An Extra-galactic Star Cluster

Abstract

IT is reported by the New York correspondent of The Times that Dr. Harlow Shapley has announced the discovery of a great cluster of stars in the southern constellation Sculptor. The discovery was accidental, the cluster being photographed with the 60-inch reflector at the Harvard Southern Station at Mazels-poort, near Bloemfontein, South Africa, on a very clear night with a highly sensitive plate. If the median absolute magnitude of the stars photographed be assumed to be –1·5, the apparent magnitude being about +18, it follows that the distance of the cluster is about 250,000 light-years. This places it well beyond the limits of our galactic system, the distance being more than double that of the Magel-lanic Clouds, and adds another to the known systems that are relatively near to our galactic system. The angular diameter of the cluster is greater than 1° and the linear diameter is estimated to be about 6,500 light-years, which indicates a size comparable with the Small Magellanic Cloud. The cluster is oval-shaped, unlike the clusters in our galactic system, which are globular. It is stated that there are about 10,000 stars in the cluster between the brightest stars and the dimmest that have yet been counted. The Magellanic Clouds are rich in stars of very high luminosity. The Sculptor cluster would undoubtedly have been discovered earlier, if it had contained such stars. It is probable that, when further photographs are available, Cepheid variables will be discovered in it and the study of their light variations will enable a more accurate determination of the distance of the cluster to be made.