NebulÆ and Clusters

In his interesting and valuable work on “The Stars,” the late Prof. Newcomb said—

Prof. Perrine thinks that the total number of the nebulÆ will ultimately be found to exceed a million.[352]

Dr. Max Wolf has discovered a number of small nebulÆ in the regions near Algol and Nova Persei (the great “new star” of 1901). He says, “They mostly lie in two bands,” and are especially numerous where the two bands meet, a region of 12 minutes of arc square containing no less than 148 of them. They are usually “round with central condensation,” and form of Andromeda nebula.[353]

Some small nebulÆ have been found in the vicinity of the globular clusters. They are described by Prof. Perrine as very small and like an “out of focus” image of a small star. “They appear to be most numerous about clusters which are farthest from the galaxy.” Prof. Perrine says, “Practically all the small nebulÆ about the globular clusters are elliptical or circular. Those large enough to show structure are spirals. Doubtless the majority of these are spirals.”[354] This seems further evidence in favour of the “spiral nebular hypothesis” of Chamberlin and Moulton.

A great photographic nebula in Orion was discovered by Prof. Barnard in 1894. In a drawing he gives of the nebula,[355] it forms a long streak beginning a little south of ? Orionis (Bellatrix), passing through the star 38 Orionis north of 51 and south of 56 and 60 Orionis. Then turning south it sweeps round a little north of ? Orionis; then over 29 Orionis, and ends a little to the west of ? Orionis. There is an outside patch west of Rigel. Barnard thinks that the whole forms a vast spiral structure; probably connected with the “great nebula” in the “sword of Orion,” which it surrounds.From calculations of the brightness of surface (“intrinsic brightness”) of several “planetary” nebulÆ made by the present writer in the year 1905, he finds that the luminosity is very small compared with that of the moon. The brightest of those examined (h 3365, in the southern hemisphere, near the Southern Cross) has a surface luminosity of only ¹/400 of that of the moon.[356] The great nebulÆ in Orion and Andromeda seem to have “still smaller intrinsic brightness.”

Arago says—

Commenting on this remark of Arago, the late Herbert Spencer says—

With reference to the small elongated nebula discovered by Miss Caroline Herschel in 1783 near the great nebula in Andromeda, Admiral Smyth says, “It lies between two sets of stars, consisting of four each, and each disposed like the figure 7, the preceding group being the smallest.”[359]

Speaking of the “nebula” Messier 3—a globular cluster in Canes Venatici—Admiral Smyth says, “This mass is one of those balls of compact and wedged stars whose laws of aggregation it is so impossible to assign; but the rotundity of the figure gives full indication of some general attractive bond of union.”[360] The terms “compact and wedged” are, however, too strong, for we know that in the globular clusters the component stars must be separated from each other by millions of miles!

Prof. Chamberlin suggests that the secondary nebula (as it is called) in the great spiral in Canes Venatici (Messier 51) may possibly represent the body which collided with the other (the chief nucleus) in a grazing collision, and is now escaping. He considers this secondary body to have been “a dead sun”—that is, a dark body.[361] This would be very interesting if it could be proved. But it seems to me more probable that the secondary nucleus is simply a larger portion of the ejected matter, which is now being gradually detached from the parent mass.

Scheiner says “the previous suspicion that the spiral nebulÆ are star clusters is now raised to a certainty,” and that the spectrum of the Andromeda nebula is very similar to that of the sun. He says there is “a surprising agreement of the two, even in respect to the relative intensity of the separate spectral regions.”[362]

In the dynamical theory of spiral nebulÆ, Dr. E. J. Wilczynski thinks that the age of a spiral nebula may be indicated by the number of its coils; those having the largest number of coils being the oldest, from the point of view of evolution.[363] This seems to be very probable.

In the spectrum of the gaseous nebulÆ, the F line of hydrogen (H) is visible, but not the C line (Ha). The invisibility of the C line is explained by Scheiner as due to a physiological cause, “the eye being less sensitive to that part of the spectrum in which the line appears than to the part containing the F line.”[364]

An apparent paradox is found in the case of the gaseous nebulÆ. The undefined outlines of these objects render any attempt at measuring their parallax very difficult, if not impossible. Their distance from the earth is therefore unknown, and perhaps likely to remain so for many years to come. It is possible that they may not be farther from us than some of the stars visible in their vicinity. On the other hand, they may lie far beyond them in space. But whatever their distance from the earth may be, it may be easily shown that their attraction on the sun is directly proportioned to their distance—that is, the greater their distance, the greater the attraction! This is evidently a paradox, and rather a startling one too. But it is nevertheless mathematically true, and can be easily proved. For, their distance being unknown, they may be of any dimensions. They might be comparatively small bodies relatively near the earth, or they may be immense masses at a vast distance from us. The latter is, of course, the more probable. In either case the apparent size would be the same. Take the case of any round gaseous nebula. Assuming it to be of a globular form, its real diameter will depend on its distance from the earth—the greater the distance, the greater the diameter. Now, as the volumes of spheres vary as the cubes of their diameters, it follows that the volume of the nebula will vary as the cube of its distance from the earth. As the mass of an attracting body depends on its volume and density, its real mass will depend on the cube of its distance, the density (although unknown) being a fixed quantity. If at a certain distance its mass is m, at double the distance (the apparent diameter being the same) it would have a mass of eight times m (8 being the cube of 2), and at treble the distance its mass would be 27 m, and so on, its apparent size being known, but not its real size. This is obvious. Now, the attractive power of a body varies directly as its mass—the greater the mass, the greater the attraction. Again, the attraction varies inversely as the square of the distance, according to the well-known law of Newton. Hence if d be the unknown distance of the nebula, we have its attractive power varying as d³ divided by d², or directly as the distance d. We have then the curious paradox that for a nebula whose distance from the earth is unknown, its attractive power on the sun (or earth) will vary directly as the distance—the greater the distance the greater the attraction, and, of course, conversely, the smaller the distance the less the attractive power. This result seems at first sight absurd and incredible, but a little consideration will show that it is quite correct. Consider a small wisp of cloud in our atmosphere. Its mass is almost infinitesimal and its attractive power on the earth practically nil. But a gaseous nebula having the same apparent size would have an enormous volume, and, although probably formed of very tenuous gas, its mass would be very great, and its attractive power considerable. The large apparent size of the Orion nebula shows that its volume is probably enormous, and as its attraction on the sun is not appreciable, its density must be excessively small, less than the density of the air remaining in the receiver of the best air-pump after the air has been exhausted. How such a tenuous gas can shine as it does forms another paradox. Its light is possibly due to some phosphorescent or electrical action.

The apparent size of “the great nebula in Andromeda” shows that it must be an object of vast dimensions. The nearest star to the earth, Alpha Centauri, although probably equal to our sun in volume, certainly does not exceed one-hundredth of a second in diameter as seen from the earth. But in the case of the Andromeda nebula we have an object of considerable apparent size, not measured by seconds of arc, but showing an area about three times greater than that of the full moon. The nebula certainly lies in the region of the stars—much farther off than Alpha Centauri—and its great apparent size shows that it must be of stupendous dimensions. A moment’s consideration will show that whatever its distance may be, the farther it is from the earth the larger it must be in actual size. The sun is vastly larger than the moon, but its apparent size is about the same owing to its greater distance. Sir William Herschel thought the Andromeda nebula to be “undoubtedly the nearest of all the great nebulÆ,” and he estimated its distance at 2000 times the distance of Sirius. This would not, however, indicate a relatively near object, as it would imply a “light journey” of over 17,000 years! (The distance of Sirius is about 88 “light years.”)

It has been generally supposed that this great nebula lies at a vast distance from the earth, possibly far beyond most of the stars seen in the same region of the sky; but perhaps not quite so far as Herschel’s estimate would imply. Recently, however, Prof. Bohlin of Stockholm has found from three series of measures made in recent years a parallax of 0·17.[365]

This indicates a distance of 1,213,330 times the sun’s distance from the earth, and a “light journey” of about 19 years. This would make the distance of the nebula more than twice the distance of Sirius, about four times the distance of a Centauri, but less than that of Capella.Prof. Bohlin’s result is rather unexpected, and will require confirmation before it can be accepted. But it will be interesting to inquire what this parallax implies as to the real dimensions and probable mass of this vast nebula. The extreme length of the nebula may be taken to represent its diameter considered as circular. For, although a circle seen obliquely is always foreshortened into an ellipse, still the longer axis of the ellipse will always represent the real diameter of the circle. This may be seen by holding a penny at various angles to the eye. Now, Dr. Roberts found that the apparent length of the Andromeda nebula is 2? degrees, or 8400 seconds of arc. The diameter in seconds divided by the parallax will give the real diameter of the nebula in terms of the sun’s distance from the earth taken as unity. Now, 8400 divided by 0·17 gives nearly 50,000, that is, the real diameter of the Andromeda nebula would be—on Bohlin’s parallax—nearly 50,000 times the sun’s distance from the earth. As light takes about 500 seconds to come from the sun to the earth, the above figures imply that light would take about 290 days, or over 9 months to cross the diameter of this vast nebula.

Elementary geometrical considerations will show that if the Andromeda nebula lies at a greater distance from the earth than that indicated by Bohlin’s parallax, its real diameter, and therefore its volume and mass, will be greater. If, therefore, we assume the parallax found by Bohlin, we shall probably find a minimum value for the size and mass of this marvellous object.

Among Dr. Roberts’ photographs of spiral nebulÆ (and the Andromeda nebula is undoubtedly a spiral) there are some which are apparently seen nearly edgeways, and show that these nebulÆ are very thin in proportion to their diameter. From a consideration of these photographs we may, I think, assume a thickness of about one-hundredth of the diameter. This would give a thickness for the Andromeda nebulÆ of about 500 times the sun’s distance from the earth. This great thickness will give some idea of the vast proportions of the object we are dealing with. The size of the whole solar system—large as it is—is small in comparison. The diameter and thickness found above can easily be converted into miles, and from these dimensions the actual volume of the nebula can be compared with that of the sun. It is merely a question of simple mensuration, and no problem of “high mathematics” is involved. Making the necessary calculations, I find that the volume of the Andromeda nebula would be about 2·32 trillion times (2·32 × 10¹⁸) the sun’s volume! Now, assuming that the nebulous matter fills only one-half of the apparent volume of the nebula (allowing for spaces between the spiral branches), we have the volume = 1·16 × 10¹⁸. If the nebula had the same density as the sun, this would be its mass in terms of the sun’s mass taken as unity, a mass probably exceeding the combined mass of all the stars visible in the largest telescopes! But this assumption is, of course, inadmissible, as the sun is evidently quite opaque, whereas the nebula is, partially at least, more or less transparent. Let us suppose that the nebula has a mean density equal to that of atmospheric air. As water is about 773 times heavier than air, and the sun’s density is 1·4 (water = 1) we have the mass of the nebula equal to 1·16 × 10¹⁸ divided by 773 × 1·4, or about 10¹⁵ times the sun’s mass, which is still much greater than the probable combined mass of all the visible stars. As it seems unreasonable to suppose that the mass of an individual member of our sidereal system should exceed the combined mass of the remainder of the system, we seem compelled to further reduce the density of the Andromeda nebula. Let us assume a mean density of, say, a millionth of hydrogen gas (a sufficiently low estimate) which is about 14·44 times lighter than air, and we obtain a mass of about 8 × 10⁷ or 80 million times the mass of the sun, which is still an enormous mass.

As possibly I may have assumed too great a thickness for the nebula, let us take a thickness of one-tenth of that used above, or one thousandth of the length of the nebula. This gives a mass of 8 million times the sun’s mass. This seems a more probable mass if the nebula is—as Bohlin’s parallax implies—a member of our sidereal system.

If we assume a parallax of say 0·01—or one-hundredth of a second of arc—which would still keep the nebula within the bounds of our sidereal system—we have the dimensions of the nebula increased 17 times, and hence its mass nearly 5000 times greater (17³) than that found above. The mass would then be 40,000 million times the sun’s mass! This result seems highly improbable, for even this small parallax would imply a light journey of only 326 years, whereas the distance of the Milky Way has been estimated by Prof. Newcomb at about 3000 years’ journey for light.

In Dr. Roberts’ photograph many small stars are seen scattered over the surface of the nebula; but these do not seem to be quite so numerous as in the surrounding sky. If the nebula lies nearer to us than the fainter stars visible on the photograph, some of them may be obscured by the denser portions of the nebula; some may be visible through the openings between the spiral branches; while others may be nearer to us and simply projected on the nebula.

To add to the difficulty of solving this celestial problem, the spectroscope shows that the Andromeda nebula is not gaseous. The spectrum is, according to Scheiner, very similar to that of the sun, and “there is a surprising agreement of the two, even in respect to the relative intensities of the separate spectral regions.”[366] He thinks that “the greater part of the stars comprising the nucleus of the nebula belong to the second spectral class” (solar), and that the nebula “is now in an advanced stage of development. No trace of bright nebular lines are present, so that the interstellar space in the Andromeda nebula, just as in our stellar system, is not appreciably occupied by gaseous matter.”[366] He suggests that the inner part of the nebula [the “nucleus”] “corresponds to the complex of those stars which do not belong to the Milky Way, while the latter corresponds to the spirals of the Andromeda nebula.”[366] On this view of the matter we may suppose that the component particles are small bodies widely separated, and in this way the mean density of the Andromeda nebula may be very small indeed. They cannot be large bodies, as the largest telescopes have failed to resolve the nebula into stars, and photographs show no sign of resolution.

It has often been suggested, and sometimes definitely stated, that the Andromeda nebula may possibly be an “external” universe, that is an universe entirely outside our sidereal system, and comparable with it in size. Let us examine the probability of such hypothesis. Assuming that the nebula has the same diameter as the Milky Way, or about 6000 “light years,” as estimated by Prof. Newcomb, I find that its distance from the earth would be about 150,000 “light years.” As this is about 8000 times the distance indicated by Bohlin’s parallax, its dimensions would be 8000 times as great, and hence its volume and mass would be 8000 cubed, or 512,000,000,000 times greater than that found above. That is, about 4 trillion (4 × 10¹⁸) times the sun’s mass! As this appears an incredibly large mass to be compressed into a volume even so large as that of our sidereal system, we seem compelled to reject the hypothesis that the nebula represents an external universe. The sun placed at the distance corresponding to 150,000 light years would, I find, shine as a star of less than the 23rd magnitude, a magnitude which would be invisible in the largest telescope that man could ever construct. But the combined light of 4 trillion of stars of even the 23rd magnitude would be equal to one of minus 23·5 magnitude, that is, 23½ magnitude brighter than the zero magnitude, or not very much inferior to the sun in brightness. As the Andromeda nebula shines only as a star of about the 5th magnitude the hypothesis of an external universe seems to be untenable.

It is evident, however, that the mass of the Andromeda nebula must be enormous; and if it belongs to our sidereal system, and if the other great nebulÆ have similar masses, it seems quite possible that the mass of the visible universe may much exceed that of the visible stars, and may be equal to 1000 million times the sun’s mass—as supposed by the late Lord Kelvin—or even much more.

With reference to the small star which suddenly blazed out near the nucleus of the Andromeda nebula in August, 1885, Prof. Seeliger has investigated the decrease in the light of the star on the hypothesis that it was a cooling body which had suddenly been raised to an intense heat by the shock of a collision, and finds a fair agreement between theory and observation. Prof. Auwers points out the similarity between this outburst and that of the “temporary star” of 1860, which appeared in the cluster 80 Messier, and he thinks it very probable that both phenomena were due to physical changes in the nebulÆ in which they appeared.

The appearance of this temporary star in the Andromeda nebula seems to afford further evidence against the hypothesis of the nebula being an external universe. For, as I have shown above, our sun, if placed at a distance of 150,000 light years, would shine only as a star of the 23rd magnitude, or over 15 magnitudes fainter than the temporary star. This would imply that the star shone with a brightness of over a million times that of the sun, and would therefore indicate a body of enormous size. But the rapid fading of its light would, on the contrary, imply a body of comparatively small dimensions. We must, therefore, conclude that the nebula, whatever it may be, is not an external universe, but forms a member of our own sidereal system.

In Sir John Herschel’s catalogue of NebulÆ and Clusters of Stars, published in 1833, in the Philosophical Transactions of the Royal Society, there are many curious objects mentioned. Of these I have selected the following:—

No. 496 is described as “a superb cluster which fills the whole field; stars 9, 10 ... 13 magnitude and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points.” This is No. 22 of Sir William Herschel’s 6th class, and will be found about 3 degrees south and a little east of the triple star 29 Monocerotis.

No. 650. This object lies about 3 degrees north of the star Leonis, the most northern of the bright stars in the well-known “Sickle,” and is thus described by Sir John Herschel: “A star 12th magnitude with an extremely faint nebulous atmosphere about 10 to 12. It is between a star 8-9 magnitude north preceding, and one 10th magnitude south following, neither of which are so affected. A curious object.”

No. 1558. Messier 53. A little north-east of the star a ComÆ Berenices. Described as “a most beautiful highly compressed cluster. Stars very small, 12th ... 20th magnitude, with scattered stars to a considerable distance; irregularly round, but not globular. Comes up to a blaze in the centre; indicating a round mass of pretty equable density. Extremely compressed. A most beautiful object. A mass of close-wedged stars 5' in diameter; a few 12th magnitude, the rest of the smallest size and innumerable.” Webb says, “Not very bright with 3⁷/10 inches; beautiful with 9 inches.” This should be a magnificent object with a very large telescope, like the Lick or Yerkes.

No. 2018. “A more than usually condensed portion of the enormous cluster of the Milky Way. The field has 200 or 300 stars in it at once.” This lies about 2° south-west of the star 6 AquilÆ, which is near the northern edge of the bright spot of Milky Way light in “Sobieski’s Shield”—one of the brightest spots in the sky.

No. 2093. “A most wonderful phenomenon. A very large space 20' or 30' broad in Polar Distance, and 1^m or 2^m in Right Ascension, full of nebula and stars mixed. The nebula is decidedly attached to the stars, and is as decidedly not stellar. It forms irregular lace-work marked out by stars, but some parts are decidedly nebulous, wherein no star can be seen.” Sir John Herschel gives a figure of this curious spot, which he says represents its “general character, but not the minute details of this object, which would be extremely difficult to give with any degree of fidelity.” It lies about 3 degrees west of the bright star ? Cygni.

Among the numerous curious objects observed by Sir John Herschel during his visit to the Cape of Good Hope, the following may be mentioned:—

h 2534 (H iv. 77). Near t⁴ Eridani. Sir John Herschel says, “Attached cometically to a 9th magnitude star which forms its head. It is an exact resemblance to Halley’s comet as seen in a night glass.”... “A complete telescopic comet; a perfect miniature of Halley’s comet, only the tail is rather broader in proportion.”[367]

h 3075. Between ? Monocerotis and ? Canis Majoris. “A very singular nebula, and much like the profile of a bust (head, neck, and shoulders) or a silhouette portrait, very large, pretty well defined, light nearly uniform, about 12' diameter. In a crowded field of Milky Way stars, many of which are projected on it.”[368]

h 3315 (Dunlop 323). In the Milky Way; about 3° east of the Eta ArgÛs nebula. Sir John Herschel says, “A glorious cluster of immense magnitude, being at least 2 fields in extent every way. The stars are 8, 9, 10, and 11th magnitudes, but chiefly 10th magnitude, of which there must be at least 200. It is the most brilliant object of the kind I have ever seen” ... “has several elegant double stars, and many orange-coloured stars.”[369] This should form a fine object in even a comparatively small telescope, and may be recommended to observers in the southern hemisphere. A telescope of 3-inches aperture should show it well.

Among astronomical curiosities may be counted “clusters within clusters.” A cluster in Gemini (N.G.C. 2331) has a small group of “six or seven stars close together and well isolated from the rest.”

Lord Rosse describes No. 4511 of Sir John Herschel’s General Catalogue of NebulÆ and Clusters (Phil. Trans., 1864) as “a most gorgeous cluster, stars 12-15 magnitude, full of holes.”[370] His sketch of this cluster shows 3 rings of stars in a line, each ring touching the next on the outside. Sir John Herschel described it as “Cluster; very large; very rich; stars 11-15 magnitude (Harding, 1827),” but says nothing about the rings. This cluster lies about 5 degrees south of d Cygni.

Dr. See, observing with the large telescope of the Lowell Observatory, found that when the sky is clear, the moon absent, and the seeing perfect, “the sky appeared in patches to be of a brownish colour,” and suggests that this colour owes its existence to immense cosmical clouds, which are shining by excessively feeble light! Dr. See found that these brown patches seem to cluster in certain regions of the Milky Way.[371]

From a comparison of Trouvelot’s drawing of the small elongated nebula near the great nebula in Andromeda with recent photographs, Mr. Easton infers that this small nebula has probably rotated through an angle of about 15° in 25 years. An examination I have made of photographs taken in different years seems to me to confirm this suspicion, which, if true, is evidently a most interesting phenomenon.

Dr. Max Wolf of Heidelberg finds, by spectrum photography, that the well-known “ring nebula” in Lyra consists of four rings composed of four different gases. Calling the inner ring A, the next B, the next C, and the outer D, he finds that A is the smallest ring, and is composed of an unknown gas; the next largest, B, is composed of hydrogen gas; the next, C, consists of helium gas; and the outer and largest ring, D, is composed—like A—of an unknown gas. As the molecular weight of hydrogen is 2·016, and that of helium is 3·96, Prof. Bohuslav Brauner suggests that the molecular weight of the gas composing the inner ring A is smaller than that of hydrogen, and the molecular weight of the gas forming the outer ring D is greater than that of helium. He also suggests that the gas of ring A may possibly be identical with the “coronium” of the solar corona, for which Mendelief found a hypothetical atomic and molecular weight of 0·4.[372]

With reference to the nebular hypothesis of Laplace, Dr. A. R. Wallace argues that “if there exists a sun in a state of expansion in which our sun was when it extended to the orbit of Neptune, it would, even with a parallax of ¹/60th of a second, show a disc of half a second, which could be seen with the Lick telescope.” My reply to this objection is, that with such an expansion there would probably be very little “intrinsic brightness,” and if luminous enough to be visible the spectrum would be that of a gaseous nebula, and no known star gives such a spectrum. But some planetary nebulÆ look like small stars, and with high powers on large telescopes would probably show a disc. On these considerations, Dr. Wallace’s objection does not seem to be valid.

It is usually stated in popular works on astronomy that the spectra of gaseous nebulÆ show only three or four bright lines on a faint continuous background. But this is quite incorrect. No less than forty bright lines have been seen and measured in the spectra of gaseous nebulÆ.[373] This includes 2 lines of “nebulium,” 11 of hydrogen, 5 of helium, 1 of oxygen (?), 3 of nitrogen (?), 1 of silicon (?), and 17 of an unknown substance. In the great nebulÆ in Orion 30 bright lines have been photographed.[374]D’Arrest found that “gaseous nebulÆ are rarely met with outside the Milky Way, and never at a considerable distance from it.”[375]

Mr. A. E. Fath thinks that “no spiral nebula investigated has a truly continuous spectrum.” He finds that so feeble is the intensity of the light of the spiral nebulÆ that, while a spectrogram of Arcturus can be secured with the Mills spectrograph “in less than two minutes,” “an exposure of about 500 hours would be required for the great nebula in Andromeda, which is of the same spectral type.”[376] Mr. Fath thinks that in the case of the Andromeda nebula, the “star cluster” theory “seems to be the only one that can at all adequately explain the spectrum obtained.”[377]

Prof. Barnard finds that the great cluster in Hercules (Messier 13) is “composed of stars of different spectral types.” This result was confirmed by Mr. Fath.[378]

From observations with the great 40-inch telescope of the Yerkes Observatory (U.S.A.), Prof. Barnard finds that the nucleus of the planetary nebula H. iv. 18 in Andromeda is variable to the extent of at least 3 magnitudes. At its brightest it is about the 12th magnitude; and the period seems to be about 28 days. Barnard says, “I think this is the first case in which the nucleus of a planetary or other nebula has been shown to be certainly variable.” “The normal condition seems to be faint—the nucleus remaining bright for a few days only. In an ordinary telescope it looks like a small round disc of a bluish green colour.” He estimated the brightness of the nebula as that of a star of 8·2 magnitude.[379] Even in a telescope of 4 inches aperture, this would be a fairly bright object. It lies about 3½ degrees south-west of the star ? AndromedÆ.

The so-called “globular clusters” usually include stars of different brightness; comparatively bright telescopic stars of the 10th to 13th magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of the Lick Observatory finds that (a) “the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects”; (b) “the globular clusters are devoid of true nebulosity”; and (c) “stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce.” He found that “exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density.” This last result confirms the late Dr. Roberts’ conclusions. Perrine finds that for clusters in the Milky Way, the faint stars (15th to 17th magnitude) “are about as numerous in proportion to the bright stars (10th to 13th magnitude) as in the globular clusters themselves.” This is, however, not the case with globular clusters at a distance from the Milky Way. In these latter clusters he found that “in the regions outside the limits of the cluster there are usually very few faint stars, hardly more than one-fourth or one-tenth as many as there are bright stars”; and he thinks that “this paucity of faint stars” in the vicinity of these clusters “gives rise to the suspicion that all regions at a distance from the Galaxy may be almost devoid of these very faint stars.” The late Prof. Keeler’s series of nebular photographs “in or near the Milky Way” tend to confirm the above conclusions. Perrine finds the northernmost region of the Milky Way “to be almost, if not entirely, devoid of globular clusters.”[380]

According to Sir John Herschel, “the sublimity of the spectacle afforded” by Lord Rosse’s great telescope of 6 feet in diameter of some of the “larger globular and other clusters” “is declared by all who have witnessed it, to be such that no words can express.”[381]

In his address to the British Association at Leicester in 1907, Sir David Gill said—

“Evidence upon evidence has accumulated to show that nebulÆ consist of the matter out of which stars have been and are being evolved.... The fact of such an evolution with the evidence before us, can hardly be doubted. I most fully believe that, when the modifications of terrestrial spectra under sufficiently varied conditions of temperature, pressure, and environment, have been further studied, this connection will be greatly strengthened.”