A new general catalogue of nebulae

By Knut Lundmark

The extremely valuable work of Dreyer, the New General Catalogue (N.G.C.), was published in 1890 and its two supplements in 1895 and 1908, respectively. The dates of issue explain why comparatively few observations based on photographic work have been included in the N.G.C.

A few years ago it was decided to collect data for some two thousand of the best observed anagalactic nebulae, and a card catalogue accordingly was written out. As has been the case many times before when I have started work with a selection of the data, it was also now found that a complete collection of existing data of nebulae was needed to meet requirements. It thus has been decided to work out another catalogue, including all existing data.

It has been thought that this catalogue should not be restricted to descriptions of objects which have not been included in N.G.C. and the index catalogues, together with recent data on N.G.C. objects. There would be obvious difficulties if the material as to nebulae were to be scattered over four different catalogues. N.G.C., as being a general catalogue, had adopted a certain description among the existing ones. Further no process of homogenization has been carried out, as far as can be inferred from the work. Besides, all the available material has not always been utilized. To mention only one such case, data of the angular dimensions as estimated in figures by the Herschels has not been included in N.G.C., where only the literal symbols of size have been inserted. It is clear that it will be of a certain value to evaluate the Herschelian symbols numerically on a basis of the numerical data of the Herschels themselves.

The main reason for going over again all published material is the wish to connect the visual and the photographic series of measures and estimates and to establish a reference system as accurate and as homogeneous as possible.

The principal objection to the construction of a new general catalogue is that such a catalogue cannot include complete

material with regard to the limiting diameter or the limiting magnitude. The same remark applies also to N.G.C. which was by no means complete, but which undoubtedly has rendered astronomy many serives. It seems that a new general catalogue ought to be of value for a number of researches, and, besides, it might at the present time be possible to select material which is complete above a certain limit. Such a collection cannot very well be made before all the available data have been compiled and worked together. I am at present engaged in the study of numerous photographs taken at Mount Wilson in order to obtain data for such a compilation. The photographs, being typical samples, may be expected to give representative data within the areas studied. The new general catalogue thus will contain a number of what might be called Selected Areas.

The data for each object are brought together on a card of size 8 x 12 inches, having heads on both sides. The positions of the objects have been reduced to 1900.0, but it is under consideration whether a reduction to 1950.0 would not be prefereable. The galactic co-ordinates will be derived, using the accurate reduction tables that now are worked out in Lund under Dr. Charlier's supervision.

The accumulation of the published data is going on at Upsala under the auspices of the Observatory and is carried out by an assistant. Some 16,000 cards have now been written out, and most of the material included in N.G.C. has already been carefully scrutinized. The complete catalogue will embrace some 35,000 objects.

The principal data included refer, of course, to such characeristics as size (major and minor axies), brightness (surface and total), form, area, position angle of major axis if the object is elongated, degree of concentration, and the different classifications of type. Besides, several new characteristics have been introduced and data sought for. Of these we may mention the following:

1. The length of the spiral arms, expressed in the major axis as unit

2. The compressibility, being a measure of the rate of the increase of light from the border of the ojbect toward the center

or, in other words, the ratio of increase in surface brightness to angular size of the object.

3. The index of asymmetry, expressing the shift of the center of luminosity from the geometrical center of the object

4. The amount of dark regions in the objects (interpreted as dark nebulae)

5. The light of secondary nuclei in the objects

The catalogue will be kept at the Observatory of Lund and future observations will be inserted from time to time. The Observatory will provide information free of cost, to workers in this field.

The data now existing in the literature, together with the ones which will be added from my own unpublished collections or from unpublished work of other astronomers, will be worked together adn published in a catalogue. The General Catalogue of Nebulae of the Observatory of Lund (L.G.C.) is intended to consist of two parts: the first being a general index catalogue, giving the descriptions and the other data finally adopted; the second giving a collection of all observations and contributions hitherto made.

Colleagues are cordially invited to give me their suggestions and criticisms in correspondence.

Pasadena, California
July, 1929