OBSERVATORY BEGINS TO EXTEND GUIDE TO STARS

Work has been begun at the University Observatory on the extension of the Henry Draper Catalogue, and immense compilation of the spectra of 225,000 stars in every part of the sky, made over a period of many years as a part of the Observatory's fundamental task of surveying and mapping the heavens.

Since the work of compiling the catalogue was undertaken, improved methods of stellar photography have made it possible to secure the spectra of fainter stars, and the new plan at the Observatory is to go for a second time over certain portions of the sky, adding the spectra of other stars than those already included.

The extent to which this new survey supplements the original one is illustrated by the example of a single photographic plate recently described at a meeting of the American Astronomical Association. The plate covers a small part of the Milky Way where the stars are especially numerous, and it shows the spectra of 140 stars, only 14 of which had been included in the Henry Draper Catalogue.

As the spectrum of a star, revealed by passing its light through an objective prism, shows of what material it is composed, such a catalogue is of fundamental importance to astronomers. The Draper Catalogue, containing as it does vast numbers of these spectra, is constantly used for reference in observatories. It consists of nine volumes. Seven of these have been published to date, and meanwhile the Observatory furnishes yearly to the astronomers of all countries upon request hundreds of spectra contained in the unpublished volumes.

The extensions to the catalogue will appear at intervals in periodical form. This new work will be supervised by Miss A. J. Cannon of the Observatory staff, who has had charge of the classifications of stars for the Henry Draper Catalogue from the outset.