IN STARRY SKIES

Evening Post, Volume CXII, Issue 8, 9 July 1931, Page 26

IN STARRY SKIES

STAR CLOUDS

FROM THE ARCHER TO THE

EAGLE

(By "Omega Oentauri.")

Between the magnificent star groups of Scorpio aud Sagittarius and those of the Ea"gle; the Swan,. and the Lyre, there ; is a wide region which, to the naked eye, is-comparatively barren and uninteresting. The' Milky Way, after expanding to fill the whole constellation of the Scorpion, and reaching its climax thero ■ and in tho star clouds of Sagittarius seems to shrink and fade. Photographs, however, show'that in this apparently barren region there aro some of the., most extraordinary parts of tho galaxy. But let us first consider the aspect presented to the naked eye. In the Scorpion ■ the galactic stream appeared to be breaking into three. Tho western branch spread from Antares over-all ftthe Claws of the Scorpion and then gradually faded away. The central branch,/crossing the middle of the Scorpion's body, passed as a' wide stream into Ophiuehus and Serpens, but after, reaching Eta Ophiuchi and Xi Serpontis, it seemed to turn and join the eastern stream, which is the only one that is carried through quite unbroken. It passes from the sting of the Scorpion through the western quad rilateral.of Sagittarius past Lambdaand then right on till it envelopes the main stars of the Eagle. The.central stream, after ah apparent break of Borne fifteen degrees, starts again between B

Ophiuchi and Eta Serpontis and continues roughly parallel to the eastern stream until the two widen so greatly in the Swai that they blend into one. Thus, to the naked eye the galactic stream is as narrow on the borders of Sagittarius and Aquila as it is bctw-een Argo and the Southern Cross. Field glasses show that even here the Galaxy is much more extensive than it looks .to the eye, but. it is photography that has revealed its marvellous beauty and complexity. "The Photographic Atlas of Selected Regions of the Milky Way," published by the Carnegie Institution of Washington, is a perfect treasure house to those who wish to study the constitution the heavens. More than a dozen of the magnificent photographs' there given, all of which were taken by Professor B. Barnard, deal with portions of the heavens shown in our map to-day. Starting at the S.W. or top left-hand corner of the map, we see Antares and M 4. Just below Antares is Bho Ophiuchi. It is situated in a bright nebula which is brought into strong relief by its intensely dark surroundings. Barnard considers this one of the most extraordinay regions'in the whole sky. His photographs clearly pr»ve that the dark parts are not holes devoid of stars, but are dark obstructing' matter which hides all the stars behind. . Dark lanes, in reality vast obscure nebulae, extend eastwards from close to Bho almost to Theta, whore the galaxy seems to be a chaotic mixture of star clouds with some brilliant and some intensely black objects all of the strangest forms. It was the careful study of these regions that convinced Barnard that "the strange dark markings indicate real opaque objects nearer to us that the main clouds of stars. A little more than a degree north of Theta one of the markings takes the, form of an' ornamental capital S; A similar grouping of strange objects continues north-eastwards from Theta into the constellation Serpens, and it is noteworthy' that it was right in the middle of this remarkable region that the Nova of 1604

appeared. In all the photographs the arrangement of the faint stars into oval curves and streams is conspicuous. The little constellation Scutum is a modern one, having been added by Helvelius in 1690. It is not adopted in all Atlases, some taking tho stars we have shown in it as part of the S.W. corner of Aquila. None of its stars are bright, the chief half-dozen lying between the fourth, and fifth magnitude, but the cloud of faint stars shown in Barnard's photographs around and to the south of the Globular cluster M 11, is as dense as' in any region of the galaxy. There is not room in the print to put a pin point between the tiny star images, except where of the cloud appears broken by tho occurrence of clearly defined dark patches', now known to bo obscuring nebulae. Similar characteristics mark the Milky Way right on into Aquila. Its chief star, Altair, lies on a particularly dense cloud of very faint stars, on which are a few astonishingly clearly defined dark markings. The lower left-hand side of our map is outside the Milky Way. Theta, Eta, Zeta, EpsJon, Ddlta,. Ophiuchi form a 'fine line of stars crossed by the line formed by Theta, Eta, and Nu Serpentis, which is continued outside our map by a clearer line of stars right to the Serpent's head. Another line leads from Zeta Serpentis down through Gamma Beta and Alpha Ophiuchi. Close to the latter is the rather fainter star Alpha Herculis. The wonderful star cluster, M 13, is 12J degrees due north of the one shown in the map as 6210. Nova Ophiuchi 1848 was on the margiii of the central galactic stream between Eta and Zeta, and Nova 1670 Vulpeculae on the edge of the eastern or main stream. THE NUMBER OF STARS IN THE GALAXY. A study of Barnard's photographs suggests that the total number of stars in our galaxy may have been consider-

ably underestimated. The usual* reasoning is that as any star is just over two and a half times as bright as one; a magnitude fainter, it should be, on au average, the square root of this number of ■ times nearer. Assuming tho stars evenly distributed ia space, the inclusion of an extra •magnitude should thus increase the space observed, and therefore the number of. stars included in it, about four times. The number actually arrived at by counting, selected areas falls very much short of this, and so the deduction is made that;: stars rapidly thin out even within the region so far explored. This conclusion: may be questioned for two reasons.". The galaxy is a very much flattened system such as a typical spirai nebula. : The space which it fills is thus much more like a section of a cylinder, say . the shape of a.penny, than that of a sphere. . The .expected ftratid should therefore be about two and a half instead of four. Next Barnard has proved the existence of enormous quantities of dark matter closer, to us than even the star clouds. The is: that there i 3 also abundance of it farther off. , The nearest dark nebulae cut off the light of all stars behind them to the utmost limits of the galaxy. Presumably all others of the same;kind act in the same way. This would fully account for the rapid diminution of the ratio in the star numbers observed. It is therefore probable that the usual estimate of 'thirty or forth thousand millions is much too low. estimate, moreover, refers to bright stars only. We have ho direct evidence, j except in the case of eclipsing variables, of the existence of dark stars; but the | fact that some have, thus been, discovered is strong evidence for the existence of very many more, for it is' only under such extremely peculiar circumstances that a dark star can make its ipresence known. The thirty or forty thousand million must be taken, therefore, as the absolute minimum, and it is probably very seriously below the true value.