Photographing the Star-Clusters
Assistant Professor of Practical Astronomy, and Superintendent of Instrument-Construction at the Yerkes Observatory
IN the constellation of Taurus, and passing a little south of the zenith in our latitude on winter nights, is that remarkable group of small stars, the Pleiades, one of the most striking objects in the heavens to the unaided eye. This group was familiar to the ancients, and is mentioned in many old Writings, including the Book of Job. The six principal stars of the group are conspicuous; indeed, eleven are readily seen by a keen eye without a telescope. With the aid of a good opera-glass nearly a hundred members of this assemblage become visible, and with a large telescope many hundreds are revealed. Although usually called a star-group instead of a star-cluster, since the latter term is generally used to designate aggregations of stars in which the components appear closer together, this assemblage is undoubtedly one in which the component stars are intimately related physically; there is good reason for believing that these stars were all developed from one great nebula.
The relative positions of the stars of the Pleiades have been measured with great accuracy by several eminent astronomers during the past half-century. In recent years excellent photographs of the group have been secured, which afford a new means of determining these relative positions with extreme precision. In 1859, Tempel discovered, visually, a large, very faint nebula around Merope, one of the principal stars of the group. Photographs made about twenty years ago, by the Henry brothers of Paris and by Roberts in England, showed a large number of wisps and streaks of nebulosity involving Merope, Maia, Alcyone, and other stars of the group. Barnard's photographs, obtained in 1893 by means of a large portrait-lens and with very long exposures, show that this nebulosity is of enormous extent, completely enveloping, and extending far beyond, the stars of the group which can be seen by the unaided eye; the latter photographs are on a small scale, and do not show the intricate filamentous structure of the nebulosity.
Recently the two-foot reflecting telescope of the Yerkes Observatory has been employed by the writer in photographing the Pleiades. This instrument is especially well adapted for photographing not only the great numbers of very faint stars of the group, but also the extremely faint extensions of nebulosity, and for depicting with the greatest sharpness the minute details of structure of the nebula. The accompanying illustration is from one of the best of the reflector photographs, which was obtained with an exposure of three and one-half hours. The bright star below the centre, to the right, with the great curved streaks of nebulosity about it, is Merope. This streaky nebulosity extends northward and involves the bright star Maia, which is seen in the photograph directly above Merope. The brightest star of the group is Alcyone, which is shown just to the left of the centre; the nebulosity about this star is very faint, and is apparently very different from the other in structure. But the long horizontal streaks, nearly straight, and parallel to each other, are the most remarkable, and are different from anything found elsewhere in the heavens.
Little is known in regard to the real meaning of such a group of stars involved in nebulosity. These objects are at such immense distances from us that any changes of form taking place in them must appear extremely slow. Photography affords the only reliable means of recording the forms of the nebulae, and consequently of detecting progressive
