After having presented all of the information which contributed to the making of the NGC, this section treats Dreyer's publication of 1888 and analyses the structure of the catalogue. It appeared in Vol. 49 of the Memoirs of the Royal Astronomical Society and is titled ‘A new catalogue of nebulae and of clusters of stars, being the catalogue of the late Sir John F. W. Herschel, Bart., revised, corrected, and enlarged’ (Dreyer 1888b). The main topics of the following Sections 10.1 and 10.2 are:

• the temporal development of discoveries (1800–1887)

• the number of independent NGC objects

• discoverers (success rate, nationality)

• telescopes (aperture, type, site, nationality)

• the distribution according to modern object types

• the presentation of special objects

• the distribution of visual brightness

• missing data

After 1888, Dreyer and other astronomers collected corrections and supplementary data regarding NGC objects. Moreover, many new non-stellar objects were found, mainly by photography. To keep track, Dreyer published two Index Catalogues, in 1895 and 1908, containing the new data (Section 10.3). NGC-numbers are mentioned in nearly all modern catalogues (e.g. of galaxies, galactic nebulae and open clusters), albeit not always correctly. This is mainly due to the fact that the original NGC is not user-friendly and thus difficult to compare with modern data. Moreover, it contains errors, which were simply reproduced.

On coming to Dunsink Observatory in 1878, Dreyer was tasked with the completion of the Birr Castle publication. The work was finished in early 1880, whereafter he had time to focus on newly discovered nebulae. The cataloguing was done in parallel with his regular work as the assistant of Robert Ball, who was familiar with this issue, due to his own stay at Birr Castle. From 1882 Dreyer, as Director of Armagh Observatory, had complete freedom to work on a new catalogue of nebulae and star clusters.

The result was presented to the RAS in 1886 as a ‘second supplement’ to the General Catalogue, a follow-up of the GCS. It was surprising for Dreyer to learn that the society had refused his new work. However, the reason was honourable: the RAS favoured a ‘new general catalogue’ instead. It should contain all known non-stellar objects, thus being a complete new edition of Herschel's GC, which it had published in 1864. Since the bulk of the work had already been done, this was not a difficult task for Dreyer – it was carried out in Armagh within a year! The NGC contains a large number of new objects (as of December 1877) and became his most important (and most popular) work.

DREYER'S UNPUBLISHED ‘SECOND SUPPLEMENT’

After the publication of the GCS (1878) and the Birr Castle observations (1880), Dreyer recurrently scanned the astronomical journals and observatory publications for new nebulae.

THE SIGNIFICANCE OF THE NEW GENERAL CATALOGUE

Besides the point-like stars, the sky offers a large number of objects showing an extended structure. Except for a few, they are not visible without the aid of a telescope. In terms of their optical appearance, there are star clusters (resolvable objects) and nebulae (unresolvable objects). In 1862 Eduard Schöfeld, an astronomer at Mannheim Observatory, gave the following definition:‘Nebulae or nebulous patches are celestial objects, which do not contrast with the sky background as shining points, like individual stars, but present the impression of a more or less extended and diffuse area of light.’

Long before the invention of the telescope, the open clusters of the Pleiades and Praesepe and the diffuse spot of the Andromeda Nebula were known. Later the telescopic exploration of the sky brought many more cases to light. Soon it became evident that some nebulae are disguised clusters of stars; the best examples are globular clusters, the compact and star-rich versions of open clusters. Other objects, such as the bright nebulae in Orion and Andromeda, could not be resolved, even with the largest telescopes. However, in 1864 the new astrophysical method of spectroscopy revealed that the Orion Nebula is a mass of gas (mainly hydrogen and helium). On the other hand, the Andromeda Nebula is a galaxy, consisting of many hundreds of billions of stars, which was eventually proved in the twentieth century.

The last chapter covers themes, spreading over a large fraction of the nineteenth century and involving many eminent visual observers. Thus it was impossible to treat them in closed form in the above sections. This concerns observation methods, measurements and presentations of some key objects. The topic ‘resolvability of nebulae’ is excluded here, because it is strongly related to William Herschel's ideas about the nature and evolution of nebulae and therefore cannot be separated from the previous text. Section 11.1 treats positional measurements, often made by professional astronomers using refractors with micrometers or meridian-circles. Inspired by the success for stars, it was hoped that it would be possible to detect proper motions of nebulae to determine their distances and spatial distribution. The result was negative, since most objects are too far away and the accuracy of the measuring method was too low. Another, much more debated topic is drawings. Section 11.2 describes the main problem: the subjectivity in perception and presentation of non-stellar objects. Prior to photography, textual descriptions or sketches were the only way to capture the appearance. An important goal was the detection of change concerning form or brightness. This attempt failed as well – the differences due to the observers were too large. A special kind of object was discovered by Lord Rosse in the year 1845: the spiral nebula. The first was M 51, a bright galaxy in Canes Venatici.

In 1841 Wilhelm Struve wrote that ‘The study of the nebular heaven seems to be the exclusive dominion of the two Herschels.’ Thus the following question arises: was there still anything to discover after the intensive observations of William and John Herschel? The clear answer is yes – for several reasons. First, the two Herschels had not completely surveyed the sky. Their sweeps left remarkable gaps, especially at high declinations and at greater distances from their reference stars. Moreover, new and better telescopes were available in the post-Herschel era, revealing much fainter objects. Finally, new campaigns were started, in which many visual observers (normally professionals) participated. Though their targets were mostly comets and minor planets, many new nebulae were found accidentally. For the search small refractors with great light-gathering power were used: the typical comet-seeker was optimal for observing large, faint nebulae too. Larger telescopes with apertures of 10–30 cm were needed to trace a comet or to create ‘ecliptical charts’ showing stars down to 13 mag – such charts were important tools by means of which to identify minor planets. Using them, occasionally faint non-stellar objects could be detected.

By virtue of these observations, the number of new nebulae slowly increased from 1845. Some discoverers published lists, but there were at first no new compilations. John Herschel's Slough catalogue was the primary reference to check the identity of objects.

My enthusiasm for nebulae and star clusters goes back a long way – they were the targets of my first telescopic explorations of the night sky. This book treats the history of their discovery, visual observation and cataloguing. It is naturally focused on the nineteenth century – the fascinating epoch of classical astronomy, characterised by precious achromatic refractors and massive metal-mirror reflectors. Only a few astronomers searched for nonstellar objects systematically – foremost among them William Herschel and his son John. We owe to both of them the first comprehensive catalogues. The development reached its climax with the New General Catalogue (NGC) by John Louis Emil Dreyer – which is still a standard source for both amateur and professional astronomers.

Initially this immense work appeared to me as a mysterious treasure, arousing my scientific curiosity. What was behind the 7840 objects and who were the discoverers? By using the NGC, I gradually became familiar with its content. However, due to erroneous and incomplete data, it was often difficult to match the entries with the real sky. Over the years, due to my research on the historical sources and visual observing, the catalogue became a close companion. Many secrets could be disclosed – and, of course, my admiration for Dreyer increased.

Dreyer's first catalogue, titled ‘A supplement to Sir John Herschel's ‘General Catalogue of Nebulae and Clusters of Stars’, appeared in March 1878 in the Transactions of the Royal Irish Academy (Dreyer 1878a). The work – henceforth abbreviated GCS – had already been received by the Academy on 26 February 1877, but afterwards was corrected by the author. Besides the new objects, it contains a large number of notes and corrections to Herschel's GC. At the same time Dreyer was working on the compilation of the Birr Castle observations made during 1848–78. These two projects influenced each other. In the GCS all objects newly found at Birr Castle were entered with calculated coordinates. The same applies for discoveries of other observers, such as Schultz and Tempel. Dreyer's supplement was the essential step towards the NGC. His first appearance as author of an important catalogue of nebulae and star clusters gives us the occasion for a detailed review of his life.

DREYER'S BIOGRAPHY

Copenhagen

Johan Ludvig Emil Dreyer (in English later: John Louis Emil) was born on 13 February 1852 in Copenhagen (Fig. 8.1). Many of his ancestors served in the army and navy. His father, Johan Christopher Friedrich Dreyer, became Danish Minister of War and the Navy in 1864.

The subject ‘nebulae and star clusters’, centred on the nineteenth century, has hitherto not historically been treated in detail. At best, a few aspects were studied and occasionally results were published. Particularly the ‘history of the NGC’ is new territory and the present work is the first of its kind. The following comprehensive summary concentrates on the following issues (see also Chapter 1):

1. (1) the subject and line of questioning

2. (2) the importance of the New General Catalogue and the motivation of the work

3. (3) objects, observers and methods

4. (4) milestones of the cataloguing of nebulae and star clusters (non-stellar objects)

5. (5) statistical analysis and the way ahead

THE SUBJECT AND LINE OF QUESTIONING

This work deals with the discovery, visual observation, description and cataloguing of nebulae and star clusters, focusing on the nineteenth century. Astronomers, sites and a large number of objects are presented. The climax of the development, starting with William and John Herschel was the publication of the New General Catalogue (NGC) by John Louis Emil Dreyer in 1888. The date also marks the transition from classical astronomy based on visual observations and positional measurements to modern astrophysics represented by spectroscopy, photography and photometry.

John Herschel's ‘Catalogue of nebulae and clusters of stars’ – commonly known as the General Catalogue (GC) – was published in 1864 in Vol. 154, Part I, of the Philosophical Transactions of the Royal Society (Herschel J. 1864). At that time Herschel was already 71. After years of low astronomical activity, he had decided to collect all known nebulae and star clusters in a common catalogue. The work started in about 1859 and was finished on 23 June 1863. This date refers to the main catalogue; the added ‘Supplementary list’ was not made until October 1863. Later in 1864a hardcover version was published by Taylor and Francis, London. The GC differs from Herschel's earlier Slough and Cape catalogues (SC and CC), which mainly contain data based on his own observations. In the case of the SC (1833) his goal was a re-examination of the objects contained in the catalogues of his father, using Caroline Herschel's reduction. The CC (1847) lists, with the exception of Dunlop's objects, mainly his own southern-sky discoveries. On the other hand, the GC presents all non-stellar objects found by 1863 for both hemispheres – therefore, it is a ‘database’ in the modern sense. As shown in the following sections, the General Catalogue lists many new objects.