 1862, Alvan Clark, jun., while testing a new 18½-inch refractor, discovered a very faint companion 10″ distant. Measures in the few subsequent years proved that the position-angle was decreasing, while the distance showed a slight extension. In 1872 it was about 11″·50, but since then the two stars have been approaching each other, and Mr. Burnham’s measures in April 1890 gave the distance as only 4″·19. It is now, therefore, a very difficult object, and only visible in large instruments. In England it is never easy, owing to its southern position, and it has been little observed, but it is satisfactory to note that the large refractors at Washington, Princeton, and Chicago, U.S.A., have been often employed on this object in recent years. Mann gives a period of 51·22 years for this interesting binary, and places the time of periastron-passage as 1890·55. This differs from Gore’s orbit, quoted in the table.

β Orionis (Rigel). A favourite test-object for small instruments. The companion has been seen with only 1½-inch aperture by experienced observers familiar with the object, and accustomed to its appearance in larger telescopes. The beginner may, however, esteem himself fortunate if he distinguishes the smaller star with 3 inches of aperture. When he has done this he may afterwards succeed with 2½ inches only, and quite possibly with 2 inches. He can ascertain his ability in this direction by inserting cardboard diaphragms of the diameters referred to in the dew-cap of his telescope. This object is not a binary; the component stars are fixed relatively to each other, and merely form an optical double. The colours are pale yellow and sapphire blue. Burnham thought the smaller star was elongated, as though a very close double, but the 36-inch at Mount Hamilton has disproved the idea.

α Lyræ (Vega). Another well-known object, and one upon which amateurs are constantly testing their means. The companion star is extremely faint, and small instruments would have no chance with it but for its comparatively wide distance from Vega. Were it much nearer it would be obliterated in the glare. This is a more difficult pair than that of Rigel, though certain lynx-eyed observers have glimpsed the minute star with ridiculously small apertures. It is no mean feat, however, to discern the star with a 3-inch telescope. Webb saw it more easily with a power of 80 than with 144 on a 3-7/10-inch. There are many other stars in the same field, though more distant than the companion alluded to. With power 60 on my 10-inch reflector, I counted eighteen stars in the field with Vega on Oct. 9, 1889, though the full Moon was shining at the time. Several faint stars have been alleged to exist much closer to Vega than the well-known comes; but these have resisted the great American refractors, and it may be safely assumed that they were ghosts produced by a faulty image.

α Ursæ Minoris (Polaris). This double, from its constant visibility in northern latitudes, from its unvarying brightness, and from the relatively stationary positions of the stars composing it, forms an excellent test for small instruments. But it is a comparatively easy object, and ought to be seen in a 2-inch telescope. With this aperture the primitive efforts of a young observer will probably be disappointing. If possible he should first look at the pair through a 3-or 4-inch, and then he will know exactly what he may expect to see with inferior means. A difficult object is often readily glimpsed in a small telescope after the eye has become acquainted with it in a larger one. Experience of this kind is very requisite, and it is by thus educating the eye that observers are gradually enabled to reach objects which appeared hopelessly beyond them at their first attempts. The companion to Polaris, like that of Rigel and Vega, though situated in nearly the same line of sight is not physically related to the larger star, the contiguity of the objects being accidental. Some dubious observations have been made of comites nearer to Polaris than the one to which we have been adverting; but Burnham does not see these, and we are forced to conclude that they have no objective existence.

α Scorpii (Antares). A fiery-red star, with a rather close, faint companion. This object being in 26° of S. declination is rarely seen with advantage in places with latitudes far north. Atmospheric disturbance usually affects the image in such degree that the smaller star is merged in the contortions of the larger. This pair is, however, interesting from the circumstance that it is frequently liable to occultation by the Moon. A night on which this double star can be distinctly seen may be regarded as an exceptional one in point of definition. It appears to have been discovered nearly half a century ago by Grant and Mitchel.

Variable Stars.—A proportion of the stars exhibit fluctuations in their visible brightness. In most cases, however, the variation is but slight, though there are instances in which the differences are considerable. The fluctuations are periodical in nature and capable of being exactly determined. But the character of the variation and the period are very dissimilar in different stars. Some are of short period, and their variations occupy a few days only; others, however, are more gradual, and twelve months or more may represent the complete cycle of their changes. These alterations of brightness generally escape the notice of casual observers of the heavens. To them the stars appear as constant in their relative magnitudes as they are in their relative positions. But a close observer of the firmament, who habitually watches and records the comparative lustre of the stars, must soon discover numerous evidences of change. He will remark certain stars which alternately grow bright and faint, and, in fact, display a regular oscillation of brilliancy. In the case of a pair of stars he may possibly notice that the superior lustre is emitted first by one and then by the other. The observation of these variables dates from a period anterior to the invention of the telescope. Nearly three centuries ago Fabricius remarked that ο Ceti (Mira) suffered a great diminution of light; for while it was of the 3rd mag. in Aug. 1596, it became invisible in the following autumn. It was re-observed by Holwarda in 1639, and as he appears to have been the first to estimate its period, some authors, including Argelander, have credited him with the discovery. The star has a period of about 331·3 days. Its variations are somewhat erratic, for at max. it is sometimes only 4th mag., while at others it is as bright as 2nd mag., and its min. are equally inconsistent.