Studies of the galactic center. IV. On the transperancy of the galactic star clouds

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ASTRONOMY: H. SHAPLEY

PR~oc. N. A. S.

STUDIES OF THE GALACTIC CENTER. IV. ON THE TRANSPARENCY OF THE GALACTIC STAR CLOUDS By HARiow SHAPiLY HARVARD OBSERVATORY, CAMBRIDGS, MASSACHUSETTS

Communicated February 14, 1929

1. Preceding papers of this series contain reports on the progress of an investigation of the dimensions and structure of the Galaxy. In the first, a program was outlined for a fifteen-year study of the distant variable stars in the Milky Way. The paper incidentally reported the discovery during the past five years of more than a thousand faint variables, most of which are more than thirty thousand light years distant.

FIGURE 1

Composite picture of the central Milky Way region, made from plates taken at Arequipa with a three-inch Ross-Tessar lens. The rings around the brighter stars are merely optical. The center of the Galaxy is indicated at 3270, 00. This picture covers a somewhat larger area than figures 2 and 3.

In the second paper was presented the evidence bearing on the existence and dimensions of a massive galactic nucleus. The star clouds in the Sagittarius region of the southern Milky Way, according to the study of the newly discovered variable stars, are at approximately the same dis-

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tance as the center of the Galaxy, previously deduced from globular clusters. These rich star fields appear, therefore, to constitute a nucleus of the Galaxy, and around it the stars in the solar neighborhood move in a period of some hundreds of millions of years. The third note gave a new determination of the average absolute magnitude of long period variables, and indicated the probable usefulness of this common type of star in future measurements of galactic dimensions. 2. The occurrence of dark nebulosities in the direction of the galactic center, shown in the composite photograph reproduced in figure 1, raises some doubt as to the security of photometric estimates of distances in that direction. It is probable that these obscuring nebulosities, roughly sketched in figure 2, are mainly within a thousand parsecs of the earth. 290

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FIGURE 2 A rough sketch to indicate the general distribution of obscuring nebulosity shown in figure 1. The patch around 324° ,+ 150, is the Rho Ophiuchi nebulosity.

If there are similar nebulous regions intermingled with the star clouds or lying beyond the nucleus, they may make completely hopeless the attempt to measure the bounds of the Milky Way beyond the center. Fortunately we can test the transparency of the Galaxy in any specific direction with the aid of the extra-galactic nebulae; at the same time we can make a definite contribution to the question of the distribution of these outside galaxies. 3. Long-exposure photographs with the Bruce 24-inch reflector are available for nearly half of the central region. Each plate covers about thirty-five square degrees, anTall are capable of showing nebulous objects for which the integrated photographic magnitude is as bright as l7'5; for most of the plates the limit is approximately 18m0. The centers of these long-exposure plates are indicated by crosses in figure 3, which

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shows also the distribution of the extra-galactic nebulae that are now known for this region. Plates of shorter exposure (60" and 120"') covering the whole of the central region have been examined, but most of them are limited to photographic magnitudes brighter than 16.5 and therefore are relatively impotent for the majority of extra-galactic nebulae. 4. The systematic examination of plates for new nebulae in the region of the galactic center has been carried out mainly by Professor S. I. Bailey, Dr. D. H. Menzel,2 and Miss Sylvia Mussells (results unpublished). The

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F^IGURZ 3 Extra-galactic nebulae in the central Milky Way region. Large dots represent N. G. C. objects; small dots, those in the I. C. or in published Harvard lists; small circles, new nebulae discovered recently at Harvard. The same symbols crossed by diagonal bars represent nebulae in each of the groups which have been definitely identified as spirals. The object at 3530,-~2O°, is N. G. C. 6822. Crosses show the centers of long-exposure plates.

total number of extra-galactic nebulae, as shown in figure 3, is 331, including those which fall somewhat outside the boundaries of the central region. All have been re-examined by the writer. Although most of the objects in lowest latitude are doubtful, as indicated by colons, there can be no doubt of the existence of a number of extra-galactic nebulae within twenty degrees of the galactic center and wthin ten degrees of the central line of the Milky Way. The object at 3530, -2O°, is N. G. C. 6822, a well-known extra-galactic system of the Magellanic Cloud type. Of the 331 objects plotted in figure 3, about twenty per cent are definitely

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spirals; an additional twelve per cent are probably spirals; and eight per cent are described doubtfully as extra-galactic nebulae. The remaining sixty per cent are unspecified extra-galactic nebulae, largely of the elliptical type or spirals in plan; their detailed classification will appear in a forthcoming catalogue of new nebulae. The doubtful objects may be remote star clusters, or planetary and irregular nebulae; or perhaps some are but fortuitous aggregations of faint stars. Larger scale photographs will be necessary to distinguish their character. 5. From the distribution of long-exposure plates and recorded nebulae, shown in figure 3, the incompleteness of the data is obvious. We may reasonably assume that when plates are available many more extragalactic objects will be found in low latitude, at least in the interval between ten and twenty degrees of latitude on both sides of the galactic circle. The effective obscuration is apparently limited to easily recognized dark nebulosities. Dependable magnitudes are not yet available for the extra-galactic nebulae of this region, but a preliminary examination shows that the relation of angular diameter to apparent magnitude is not greatly different from that in Coma-Virgo. For the 2775 objects in the Coma-Virgo region (high galactic latitude) for which measures are available, we have recently shown that the diameter-magnitude relation indicates that space is effectively transparent up to a distance of something like a hundred million light years. The same now appears to be true in low latitudes as well, for in the places where the outside galaxies are seen the apparent magnitudes are not seriously dimmed by either general or differential space absorption. In summary, we conclude that though readily-observed obscuring matter effectively conceals some of the galactic star clouds in the central region, and even hides the center itself, the Galaxy is completely transparent at the borders of these obscuring nebulae, permitting the observation of not only the most distant stars of our system but also the outside galaxies, probably millions of light years beyond the farthest edge of our Galaxy. 1 Ham,. Ann., 72, No. 2, 1913. 2Harm. Ann., 85, No. 6, 1924. 3 Shapley and Miss Ames, Harv. Obs. Bull. 864, 1929.