Part 21 (1/2)
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--
”So far as I am able at present to judge, under the atmospheric conditions prevalent in this country, the limit of the photographic method of delineation will be reached at stellar, or nebular, light of the feebleness of about 18th-magnitude stars. The reason for this inference is that the general illumination of the atmosphere by starlight concentrated upon a film by the instrument will mask the light of objects that are fainter than about 18th-magnitude stars.”[462]
With reference to blank s.p.a.ces in the sky, the late Mr. Norman Pogson remarked--
”Near S Ophiuchi we find one of the most remarkable vacuities in this hemisphere--an elliptic s.p.a.ce of about 65' in length in the direction of R.A., and 40' in width, in which there exists _no_ star larger than the 13th magnitude ... it is impossible to turn a large telescope in that direction and, if I may so express it, view such black darkness, without a feeling that we are here searching into the remote regions of s.p.a.ce, far beyond the limits of our own sidereal system.”[463]
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--
”The lifetime of a man seems fairly long, and the epoch when Troy was besieged, or when the Pharaohs piled up the pyramids in the valley of the Nile, or when our ancestors separated on the high plateaux of Asia, seems extremely remote, but these intervals are only moments compared to the immense periods required for geological evolutions and the enormously greater ones consumed in the developement of worlds from widely extended nebulous ma.s.ses. We recognize the existence of only those forces whose immediate consequences are appreciable, and it may be that those whose effects are yet unseen are really of the highest importance. A little creature whose life extended over only two or three hours of a summer's day might be led, if he were sufficiently endowed with intelligence, to infer that pa.s.sing clouds were the chief influence at work in changing the climate instead of perceiving that the sun's slow motion across the sky would bring on the night and its southward motion the winter.”[465]
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 a.s.sociation, Prof. G. H. Darwin (now Sir George Darwin) said--
”Man is but a microscopic being relatively to astronomical s.p.a.ce, and he lives on a puny planet circling round a star of inferior rank. Does it not, then, seem futile to imagine that he can discover the origin and tendency of the Universe as to expect a housefly to instruct us as to the theory of the motions of the planets? And yet, so long as he shall last, he will pursue his search, and will no doubt discover many wonderful things which are still hidden. We may indeed be amazed at all that man has been able to find out, but the immeasurable magnitude of the undiscovered will throughout all time remain to humble his pride. Our children's children will still be gazing and marvelling at the starry heavens, but the riddle will never be read.”
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 ma.s.ses of stars, sometimes leave s.p.a.ces 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 65 to 85; and a third, farther off, in stars of magnitudes 76 to 8. These may be called opera-gla.s.s, or field-gla.s.s stars.
Stratonoff finds that stars with spectra of the first type (cla.s.s A, B, C, and D of Harvard) which include the Sirian and Orion stars, are princ.i.p.ally situated near the Milky Way, while those of type II. (which includes the solar stars) ”are princ.i.p.ally 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 s.p.a.ce, 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 s.p.a.ce, and I think that even a good guess as to the order of its amount can be made. For, first we know that s.p.a.ce contains an enormous ma.s.s 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 0016 (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 quant.i.ty 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 9 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 (10{3}), and this great difference is not perhaps improbable.
The const.i.tution 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 s.h.i.+eld,” closely south of ?