On the Perseus Cluster of Nebulae

Abstract

The Perseus cluster of nebulae (R.A. 3h 15m, Dec. +41° 15', 1930; gal. long. 118°, lat. - 13°) according to Hubble and Humason [1] contains about 500 nebulae scattered over an area nearly 20 in diameter and lies at a distance of 11 million parsecs. As is the case with most clusters investigated, counts of nebulae made on photographs which were obtained with the 18-inch Schmidt telescope show that the Perseus cluster is considerably larger than was originally derived from the photographs taken with the large reflectors whose very severely restricted fields make them unsuitable for the efficient analysis of objects subtending large angles. For quite a different reason the analysis of the spatial distribution of the nebulae in the Perseus cluster with the 18-inch Schmidt telescope also presents considerable difficulties. The cluster is projected on a field of the Milky Way so rich in stars, that, because of the small scale of the telescope, the blurred images of close pairs and groups of stars may easily be mistaken for extragalactic nebulae. This error can partly be avoided by taking a number of well-focused photographs while the telescope is being drifted slightly in a different direction for each photograph. Groups of stars are likely to betray themselves by an image containing sharp streaks such as should not be expected in a drifted image of a nebula. Also, the precaution was taken to identify all of the nebulae involved several times through a repeated analysis conducted during several years and making use of many photographs taken on various emulsions. It is therefore felt that the results presented here can be viewed with more confidence than might have been originally hoped for.

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