Chapter VI: Part 6

Another fine example of the globular class is 5 Messier, which lies closely north, preceding the fifth magnitude star, 5 Serpentis. It is considerably compressed at the centre. Sir William Herschel counted 200 stars, but failed to resolve the central nebulosity. Messier, its discoverer, found it visible with a telescope only one foot long.

Another fine object is 3 Messier, in Boötes. Admiral Smyth describes it as “a brilliant and beautiful globular aggregation of not less than 1,000 small stars.” It is beyond the power of small telescopes, but it was resolved by Buffham, even in the centre, with a 9-inch reflector.

Numerous fine examples of the globular class are found in the Southern Hemisphere, which indeed seems to be richer in these marvelous objects than the northern sky. Of these the most interesting are those known as Omega Centauri and 47 Toucani. Omega Centauri, from its great apparent size—about two-thirds of the moon’s diameter—and its visibility to the naked eye, may perhaps be considered as the most remarkable object of its kind in the heavens. It shines as a hazy star of the fourth magnitude, and I have often so seen it in the Punjab sky. Its large size and globular form are clearly visible in a binocular field-glass, but, of course, its component stars are far beyond the reach of such an instrument. Sir John Herschel, observing it with his large telescope at the Cape of Good Hope, found it a “truly astonishing object. All clearly resolved into stars of two magnitudes, viz., thirteen and fifteen, the larger lying in lines and ridges over the smaller;... the larger form rings like lace-work on it.” If we take the average magnitude of the components at thirteen and a half, the apparent brightness of the cluster would imply that it contains about 15,000 stars.

The other wonderful cluster is that known as 47 Toucani, which lies close to the smaller Magellanic Cloud. It is smaller in apparent size than Omega Centauri, but Dr. Gould, observing it at Cordoba, speaks of it as “one of the most impressive and perhaps the grandest of its kind in either hemisphere,” and he estimates its magnitude at four and a half, as seen with the naked eye. It is thus described by Sir John Herschel: “A most magnificent globular cluster. It fills the field with its outskirts, but within its more compressed part I can insulate a tolerably defined, circular space, of 90″ diameter, wherein the compression is much more decided, and the stars seem to run together, and this part, I think, has a pale pinkish or rose color, which contrasts, evidently, with the white light of the rest. The stars are equal, fourteen magnitude, immensely numerous and compressed.... Condensation in three distinct stages.... A stupendous object.” Sir John Herschel’s drawing of this cluster reminds one of a swarm of bees, and perhaps suggested to Tennyson the lines:

“Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”

There are other interesting specimens of the globular class in the Southern Hemisphere, but not of such large apparent dimensions as those already described. Of these may be mentioned 22 Messier, which lies about midway between the stars Mu and Sigma Sagittarii. It is described by Sir John Herschel as a fine globular cluster, with stars of two magnitudes, namely eleven or twelve, and fifteen or sixteen, the larger being visibly reddish, and he suggested that it consists of “two layers, or one shell over another.” Owing to the comparative brightness of the larger components, this cluster forms a good object for small telescopes. I saw the brighter stars well with a 3-inch refractor in the Punjab sky, but, of course, the greater portion of the cluster has a nebulous appearance in a telescope of this size.

Between Alpha and Beta Scorpii there is a condensed globular cluster. With small telescopes it very much resembles a telescopic comet, but with larger instruments its true character is revealed. Sir William Herschel considered it “the richest and most condensed mass of stars in the firmament.” In May, 1860, a “temporary star” of the seventh magnitude suddenly appeared in the centre, almost blotting out the cluster by its superior light. The star faded away before the end of June of the same year, and has not been seen with any certainty since. It has been suggested that this temporary star lay _between_ the cluster and the earth, but it seems to me much more probable that the outburst took place _in_ the cluster itself, and that it was possibly caused by a collision between two of the component stars, or by a swarm of meteors rushing with a high velocity through the cluster.

The beauty and sublimity of the spectacle presented by these globular clusters, when viewed with a powerful telescope, is such as can not be adequately described, and it has been said that when seen for the first time, “few can refrain from a shout of rapture.” The component stars, although distinctly visible as points of light, defy all attempts at counting them, and seem literally innumerable. Placed like a mass of glittering diamond-dust on the dark background of the heavens, they impress us forcibly with the idea that if each of these lucid points is a sun, the thousands which seem massed together in so small a space must be in reality either relatively close and individually small, or else the system of suns must be placed at a distance almost approaching the infinite.

The distance of these globular clusters from the earth is, however, certainly very great. Attempts to accurately determine their position in space have not been attended with success. As the component stars are at practically the same distance from the eye, we have no comparison stars to measure from, and their exact distance, therefore, remains unknown. We may, however, estimate their probable distance with some show of plausibility. We may assume that the stars of the Hercules cluster would, if concentrated in a point, shine as a star of about the fourth magnitude. As the components are of about the twelfth and thirteenth magnitudes, this would imply that the cluster consists of about 2,500 stars. With the data assumed, we may therefore conclude that the components of the Hercules cluster are suns of comparatively small size, separated by considerable distances, but apparently massed together by the effect of distance.

Among less condensed star clusters there are many interesting objects. The Pleiades have been already referred to. On a photograph of this remarkable group, taken at the Paris Observatory, over 2,000 stars can be counted of all degrees of brilliancy, from those visible without optical aid down to points of light so faint as to be invisible to the eye in the telescope with which they were photographed. Here we have a cluster of probably larger size than that in Hercules, probably at a greater distance from the earth, and with its larger components of considerably greater mass than our sun.

Near the bright star Pollux, I see a small cluster of stars of about the seventh and eight magnitudes, which, with a binocular field-glass, very much resembles the Pleiades as seen with the naked eye. A smaller cluster (known as 39 Messier) may be seen near the star Pi Cygni.

The well-known Chi Persei may be also seen with an opera-glass, but a telescope is necessary to show the component stars to advantage, and the larger the telescope the greater the number of faint stars in these wonderful objects.

The cluster known as 35 Messier, a little north of the star Eta Geminorum, is visible in an opera-glass, but a small telescope is required to see the component stars. A well-marked clustering tendency is visible among the brighter stars of the group, two, three, four, and sometimes five stars being grouped together in subordinate collections. Admiral Smyth says: “It presents a gorgeous field of stars from the ninth to the sixteenth magnitude, but with the centre of the mass less rich than the rest. From the small stars being inclined to form curves of three or four, and often with a large one at the root of the curve, it somewhat reminds one of the bursting of a sky-rocket.” This tendency to “stream” formation in the components of star clusters is also well marked in a photograph of the cluster 38 Messier (kindly sent to me by MM. Henry of the Paris Observatory). It was described by Webb as “a noble cluster arranged in an oblique cross,” and Smyth says: “The very unusual shape of this cluster recalls the sagacity of Sir William Herschel’s speculations upon the subject, and very much favors the idea of an attractive power lodged in the brightest part. For although the form is not globular, it is plainly to be seen that there is a tendency toward sphericity, by the swell of the dimensions as they draw near the most luminous part, denoting, as it were, a stream or tide of stars, setting toward the centre.”

Sir W. Herschel, speaking of a compressed cluster in Perseus, says “the large stars are arranged in lines like interwoven letters,” and Webb says “it is beautifully bordered by a brighter foreshortened pentagon.”

Observing with a 3-inch telescope in India, I noticed a beautiful cluster of stars, about 4° north of Gamma and Upsilon Scorpii, resembling in shape a bird’s foot, with remarkable streams of stars. This cluster is visible to the naked eye as a star of about the fifth magnitude.

Although these loosely associated star clusters do not show such evidence in favor of family connection as the more closely compacted globular clusters, still we can hardly escape from the conviction that their apparent aggregation is really due to some physical bond of union, and not merely the result of a fortuitous scattering of stars at different distances in the line of sight.

THE GREAT NEBULA OF ORION.—SIR ROBERT S. BALL

The telescope, ever an ally in the study of the heavens, is in this part of the science absolutely indispensable. In other branches of astronomy we can learn something without its aid. Indeed, many great astronomical discoveries were made long before the telescope was invented. But ere this memorable event in the history of science it was impossible for us to know anything of the existence of the nebulæ. It is indeed true that there is one of these objects which can be just detected by the naked eye. It lies in the constellation of Andromeda, where, on a clear and dark night, a faint spot of light can just be discerned by a good eye. But a mere glimpse gives us really no adequate notion of the true character of the object. It might only, so far as the naked eye discloses its nature, be a cluster of stars like that we have already discerned in Perseus, or like the similar group that, under the name of the Beehive, is comparatively familiar in the constellation of Cancer. With the single exception of the nebula in Andromeda, all the objects so called are entirely telescopic, yet how important a constituent the nebulæ form in the contents of the heavens will be shown by a look at some of the lists of these objects. There are now several thousands of nebulæ known, and their positions in the sky, as well as the details of their appearances, are set forth in the catalogues.

The most glorious constellation of stars in the firmament is undoubtedly that of Orion. This splendid group is seen in the south during the winter months, and toward the close of January it is situated in a very convenient position for observing early in the evening. The group is specially characterized by the number of unusually bright stars which it includes, and the three stars in the centre, forming the so-called Belt of Orion, is as well known a celestial figure as the sky contains. Directly under the belt are three much smaller stars nearly in a line, which points straight upward to the middle star of the belt. These three lower stars are usually known as the sword handle of Orion, this being the position which they occupied in the fanciful old sketches of the constellation. The three stars of the sword handle of Orion are plunged in the Great Nebula. This object can not be seen by the unassisted eye, though doubtless around the central star a little haziness is perceptible, and even the slightest telescopic aid will suffice to indicate that the central star of the sword handle is attended by a surrounding glow of light, which renders it quite unlike other stars. This can indeed be sufficiently shown with an ordinary opera-glass, one glance through which will awaken in the beholder a keen desire to study the object under more favorable conditions. But to do justice to the object, telescopes of large power are desirable.

To realize fully the magnificence of the Great Nebula, the observer who is being introduced to the object for the first time should not, strange to say, direct the telescope at the nebula; the instrument should rather be pointed at the heavens, just a little to the west of the nebula. The clock driving the equatorial should not be started, and the observer should take his seat and look through the eye-piece before the nebula has entered the field. He will see, no doubt, a few stars on the black background, which gradually pass in procession across his field of view. This is merely the ordinary diurnal journey of the heavens, by which all the objects move slowly from east to west; I ought rather to say _appear_ to move, for, of course, the motion on the heavens is only apparent, the fact being that it is the earth which is turning round.

After the observer’s eye for a minute or so has become familiarized with the dark aspect of the heavens under ordinary circumstances, he will begin to perceive on the eastern side (it will appear in the telescope no doubt as on the western side) a faint dawn of light. Gradually there will steal across his field of view a sort of ghostlike luminosity that is in marked contrast to the darkness in the rest of the field; as the seconds move on, this object will disclose itself until the full splendor of the Great Nebula comes into view; then the entire field will be filled with the light, and then it will gradually advance and gradually pass away again to emphasize the contrast between the brilliance of the nebula and the darkness of the sky. Unless this method is adopted, the full interest of a telescopic view of the Great Nebula is not attained, for when the entire field is full of the glow the beginner will hardly recognize the nebula. He will be apt to think that the fainter part of the field he sees is the ordinary groundwork of the sky, and this illusion can only be dispelled by enabling him to witness the actual contrast in the way I have described. The central portions of the nebula are, however, so brilliant and so wonderfully marked with interesting detail, that even a small instrument will suffice to reveal much of its beauties.

In the centre of the nebula is the star known to astronomers at Theta Orionis, the most prominent star of the sword handle. To the eye this looks like an ordinary star, but the telescope speedily dispels that notion. Theta Orionis is found to consist of four, or rather six, stars all so close together that the unaided eye fails to distinguish them separately. A structure so complex gives to this star quite a special, indeed a unique, interest, wholly apart from the marvelous nebula of which it is the focus. We must dwell a little on the peculiarities of this star. We are familiar with stars which are called double; there are indeed some ten thousand objects so designated known to astronomers and duly registered in catalogues.

Many of these double stars are objects of extreme telescopic beauty; sometimes they offer to our admiration a delightful contrast of colors; perhaps one will be topaz color and the other bluish, or on rare occasions a pair of emerald gems will be seen with an invisible band of mutual connection. Sometimes triple stars are found, in which three stars are obviously in alliance; but multiple stars of greater complexity are comparatively rare; and so marvelous a spectacle as Theta Orionis, in which no fewer than six stars are obviously an allied group, is almost unique. It is not a little remarkable that we find the most exquisite multiple star which the sky can show, beautifully framed or set in the centre of the grandest of the nebulæ. Of course it might conceivably happen that the apparent concourse of these objects was fortuitous. The actual phenomenon could be accounted for by the belief that the Great Nebula was either very much nearer or very much further than the multiple star, and that they chanced to lie in the same line of sight, and had no other connection. But to me it appears that this view is quite at variance with every reasonable probability; that the most wondrous multiple star should have happened to lie in line with the very centre of the most wondrous nebula would have been a coincidence against the occurrence of which the probabilities were almost infinite. There can scarcely be any doubt that the multiple star and the Great Nebula are part of the same system, and that the star is, in truth, placed in the middle of the nebula, as it actually appears to be.

And now as to the composition of this mysterious object.

The word nebula means, of course, a little cloud, but the expression is apt to be a misleading one. In a sense no doubt they are little, inasmuch as the patch of the sky which a nebula covers would be small compared with one of our ordinary clouds. Indeed, a nebula which covered as large an apparent part of the sky as the size of the moon would be ranked as a large object of its class, while even the greatest of them is perhaps not more than ten or twelve times as great. Nor is the word cloud, as applied to nebula, an appropriate one. What we mean by a cloud is only a vast mass of watery vapor raised by the sun from the sea, and poised aloft until such time as it shall be again dispersed into invisible water, or until it shall descend to the earth as rain. Such clouds are, of course, within the limits of our atmosphere, and are rarely more than a few miles above the earth’s surface. The light which renders clouds visible only comes from reflected sunbeams, and consequently at night clouds become invisible, though the astronomer is often only too unpleasantly made acquainted with their presence by the opacity with which they shut out the stars from his view.

Utterly different in all respects are the nebulæ. They are not masses of watery vapor. It may no doubt possibly be that water in some form is there, but it is not water which we see. We are looking at some gaseous material of a bluish hue. The light with which it glows is no reflected sunlight. The nebula is indeed indebted to no foreign source for that weird—I had almost said ghostlike—radiance which it gives forth. The light comes from the nebula itself. But how, it may well be asked, should a purely gaseous substance be able to radiate forth light? It is easy for us to comprehend how stars or suns or comparatively solid bodies can, in virtue of their tremendous temperature, glow with heat like red-hot or white-hot iron. It is true that flame is gas in an incandescent state, but in flame a vehement chemical union of oxygen with some other substance is in progress, and this is the source of the heat and the light that flame gives forth. We can not regard the Great Nebula in Orion as originating in anything resembling flame.

We can, however, in our physical laboratories arrange an experiment which seems to throw some light on the composition of the nebula. Into a glass tube a small quantity of hydrogen gas is admitted, the air having been previously extracted. Then, by means of two wires, one at each end of the tube, an electric current is transmitted through the gas. Here there is no combustion; the gas is merely the vehicle by which the electricity flows from one pole to the other. In doing so the gas instantly begins to glow with an intense bluish light, and a very beautiful effect is produced, which can be renewed or terminated at will by simply making or breaking the electric current. It would seem as if the gas we see in the nebula were in a condition somewhat analogous to the gas in the tube. I do not mean that the passage of electricity through the nebula is the source of its luminosity. There is, indeed, no ground for such a supposition. It is the property of electricity when passing through a conductor to warm that conductor; thus we know that if a powerful current be transmitted through a wire of the most infusible of all metals, platinum, the wire will not only get warm, but it may become red hot, white hot, and even melt under the influence of the heat which is generated. In those beautiful incandescent electric lamps which are now happily coming into extensive use a current of electricity flows through a filament of carbon, and kindles that exquisite incandescence which is maintained while the current flows. It would appear that so long as the electricity is flowing through the glass tube its action on the gas is to impart a very high temperature. It is in consequence of this temperature that the gas glows. Now we can offer a reasonable account of the luminosity of the Great Nebula in Orion. The particles of gaseous or vaporous material of which it is formed are of an extremely high temperature, sufficient to enable them to glow with the brilliancy which renders them visible.

It is now almost twenty years since a marvelous accession to our knowledge of such objects as the Great Nebula in Orion was made by Dr. Huggins. I have used our gas hydrogen as an illustration in describing the character of the nebula, but I have now to add that the presence of hydrogen is no mere fiction but a substantial verity. Truly we here open up one of the most marvelous chapters which science has to disclose. The chemist can analyze the different substances on the earth with his test tubes, and he can tell the elements of which they are composed. But in this old-fashioned chemistry it was at least reasonable for the chemist to demand a portion of the substance he was expected to analyze. Unless he were provided with a sample, how could it be possible for him to grind it up or submit it to the various operations of his laboratory? In these modern days the chemist can perform operations of which his predecessors never even dreamed. No doubt the old method is still used—nay, is indeed at this moment cultivated with greater skill and means than in any previous age—but side by side with the old method, and as an invaluable supplement thereto, the new method of chemical research, called spectrum analysis, has been created, and has already conducted to many profoundly interesting discoveries in the most varied branches of science.

In the application of the spectroscopic method it is not indispensably necessary that we actually have a fragment of the substance; all we require is a beam of light which that substance can be made to yield when heated to a sufficiently high temperature.

When a beam of the nebular light is transmitted through the prisms, it declares at once that the object from which that light has come is totally different from a star like the sun. Instead of the beautifully colored band, decked in all the glowing hues of the rainbow, the nebular beam is seen to be composed simply of six or seven widely separated strips. It is important to test the character of the light in these strips. Fortunately this can be done in a way that is completely satisfactory. We can produce artificial lights from known sources, and observe them through the spectroscope simultaneously with the light of the nebula.

There are in the composition of this globe some sixty or seventy different elementary substances, and under suitable conditions each one of these substances can afford a perfectly characteristic spectrum. Thus the way of making the comparison with the nebula is to try the different elements one after another, until one can be discovered which pours forth a light that behaves under the prism as does the light from the nebula. Pursuing this inquiry, Dr. Huggins found that when hydrogen gas was ignited to incandescence by the passage of electricity, it emitted light which, after passage through the prisms, came to coincidence with one of the lines in the spectrum of nebula; and the hydrogen character of two of the other lines has been since demonstrated. It was thus established that hydrogen is one of the constituents of the Great Nebula in Orion. Further confirmation of this important discovery was forthcoming when the photographs of the spectrum of the Great Nebula were subsequently obtained. On these photographs lines were present which are constituted by light of such a nature as to be wholly invisible to the eye, though perceptible on the photographic plate. It is of the greatest interest to discover that these invisible rays from the nebula are also indicative of the presence of hydrogen. Thus we obtain a beautiful confirmation of the fact that the nebula is partly composed of glowing hydrogen.

There are, however, some remaining lines, the character of which has not yet been ascertained.

It would be a little premature to assert that there must be some substance in the Great Nebula not at present known to us on the earth. This would be, no doubt, one interpretation of the facts. We must, however, admit the possibility of another explanation. It is frequently found that the lines yielded by an incandescent material vary to some extent when the physical conditions of temperature and of pressure are modified. It is, therefore, not impossible that the unknown lines in the spectrum of the Great Nebula may be due to some element known to us, but which has not yet been tested under the conditions which would make it yield the particular rays we are speaking of.

The composition of a nebula as disclosed to us by these researches is very instructive. Here we are looking at an object which seems to lie at the very limits of the visible universe—an object so remote that our attempts to fathom its distance are quite unsuccessful; yet in this inconceivably distant part of our system we find at least one ingredient which we know well on the earth. Previous to actual trial no one would have expected, I think, to find the Great Nebula largely constituted from such a familiar element as hydrogen. This gas enters into the composition of water, and is thus an element of extreme abundance on the earth. That an element so common with us here should also be abundant in these awfully distant regions of the universe is one of the most astonishing facts that modern science has revealed.

As the eye follows these ramifications of the Great Nebula, ever fading away in brightness until it dissolves in the blackness of the sky; as we look at the multitudes of bright stars which sparkle out from the depths of the great glowing gas; as we ponder on the marvelous outlines of a portion of the nebula, we are tempted to ask what the true magnitude of this object must really be. Here, again, we have to confess that science is unable to satisfy this very legitimate curiosity. The only means of learning the true length and breadth of a celestial object depends upon our first having discovered the distance from us at which the object is situated. Unhappily we are, as I have said, entirely ignorant of what this distance may be in the case of the Great Nebula in Orion. Our ordinary methods of conducting such an inquiry are hardly applicable to such an object, and its position so near the Equator introduces fresh difficulties into the problem. We shall, however, certainly not err on the side of exaggeration if we assert that the Great Nebula must be many millions of times larger than that group of bodies which we call the Solar System.

COLORED, DOUBLE, MULTIPLE, BINARY, VARIABLE AND TEMPORARY STARS.
—J. E. GORE

On a clear night a careful observer will notice a marked difference in the colors of the brighter stars. The brilliant white or bluish-white light of Sirius, Rigel, and Vega contrasts strongly with the yellowish color of Capella, the deeper yellow, or orange, of Arcturus, and the ruddy light of Aldebaran and Betelgeuse. These colors are, however, limited to various shades of yellow and red. No star of a _decided_ blue or green color is known, at least among those visible to the naked eye in the Northern Hemisphere. The third magnitude star Beta Libræ is described by Webb as of a “beautiful pale green hue,” but probably such a tint in the light of this star will to most people prove quite imperceptible. Dr. Gould, observing it in the Southern Hemisphere—under, of course, more favorable conditions—says: “There is a decidedly greenish tinge to the light of Beta Libræ, although its color can not properly be called conspicuous.”

Among the ruddy stars visible to the naked eye, Mu Cephei, Herschel’s “garnet star,” is generally admitted to be the reddest, but it is not sufficiently bright to enable its color to be well distinguished without the aid of an opera-glass. With such an instrument, however, its reddish hue is striking and beautiful, and very remarkable when compared with other stars in its vicinity. Like so many of the red stars, Mu Cephei is variable in its light, but seems to have no regular period, and often remains for many weeks without perceptible change. It may be seen near the zenith in the early evening hours toward the end of October, and when in this position its ruddy color is very conspicuous.

Among the brightest stars, Betelgeuse is perhaps the reddest, and the contrast between its ruddy tint and the white color of Rigel in the same constellation (Orion) is very noticeable. Like Mu Cephei, Betelgeuse is irregularly variable in its light, but not to such an extent, and, like the “garnet star,” it frequently remains for protracted periods nearly constant in brightness. There are other cases of reddish color among the naked-eye stars. Among these may be mentioned Antares (Alpha Scorpii), Alphard (Alpha Hydræ), noted as red by the Persian astronomer Al-Sûfi, in the Tenth Century, and called by the Chinese “The Red Bird”; Eta and Mu Geminorum; Mu and Nu Ursæ Majoris; Delta and Lambda Draconis; Beta Ophiuchi; Gamma Aquilæ, and others in the Southern Hemisphere.

But it is among the stars below the limit of naked-eye vision that we meet with the finest examples of the red stars. Some of these are truly wonderful objects. The small star, No. 592 of Birmingham’s Catalogue of Red Stars (No. 713 of Espin’s edition), which lies a little south of the 5½ magnitude star 79 Cygni, was described as “splendid red” by Birmingham, “very deep red” by Copeland and Dreyer, and “orange vermilion” by Franks. The star 248 Birmingham, which lies about 5° south of Gamma Hydræ, is another fine specimen. Birmingham described it as “fine red” and “ruby”; Copeland as “brown red”; Dreyer as “copper red”; and Espin as “magnificent blood red.” This star is variable in light, as the estimates of magnitude range from 6.7 to below 9. About 3° to the northeast of this remarkable object is another highly-colored star, known as R Crateris. It is easily found, as it lies in the same telescopic field of view with Alpha Crateris, a 4½ magnitude star. Sir John Herschel described it as “scarlet, almost blood-color; a most intense and curious color.” Birmingham called it “crimson”; and Webb “very intense ruby.” Observing it with a 3-inch refractor in India in 1875, I noted it as “full scarlet.” It varies in light from above the eighth magnitude to below the ninth, and has near it a star of the ninth magnitude of a paler blue tint.

Another very red star is No. 4 of Birmingham’s Catalogue, which will be found about 5° north, preceding the great nebula in Andromeda. It is of about the eighth magnitude, and may be well seen with a 3-inch refractor. Krüger describes it as “_intensiv roth_”; Birmingham as “fine red” and “crimson”; Franks as “fine color, almost vermilion”; and Espin as “intense red color, most wonderful.”

Another fine object is R Leporis, which forms roughly an equilateral triangle with Kappa and Mu Leporis. This is also variable from 6½ to 8½ magnitude. It was discovered by Hind in 1845, and described by him as “of the most intense crimson, resembling a blood-drop on the background of the sky; as regards depth of color, no other star visible in these latitudes could be compared with it.” Schönfeld called it “_intensiv blutroth_,” but Dunér, observing its spectrum in 1880, gives its color as a less intense red than that of other stars. Possibly it may vary in color as well as in light.

The variable star U Cygni, which lies between Omicron and Omega Cygni, is also very red. Webb described it as showing “one of the loveliest hues in the sky.” It varies from about the seventh to 11½ magnitude, with a period of about 461 days.

Another deeply colored star is the well-known variable R Leonis. Hind says: “It is one of the most fiery-looking variables on our list—fiery in every stage from maximum to minimum, and is really a fine telescopic object in a dark sky about the time of greatest brilliancy, when its color forms a striking contrast with the steady white light of the sixth magnitude a little to the north.” This latter star is 19 Leonis.

In the Southern Hemisphere there are some fine examples of red stars. Epsilon Crucis, one of the stars in the Southern Cross, is very red. Mu Muscæ is described by Dr. Gould as of “an intense orange red.” Delta^2 Gruis is a very reddish star of about the fourth magnitude. Pi^1 Gruis was observed by Gould as “deep crimson,” and forming a striking contrast with its white neighbor Pi^2 Gruis, which he notes as “conspicuously white.” The variable L_{2} Puppis is described as “red in all its stages, and remarkably so when faint.” Miss Clerke, observing—at the Cape of Good Hope—R Doradûs, another southern variable, says: “This extraordinary object strikes the eye with the glare of a stormy sunset,” and with reference to the variable R Sculptoris, described by Gould as “an intense scarlet,” she says: “The star glows like a live coal in the field,” a description I have found myself very applicable to other small red stars.

An eighth magnitude star about 5° north of Beta Pictoris is noted by Sir John Herschel, in his _Cape Observations_, as “vivid sanguine red, like a blood-drop. A superb specimen of its class.” With reference to a star of about 8½ magnitude in the field with Beta Crucis, Herschel says: “The fullest and deepest maroon red; the most intense blood-red of any star I have seen. It is like a drop of blood when contrasted with the whiteness of Beta Crucis.”

Of stars of other colors, the asserted green tint of Beta Libræ has already been referred to. Among the brighter stars of the Southern Hemisphere, Theta Eridani, Epsilon Pavonis, Upsilon Puppis, and Gamma Tucanæ are said to be decidedly blue. The wonderful cluster surrounding the star Kappa Crucis contains several bluish, greenish and red stars, and is described by Sir John Herschel as resembling “a superb piece of fancy jewelry.”

Among the double stars we find many examples of colored suns. Of these may be mentioned Epsilon Boötis, of which the colors are “most beautiful yellow” and “superb blue,” according to Secchi; Beta Cephei, “yellow and violet”; Beta Cygni, “golden yellow and smalt blue”; Gamma Delphini, of which I noted the colors in 1874 as “reddish yellow and grayish lilac”; Alpha Herculis, “orange and emerald or bluish green,” and described by Admiral Smyth as “a lovely object, one of the finest in the heavens”; Zeta Lyræ, “pale yellow and lilac” (Franks); and Beta Piscis Australis, of which I observed the colors in India as white and reddish lilac.

Some distant telescopic companions to red stars have been described as blue. This may be in some case due, partly at least, to the effect of contrast. In others the blue color seems to be real. This has been shown spectroscopically to be the case with the bluish companions of Beta Cygni.

The physical cause of the difference of color is still more or less a matter of mystery. Although we can not consider it proved that the red stars are cooling and “dying out” suns, as has been suggested, we may, I think, conclude that their temperature, although doubtless very high, must be lower than that of the white stars. We know that a bar of iron when heated to redness is not so hot as when raised to “white heat,” and although the analogy between hot iron and stellar photospheres may not be a perfect one, it seems probable that the higher the temperature of a star, the whiter its color will be. Most of the white stars, as Sirius, Vega, and those only yellow or slightly colored, show spectra of Secchi’s first and second types, while the great majority of the red stars exhibit banded spectra of the third and fourth types.

To this rule there are, however, like other rules, some notable exceptions. For instance, Aldebaran, Alpha Hydræ, Xi Cygni, and 31 Orionis, although distinctly reddish stars, show well-marked spectra of the second or solar type. On the other hand, Rho Ursæ Majoris and Omega Virginis, which, according to Dunér, are only slightly yellow, have well-marked spectra of the third type.

An apparent change of color seems in some cases to be well established. The supposed red color of Sirius in ancient times is well known. A certain established change is found in the case of the famous variable star Algol, which is distinctly described as red by Al-Sûfi in the Tenth Century. It is now pure white, or nearly so, and this is probably the best attested instance on record of change of color in a bright star.

Schmidt’s Nova Cygni of 1876 was noted as “golden yellow” on the night of its discovery. When it had faded to the eighth magnitude, Dr. Copeland called it “decided red,” but when examined at Lord Crawford’s observatory in September, 1877, its color was recorded as “faint blue”! The new star in the Andromeda nebula was considered to be yellowish or reddish by most observers when near its maximum, but about a month later its color was noted as “bluish.”

Among the red and variable stars, there are many suspected cases of color variation. Espin and other observers have noted that the wonderful variable Mira Ceti is much less red at maximum than at minimum. My own observations confirm this. When at its maximum brightness, Mira does not seem to me a very highly-colored star, while at one of its minima I noted it as “fiery red.” Possibly, however, the great difference between its maximum and minimum brilliancy may have an influence on estimations of its color. The remarkable variable Chi Cygni is said to be “strikingly variable in color.” Espin’s observations in different years show it “sometimes quite red, at others only pale orange red.” The star Birmingham 118 was described by Schjellerup in 1863 as “decided red,” but it was found yellow by Secchi in 1868; “bluish” by Birmingham, 1873-76; “no longer red” by Schjellerup in March, 1876; and “white” by Franks in 1885. Espin omits it from his revised edition of Birmingham’s Catalogue.

Birmingham 169 was found red by Struve, blue or bluish-white by Birmingham in 1874, and white at Greenwich in the same year. Espin also saw it white in March, 1888. The star Birmingham 30, which lies close to Phi Persei (54 Andromedæ), was described by Schweizer as a “red star with a little disk” in January, 1843; Birmingham noted it as “light red” in December, 1875; Copeland “deep red” in January, 1876; and Dreyer “reddish” in September, 1878; but Espin, in November and December, 1887, found it “certainly not red, and nothing peculiar in the star’s appearance.” It might be expected that these curious changes of color, if real, would be accompanied by corresponding changes in the star’s spectrum. Such may be the case, and observations in this direction would probably lead to some interesting results.

There seems to be some law governing the distribution of the colored stars. The white stars appear to be most numerous, as a rule, in those constellations where bright stars are most abundant, for instance, in Orion, Cassiopeia, and Lyra; yellow and orange stars in large and ill-defined constellations, such as Cetus, Pisces, Hydra, Virgo, etc. The very reddish stars are most numerous in or near the Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and Cygnus—was termed by Birmingham “the red region in Cygnus.”

Many of the stars when examined with a good telescope are seen to be double, some triple, and a few quadruple, and even multiple. These when viewed with the naked eye, or even a powerful binocular, seem to be single, and show no sign of consisting of two components. These telescopic double stars should be carefully distinguished from those which appear very close together with the naked eye, and which in opera-glasses or telescopes of small power might be mistaken for wide double stars by the inexperienced observer. These latter stars, such as Mizar—the middle star in the tail of the Great Bear—and its small companion, Alcor, have been called “naked-eye doubles,” but they are not, properly speaking, double stars at all. Telescopic double stars are far closer, and even the widest of them could not possibly be seen double without optical aid, even by those who are gifted with the keenest vision. Of these so-called “naked-eye doubles,” we may mention Alpha Capricorni, which on a very clear night may be seen with the naked eye to consist of two stars. On a very fine night two stars may be seen in Iota Orionis, the most southern star in Orion’s Sword. The star Zeta Ceti has near it a fifth magnitude star, Chi, which may be easily seen with the unaided vision. The star Epsilon Lyræ (near Vega) is a severe test for naked-eye vision. Bessel, the famous German astronomer, is said to have seen it when thirteen years of age. Omicron Cygni (north of Alpha and Delta Cygni) forms another naked-eye double, and other objects of this class may be noticed by a sharp-eyed observer.

The star Mizar, already referred to, is itself a wide telescopic double, and it seems to have been the first double star discovered with the telescope (by Riccioli in 1650). It consists of two components, of which one is considerably brighter than the other. It will give an idea of the closeness of even a “wide” telescopic double when we say that the apparent distance between Mizar and Alcor is nearly forty times the distance which separates the close components of the bright star. From this it will be seen that even a powerful binocular field-glass would fail to show Mizar as anything but a single star. The components may, however, be well seen with a 3-inch telescope, or even with a good 2-inch. The colors of the two stars are pale green and white. Between Mizar and Alcor is a star of the eighth magnitude, and others fainter. Mizar was the first double star photographed by Bond.

The Pole Star has a small companion at a little greater distance than that which separates the components of Mizar, but owing to the faintness of this small star, the object is not so easy as Mizar.

The star Beta Cygni is composed of a large and small star, of which the colors are described as “golden yellow and smalt blue.” This is a very wide double, and may be seen with quite a small telescope. Another fine double star is that known to astronomers as Gamma Andromedæ. The magnitudes of the components are about the same as those of Mizar, but a little closer. Their colors are beautiful (“gold and blue”). This is one of the prettiest double stars in the heavens. It is really a triple star, the fainter of the pair being a very close double star; but this is beyond the reach of all but the largest telescopes. The star Gamma Delphini is another beautiful object, the components being a little more unequal in magnitude, but the distance between them about the same as in Gamma Andromedæ. I have noted the colors with a 3-inch telescope as “reddish yellow and grayish lilac.” Gamma Arietis, the faintest of the three well-known stars in the head of Aries, is another fine double star, a little closer than Gamma Delphini. This is an interesting object, from the fact that it was one of the first double stars discovered with the telescope—by Hooke, in 1664, when following the comet of that year.

Another beautiful double star is Eta Cassiopeiæ, the components being about equal in brightness to those of Gamma Delphini, but the distance less than one-half. The colors are, according to Webb, yellow and purple; but other observers have found the smaller star garnet or red. This is a very interesting object, the components revolving round each other, and forming what is called a binary star.

Another fine double star is Castor, which is composed of two nearly equal stars separated by a distance about half that between the components of Gamma Andromedæ. This is also a binary, or revolving double star, but the period is long. Gamma Virginis is another fine double star, with components at about the same distance as those of Castor, and the colors very similar. It is also a remarkable binary star.

Among double stars of which the components are closer than those mentioned above, but which are within the reach of a good 3-inch telescope—a common size with amateur observers—the following may be noticed: Alpha Herculis, colors, orange or emerald green; the light of this star is slightly variable. Gamma Leonis, another binary star with a long period; colors, pale yellow and purple. Epsilon Boötis, a lovely double star, the colors of which Secchi described as “most beautiful yellow, superb blue.”

For observers in the Southern Hemisphere, the following fine double stars may be seen with a 3-inch telescope: Alpha Centauri; this famous star, the nearest of all the fixed stars to the earth, is also a remarkable binary; its period, as recently computed by Dr. See, is eighty-one years. Theta Eridani is a splendid pair, but closer than Alpha Centauri. It is, however, an easy object with a 3-inch telescope, and with a telescope of this size I noted the colors in India as light yellow and dusky yellow. The star known as ƒ Eridani is a very similar double to Theta, but the components are fainter. I noted the colors in India as yellowish-white and very light green.

Of triple, quadruple, and multiple stars, there are several which may be well seen with a small telescope. Of these may be mentioned Iota Orionis, the lowest star in the Sword of Orion, which consists of a bright star accompanied by two small companions. In Theta Orionis, the middle star of the Sword, four stars may be seen forming a quadrilateral figure, known to observers as the “trapezium.” There are two fainter stars in this curious object, which lie in the midst of the Orion nebula, but a somewhat larger telescope is required to see them. Within the trapezium are two very faint stars, which are only visible in the largest telescopes. In Sigma Orionis—a star closely south of Zeta, the lowest star in Orion’s Belt—six stars may be seen with a 3-inch telescope.

Double and multiple stars may be either optical or real. Optical double stars are those in which the component stars are merely apparently close together, owing to their being seen in nearly the same direction in space. Two stars may _seem_ to be close together, while, in reality, one of them may be placed at an immense distance behind the other. Just as two lighthouses at sea may, on a dark night, appear close together when viewed from a certain point, whereas they may be really miles apart. In the case of double stars it is, of course, always difficult to determine whether the apparent closeness of the stars is real or merely optical. But when, from a long series of observations of their relative position, we find that one is apparently moving round the other, we know that the stars must be comparatively close, and linked together by some physical bond of union. These most interesting objects are known to astronomers as binary, or revolving double stars. The probable existence of such objects was predicted from abstract reasoning by Mitchell in the Eighteenth Century; but the discovery of their actual existence was made by Sir William Herschel, while engaged on an attempt to determine the distance of some of the double stars from the earth. Unlike the planetary orbits, which are nearly circular, at least those of the larger planets of the Solar System, it is found that the orbits of these double stars differ, in many cases, widely from the circular form, in some cases, indeed, approaching in shape more the orbit of a comet than a planet.

The binary stars are among the most interesting objects in the heavens. The number now known probably amounts to nearly one thousand. In most of them, however, the motion is very slow, and in only about seventy cases has the change of position, since their discovery, been sufficient to enable an orbit to be computed.

Savary, in 1830, was the first astronomer who attempted to compute the orbit of a binary star, namely, the star Xi Ursæ Majoris. This remarkable pair was discovered by Sir William Herschel in 1780, and as the period of revolution is about sixty-one years, a considerable portion of the ellipse had been described in 1830, when it was attacked by Savary.

The binary star with the shortest period known at present seems to be the fourth magnitude star Kappa Pegasi. It was discovered as a wide double star by Sir William Herschel in 1786, the companion star being of the ninth magnitude. In August, 1880, Mr. Burnham, the famous American double star observer, examining the star with the 18½-inch refractor of the Dearborn Observatory, found the brighter star to be a very close double, with a distance between the components of only a quarter of a second of arc. A few years’ observations showed that this pair were in rapid motion round each other (about eleven years).

Another binary star, with a period of about the same length, is Delta Equulei, which was discovered to be a close double by Otto Struve in 1851. Next in order of shortness of period comes the southern binary star Zeta Sagittarii, for which an orbit was first computed in the year 1886 by the present writer. The orbit of this star will, I think, require still further revision, but the period of about eighteen years is probably not far from the truth.

Another remarkably rapid binary star is 85 Pegasi. Next in order of rapidity of motion we have the southern binary star 9 Argûs.

The star 42 Comæ Berenices has a period of about 25¾ years, according to Otto Struve. The orbit is remarkable from the fact that its plane passes through, or nearly through, the earth, and is, therefore, projected into a straight line, the companion star oscillating backward and forward on each side of its primary.

The star Beta Delphini—the most southern of the four stars in the “Dolphin’s Rhomb”—is also a fast-moving binary, discovered by Burnham in 1873. Burnham thinks the period will prove to be about twenty-eight years. The spectrum of the light of Beta Delphini is similar to that of our sun, so that the two bodies should be comparable in intrinsic brilliancy.

Another remarkable binary star with a comparatively short period is Zeta Herculis. This pair have now performed three complete revolutions since their discovery in 1782 by Sir William Herschel. Several orbits have been computed, but Dr. See’s period of thirty-five years is probably the best. The companion is, however, rather faint, being only 6½ magnitude, while the primary star is of the third.