J. Ellard Gore

The Visible Universe

Some researches on the distribution of stars in the sky have recently been made at the Harvard Observatory (U.S.A.). The principal results are:—(1) The number of stars on any “given area of the Milky Way is about twice as great as in an equal area of any other region.” (2) This ratio does not increase for faint stars down to the 12th magnitude. (3) “The Milky Way covers about one-third of the sky and contains about half of the stars.” (4) There are about 10,000 stars of magnitude 6·6 or brighter, 100,000 down to magnitude 8·7, one million to magnitude 11, and two millions to magnitude 11·9. It is estimated that there are about 18 millions of stars down to the 15th magnitude visible in a telescope of 15 inches aperture.[456]

According to Prof. Kapteyn’s researches on stellar distribution, he finds that going out from the earth into space, the “star density”—that is, the number of stars per unit volume of space—is fairly constant until we reach a distance of about 200 “light years.” From this point the density gradually diminishes out to a distance of 2500 “light years,” at which distance it is reduced to about one-fifth of the density in the sun’s vicinity.[457]

In a letter to the late Mr. Proctor (Knowledge, November, 1885, p. 21), Sir John Herschel suggested that our Galaxy (or stellar system) “contained within itself miniatures of itself.” This beautiful idea is probably true. In his account of the greater “Magellanic cloud,” Sir John Herschel describes one of the numerous objects it contains as follows:—

“Very bright, very large; oval; very gradually pretty, much brighter in the middle; a beautiful nebula; it has very much the resemblance to the Nubecula Major itself as seen with the naked eye, but it is far brighter and more impressive in its general aspect as if it were doubled in intensity. Note—July 29, 1837. I well remember this observation, it was the result of repeated comparisons between the object seen in the telescope and the actual nubecula as seen high in the sky on the meridian, and no vague estimate carelessly set down. And who can say whether in this object, magnified and analysed by telescopes infinitely superior to what we now possess, there may not exist all the complexity of detail that the nubecula itself presents to our examination?”[458]

The late Lord Kelvin, in a remarkable address delivered before the Physical Science Section of the British Association at its meeting at Glasgow in 1901, considered the probable quantity of matter contained in our Visible Universe. He takes a sphere of radius represented by the distance of a star having a parallax of one-thousandth of a second (or about 3000 years’ journey for light), and he supposes that uniformly distributed within this sphere there exists a mass of matter equal to 1000 million times the sun’s mass. With these data he finds that a body placed originally at the surface of the sphere would in 5 million years acquire by gravitational force a velocity of about 12½ miles a second, and after 25 million of years a velocity of about 67 miles a second. As these velocities are of the same order as the observed velocities among the stars, Lord Kelvin concludes that there is probably as much matter in our universe as would be represented by a thousand million suns. If we assumed a mass of ten thousand suns the velocities would be much too high. The most probable estimate of the total number of the visible stars is about 100 millions; so that if Lord Kelvin’s calculations are correct we seem bound to assume that space contains a number of dark bodies. The nebulÆ, however, probably contain vast masses of matter, and this may perhaps account—partially, at least—for the large amount of matter estimated by Lord Kelvin. (See Chapter on “NebulÆ.”)

In some notes on photographs of the Milky Way, Prof. Barnard says with reference to the great nebula near ? Ophiuchi, “The peculiarity of this region has suggested to me the idea that the apparently small stars forming the ground work of the Milky Way here, are really very small bodies compared with our own sun”; and again, referring to the region near Cygni, “One is specially struck with the apparent extreme smallness of the general mass of the stars in this region.” Again, with reference to ? Cygni, he says, “The stars here also are remarkably uniform in size.”[459]

Eastman’s results for parallax seem to show that “the fainter rather than the brighter stars are nearest to our system.” But this apparent paradox is considered by Mr. Monck to be very misleading;[460] and the present writer holds the same opinion.

Prof. Kapteyn finds “that stars whose proper motions exceed 0·05 are not more numerous in the Milky Way than in other parts of the sky; or, in other words, if only the stars having proper motions of 0·05 or upwards were mapped, there would be no aggregation of stars showing the existence of the Milky Way.”[461]With reference to the number of stars visible on photographs, the late Dr. Isaac Roberts says—

With reference to blank spaces in the sky, the late Mr. Norman Pogson remarked—

Prof. Barnard describes some regions in the constellation Taurus containing “dark lanes” in a groundwork of faint nebulosity. He gives two beautiful photographs of the regions referred to, and says that the dark holes and lanes are apparently darker than the sky in the immediate vicinity. He says, “A very singular feature in this connection is that the stars also are absent in general from the lanes.” A close examination of these photographs has given the present writer the impression that the dark lanes and spots are in the nebulosity, and that the nebulosity is mixed up with the stars. This would account for the fact that the stars are in general absent from the dark lanes. For if there is an intimate relation between the stars and the nebulosity, it would follow that where there is no nebulosity in this particular region there would be no stars. Prof. Barnard adds that the nebulosity is easily visible in a 12-inch telescope.[464]

With reference to the life of the universe, Prof. F. R. Moulton well says—

In a review of my book Astronomical Essays in The Observatory, September, 1907, the following words occur. They seem to form a good and sufficient answer to people who ask, What is there beyond our visible universe? “If the stellar universe is contained in a sphere of say 1000 stellar units radius, what is there beyond? To this the astronomer will reply that theories and hypotheses are put forward for the purpose of explaining observed facts; when there are no facts to be explained, no theory is required. As there are no observed facts as to what exists beyond the farthest stars, the mind of the astronomer is a complete blank on the subject. Popular imagination can fill up the blank as it pleases.” With these remarks I fully concur.

In his address to the British Association, Prof. G. H. Darwin (now Sir George Darwin) said—

The ancient philosopher Lucretius said—

“Globed from the atoms falling slow or swift
I see the suns, I see the systems lift
Their forms; and even the system and the suns
Shall go back slowly to the eternal drift.”[466]

But it has been well said that the structure of the universe “has a fascination of its own for most readers quite apart from any real progress which may be made towards its solution.”[467]

The Milky Way itself, Mr. Stratonoff considers to be an agglomeration of immense condensations, or stellar clouds, which are scattered round the region of the galactic equator. These clouds, or masses of stars, sometimes leave spaces between them, and sometimes they overlap, and in this way he accounts for the great rifts, like the Coal Sack, which allow us to see through this great circle of light. He finds other condensations of stars; the nearest is one of which our sun is a member, chiefly composed of stars of the higher magnitudes which “thin out rapidly as the Milky Way is approached.” There are other condensations: one in stars of magnitudes 6·5 to 8·5; and a third, farther off, in stars of magnitudes 7·6 to 8. These may be called opera-glass, or field-glass stars.

Stratonoff finds that stars with spectra of the first type (class A, B, C, and D of Harvard) which include the Sirian and Orion stars, are principally situated near the Milky Way, while those of type II. (which includes the solar stars) “are principally condensed in a region coinciding roughly with the terrestrial pole, and only show a slight increase, as compared with other stars, as the galaxy is approached.”[468]

Prof. Kapteyn thinks that “undoubtedly one of the greatest difficulties, if not the greatest of all, in the way of obtaining an understanding of the real distribution of the stars in space, lies in our uncertainty about the amount of loss suffered by the light of the stars on its way to the observer.”[469] He says, “There can be little doubt in my opinion, about the existence of absorption in space, and I think that even a good guess as to the order of its amount can be made. For, first we know that space contains an enormous mass of meteoric matter. This matter must necessarily intercept some part of the star-light.”

This absorption, however, seems to be comparatively small. Kapteyn finds a value of 0·016 (about ¹/60th) of a magnitude for a star at a distance corresponding to a parallax of one-tenth of a second (about 33 “light years”). This is a quantity almost imperceptible in the most delicate photometer. But for very great distances—such as 3000 “light years”—the absorption would evidently become very considerable, and would account satisfactorily for the gradual “thinning out” of the fainter stars. If this were fully proved, we should have to consider the fainter stars of the Milky Way to be in all probability fairly large suns, the light of which is reduced by absorption.

That some of the ancients knew that the Milky Way is composed of stars is shown by the following lines translated from Ovid:—

“A way there is in heaven’s extended plain
Which when the skies are clear is seen below
And mortals, by the name of Milky, know;
The groundwork is of stars, through which the road
Lies open to great Jupiter’s abode.”[470]

From an examination of the distribution of the faint stars composing the Milky Way, and those shown in Argelander’s charts of stars down to the 9½ magnitude, Easton finds that there is “a real connection between the distribution of 9th and 10th magnitude stars, and that of the faint stars of the Milky Way, and that consequently the faint or very faint stars of the galactic zone are at a distance which does not greatly exceed that of the 9th and 10th magnitude stars.”[471] A similar conclusion was, I think, arrived at by Proctor many years ago. Now let us consider the meaning of this result. Taking stars of the 15th magnitude, if their faintness were merely due to greater distance, their actual brightness—if of the same size—would imply that they are at 10 times the distance of stars of the 10th magnitude. But if at the same distance from us, a 10th magnitude star would be 100 times brighter than a 15th magnitude star, and if of the same density and “intrinsic brightness” (or luminosity of surface) the 10th magnitude would have 10 times the diameter of the fainter star, and hence its volume would be 1000 times greater (10³), and this great difference is not perhaps improbable.

The constitution of the Milky Way is not the same in all its parts. The bright spot between and ? Cygni is due to relatively bright stars. Others equally dense but fainter regions in Auriga and Monoceros are only evident in stars of the 8th and 9th magnitude, and the light of the well-known luminous spot in “Sobieski’s Shield,” closely south of ? AquilÆ, is due to stars below magnitude 9½.

The correspondence in distribution between the stars of Argelander’s charts and the fainter stars of the Milky Way shows, as Easton points out, that Herschel’s hypothesis of a uniform distribution of stars of approximately equal size is quite untenable.

It has been suggested that the Milky Way may perhaps form a ring of stars with the sun placed nearly, but not exactly, in the centre of the ring. But were it really a ring of uniform width with the sun eccentrically placed within it, we should expect to find the Milky Way wider at its nearest part, and gradually narrowing towards the opposite point. Now, Herschel’s “gages” and Celoria’s counts show that the Galaxy is wider in Aquila than in Monoceros. This is confirmed by Easton, who says, “for the faint stars taken as a whole, the Milky Way is widest in its brightest part” (the italics are Easton’s). From this we should conclude that the Milky Way is nearer to us in the direction of Aquila than in that of Monoceros. Sir John Herschel suggested that the southern parts of the galactic zone are nearer to us on account of their greater brightness in those regions.[472] But greater width is a safer test of distance than relative brightness. For it may be easily shown than the intrinsic brightness of an area containing a large number of stars would be the same for all distances (neglecting the supposed absorption of light in space). For suppose any given area crowded with stars to be removed to a greater distance. The light of each star would be diminished inversely as the square of the distance. But the given area would also be diminished directly as the square of the distance, so we should have a diminished amount of light on an equally diminished area, and hence the intrinsic brightness, or luminosity of the area per unit of surface, would remain unaltered. The increased brightness of the Milky Way in Aquila is accounted for by the fact that Herschel’s “gages” show an increased number of stars, and hence the brightness in Aquila and Sagittarius does not necessarily imply that the Milky Way is nearer to us in those parts, but that it is richer in small stars than in other regions.

Easton is of opinion that the annular hypothesis of the Milky Way is inconsistent with our present knowledge of the galactic phenomena, and he suggests that its actual constitution resembles more that of a spiral nebula.[473] On this hypothesis the increase in the number of stars in the regions above referred to may be due to our seeing one branch of the supposed “two-branched spiral” projected on another branch of the same spiral. This seems supported by Sir John Herschel’s observations in the southern hemisphere, where he found in some places “a tissue as it were of large stars spread over another of very small ones, the immediate magnitudes being wanting.” Again, portions of the spiral branches may be richer than others, as photographs of spiral nebulÆ seem to indicate. Celoria, rejecting the hypothesis of a single ring, suggests the existence of two galactic rings inclined to each other at an angle of about 20°, one of these including the brighter stars, and the other the fainter. But this seems to be a more artificial arrangement then the hypothesis of a spiral. Further, the complicated structure of the Milky Way cannot be well explained by Celoria’s hypothesis of two distinct rings one inside the other. From analogy the spiral hypothesis seems much more probable.

Considering the Milky Way to represent a colossal spiral nebula viewed from a point not far removed from the centre of the spiral branches, Easton suggests that the bright region between and ? Cygni, which is very rich in comparatively bright stars, may possibly represent the “central accumulations of the Milky Way,” that is, the portion corresponding to the nucleus of a spiral nebula. If this be so, this portion of the Milky Way should be nearer to us than others. Easton also thinks that the so-called “solar cluster” of Gould, Kapteyn, and Schiaparelli may perhaps be “the expression of the central condensation of the galactic system itself, composed of the most part of suns comparable with our own, and which would thus embrace most of the bright stars to the 9th or 10th magnitude. The distance of the galactic streams and convolutions would thus be comparable with the distances of these stars.” He thinks that the sun lies within a gigantic spiral, “in a comparatively sparse region between the central nucleus and Orion.”

Scheiner thinks that “the irregularities of the Milky Way, especially in streams, can be quite well accounted for, as Easton has attempted to do, if they are regarded as a system of spirals, and not as a ring system.”

Evidence in favour of the spiral hypothesis of the Milky Way, as advocated by Easton and Scheiner, may be found in Kapteyn’s researches on the proper motions of the stars. This eminent astronomer finds that stars with measurable proper motions—and therefore in all probability relatively near the earth—have mostly spectra of the solar type, and seem to cluster round “a point adjacent to the sun, in total disregard to the position of the Milky Way,” and that stars with little or no proper motion collect round the galactic plain. He is also of opinion that the Milky Way resembles the Andromeda nebula, “the globular nucleus representing the solar cluster, and the far spreading wings or whorls the compressed layer of stars enclosed by the rings of the remote Galaxy.”

With reference to the plurality of inhabited worlds, it has been well said by the ancient writer Metrodorus (third century B.C.), “The idea that there is but a single world in all infinitude would be as absurd as to suppose that a vast field had been formed to produce a single blade of wheat.”[474] With this opinion the present writer fully concurs.