Star catalogues in earlier times usually included some "nebulous stars" or "nebulae". Almagest contains seven of them. Tycho Brahe, in his Astronomiae Instaurate Progymnasmata, listed six, but only one (Praesepe) was also listed in the catalogue of Almagest. In Prodromus Astronomiae Johannes Hevelius listed sixteen entries as nebulous. This ancient term "nebula" or "nebulosa" covers - as was gradually recognized up to 1925 - a variety of phenomena: gaseous nebulae in the Milky Way, clusters of stars, galaxies, small groups of stars, and sometimes separate stars, which the observer for some reason had perceived as "nebulous". Only three extragalactic objects had been observed before the invention of the telescope: the Andromeda nebula and the Magellanic Clouds. Thus, when gaiaxies (as we now recognize them) were recorded by and by in the astronomical annals, it was as a part of the whole process in which non-stellar, "nebulous" objects of all kinds were discovered and described.
The Andromeda nebula was mentioned by Al-Sufi in the 10th century (and its discovery is usually ascribed to him). It was re-discovered by Simon Marius in 1612. Al-Sufi probably also knew the Large Magellanic Cloud, apparently mentioned by him as "El-Bakar", "the (White) Ox" (cf. Humboldt 1850).
The first separate list of nebulae was compiled by Edmond Halley. In 1715, he published in the Philosophical Transactions of the Royal Society a memoir called "Of Nebulae or lucid Spots among the Fix't Stars". Only six objects were included, and the Andromeda nebula has turned out to be the sole extragalactic object among them. The remaining ones are the Orion nebula (M 42, discovered by Nicholas Pieresc in 1610), the globular cluster M 22 (discovered by Abraham Ihle in 1665 or possibly somewhat earlier by Hevelius), the globular clusters Centauri and M 13 (discovered by Halley in 1677 and 1714), and "Kirch's nebula", the open cluster M 11 (discovered by Gottfried Kirch in 1681).
The second early list of nebulae alone was prepared by the English divine William Derham in 1733; it included Halley's six nebulae and sixteen other objects listed in Hevelius's Prodromus Astronomiae. As was pointed out by Herman Schultz in 1866, only thirteen of these "Hevelian" nebulae are really to be found in the work of the Polish astronomer, and only two of them (the Andromeda nebula and Praesepe) are in the classical sense "nebulous". Derham states that he had made "some good Observations" of five of Halley's objects, but he never observed the remaining ones from Hevelius's catalogue. This list was a poor work (from an observer's point of view), but it appears to have inspired others to make their own observations of nebulae. It also attained a somewhat undeserved credit, as it was reprinted by Maupertuis in his Discours sur les différents Figures des Astres (2nd edition, 1742) and was commended by Le Gentil, who also claimed that most of the objects had been observed by himself and by the Swiss astronomer Philippe Luis de Chésaux. Derham's list reached Immanuel Kant through Maupertuis, and this information supported the philosopher in his speculations concerning the structure of the Universe.
De Chésaux - also well known in the early history of Olber's paradox - is a name of interest in this connection, as he took the first step towards a classification of the objects. He divided them roughly into two groups: the star clusters, clearly resolvable into faint stars through the telescope, and the true nebulae, those which never looked like anything else than "white clouds". De Chésaux observed at Lausanne, and he reported on his observations of 20 nebulae in a letter written about 1746 to his grandfather in Paris (Hogg 1947). According to Kenneth Glyn Jones, eight or possibly nine of these objects were discoveries by de Chésaux himself.
Up to the middle of the 18th century, a total of about 60 nebulous objects had been discovered. Among the newcomers since Halley's and Derham's lists, only two later on turned out to be extragalactic: M 32, discovered by Le Gentil in 1749, and the bright southern spiral M 83, discovered by Lacaille about 1752. Lacaille, a remarkably assiduous observer, worked at the Cape of Good Hope from 1751 to 1753, and his observations resulted in a catalogue of some 10,000 stars, as well as in the first catalogue of southern nebulous objects, published in the Mémoires del'Academie Royale des Sciences in 1755. It was reprinted in Connoissance des Temps for 1783, 1784 and 1787, along with Messier's lists of nebulae. Lacaille divided his objects into three groups: "Nebulae of the First Class" (i.e. nebulae without stars), "Nebulous Stars in Clusters", and "Stars accompanied by Nebulosity". This division happens to agree approximately with the modern division into globular clusters, open clusters and gaseous nebulae. He did not intentionally look for nebulae; like Messier, he was occupied with other observations when the nebulous objects happened to enter his field of view. He made the illustrative statement that his "Nebulae of the First Class" might all be faint comets, as time did not allow him to check whether the objects remained in the same position.
The famous Messier catalogue originated from a list of 45 nebulae, which Charles Messier in 1771 published in the Mémoires del'Academie Royale des Sciences. It is probably well known that Messier's interest in nebulae (or dislike of them, as he mistook them for comets) originated in 1758, when he accidentally found the Crab nebula during his observations of a comet. (The nebula had been discovered by John Bevis at Greenwich in 1731.) Three of these early Messier objects have proved to be galaxies: to M 31 and M 32 was added the Triangulum spiral M 33, which Messier had discovered in 1764. A similar list was published six years later by J.E. Bode in Astronomisches Jahrbuch for 1779: "Vollständiges Verzeichnis der bisher bemerkten Nebelsterne und Sternhäuflein bis zur 38sten Grad sudlicher Abweichung". It contains 75 entries, but several of the objects are not nebulous, and, moreover, there are many obvious errors in the list and some objects are impossible to identify. However, Bode had in 1774 discovered the Ursa Major galaxies M 81 and M 82 ("Bode's nebula"), and later on, in 1779, he also discovered M 64 in Coma Berenices. He claimed to have discovered M 51, but this galaxy was observed by Messier for the first time in 1773.
During the following years, several new nebulae were subsequently found, and some of them later on turned out to be extragalactic: M 59 and M 60 in Virgo were discovered by Koehler in Dresden (cf. Bode 1779), and M 61 (also in Virgo) by Oriano in Milan. Messier included these discoveries, and several others, in his second list, which appeared in Connoissance des Temps for 1783, published in 1780. The southern nebulae observed by Lacaille were not listed among Messier's objects, but Lacaille's list was reprinted along with Messier's own. Among the Messier objects, three nebulae discovered by Pierre Mechain proved to be galaxies: the spiral M 63 in Canes Venatici and the pair M 65 and M 66 in Leo.
The final Messier catalogue was published in 1781 (Connoissance des Temps for 1784). It included 103 objects, of which we now recognize 32 to be galaxies. However, one object is duplicated (M 102 is identical with M 101), one is a double star (M 40), one is merely an "asterism" (M 73; four stars close together), and one is non-existent (M 91). The same list appeared again in the same calendar three years later. Most of the new nebulae were, in fact, discoveries by Pierre Méchain, and the last three entries were listed under the note "Par M. Méchain, que M. Messier n'a pas encore vue" ([Objects reported] "By Mr. Méchain, which have not yet been seen by Mr. Messier"). Méchain, in a letter published in Astronomisches Jahrbuch for 1786, first pointed out that nebula No. 102 is the same objects as No. 101. Moreover, he listed in this letter six more nebulae, among them the "Sombrero" spiral, now recognized as M 104. These additional objects have been provided with Messier numbers by Camille Flammarion, Helen Sawyer Hogg and Owen Gingerich, so the number of Messier objects, including this "third supplement", is 109. Finally, in 1967, Kenneth Glyn Jones argued that the M 31 companion NGC 205 should bear the Messier number 110. This identification has been accepted in the present Catalogue. Thus 106 separate Messier objects ("nebulae and star clusters") exist, of which 39 are galaxies.
The publication of Messier's final list in 1781 marked the end of an epoch in the search for nebulae. In March of that year, William Herschel discovered the planet Uranus. When he got Connoissance des Temps in his hands, the list by Messier and Méchain inspired him to start his famous "sweeps" for nebulae and clusters of stars, which were to make him the most successful nebula discoverer in the prephotographic period. Messier himself had found 39 of the nebulae he listed. Mechain discovered 28, Lacaille 24, de Chéseaux 8 or 9, Bode 5 and Le Gentil 4. Including Mechain's "third supplement" and the southern objects found by Lacaille, a total of 137 nebulae and clusters of stars were known before the beginning of Herschel's work. It may be of some interest to sum up the total number of extragalactic objects - as we now recognize them - known at different times:
|Year||Event||Number of extragalactic objects|
|1749||Le Gentil's discovery of M 32||4|
|1771||Messier's first list||6|
|1780||Messier's second list||19|
|1781||Messier's first list, including |
Mechain's discoveries but
before Herschel's sweeps
As concerns nebula discoveries in general, the reader is referred to the more extensive table and diagram by Kenneth Glyn Jones (1969e, p. 453).
William Herschel's sweeps for nebulae started on 28 October 1783, and, after some experiments, his long series of nebula observations was commenced in December that year and continued until 30 December 1802. They ended with sweep No. 1112. During those years, Herschel discovered about 2500 nebulae or clusters of stars, i.e. the number of such objects grew by 1800 per cent during this period of Herschel's work.
His first catalogue appeared in the Philosophical Transactions, vol. LXXVI, in 1786. It contained 1000 objects divided into eight classes: "bright nebulae", "faint nebulae", "very faint nebulae", "planetary nebulae", "very large nebulae","very compressed and rich clusters of stars", "pretty much compressed and rich clusters of stars", and "coarsely scattered clusters of stars". The second and third classes contained the main part (778 objects). In this catalogue, Herschel introduced a notation system that has been used (in slightly revised versions) by most nebula observers up to the present time: "B" for "bright", "S" for "small", and so on. Herschel's next catalogue was published in the Philosophical Transactions three years later and also included 1000 entries, and his third, with 500 entries, in 1802. The total numbers of objects in the eight Herschelian classes were 288, 910, 985, 78, 52, 42, 67 and 88. The objects in Herschel's catalogues are identified by the number of the class (in Roman numerals) and the number inside the class, for example III 868. (Later on, William Herschel's discoveries were usually identified by an "H", to distinguish them from those made by his son, which were denoted by an "h".) The number of separate objects seen by Herschel is, of course owing to duplications and other errors not quite as high as 2500.
William Herschel's opinion concerning the nature of the nebulae changed quite radically towards the end of his life. He originally regarded them as separate stellar systems outside the Milky Way and thus as evidence supporting the theory of "island universes", but the appearance of some planetary nebulae made him subsequently change his mind and he come to believe that they were all "true nebulae" inside our own stellar system. His publications after 1802 contain no new discoveries, although he discussed the nature of the objects in papers dealing with "the Construction of the Heavens". The next large surveys for nebuiae were carried out by John Herschel, who observed at Slough from 1825 to 1833. With a 20-ft. reflector, he found about 500 objects which had not been recognized by his father. His observations in those years covered in all 2307 objects (some duplications); they are published in a catalogue which appeared in the Philosophical Transactions for 1833. His descriptions of the nebulae are in general more detailed than his father's, and he included 91 drawings of interesting or typical objects. William Herschel had also published drawings of nebulae and star clusters; the oldest such pictures in the astronomical literature are probably those published by Le Gentil in 1755. However, the drawings by John Herschel are those which appear to have been most frequently reproduced in astronomical textbooks up to the beginning of this century.
William Herschel had measured the positions of his nebulae in relation to nearby stars. John Herschel arranged the objects in right ascension and used the epoch 1830.0. As in most early catalogues of nebulae and star clusters, the positions are given with an unrealistic precision (to tenth of seconds in right ascension and to seconds of arc in declination). Of course, many of these positions were very erroneous due to misidentifications, writing errors and instrumental shortcomings. The positions were later on critically studied, especially by Henrik Ludvig D'Arrest in Leipzig (later in Copenhagen). In 1855, he began a survey of objects in this first catalogue by John Herschel, and these observations were published with detailed descriptions in 1856. About 320 objects were included, but some of them were discoveries by D'Arrest himself. In Copenhagen he continued these observations in a very careful manner, of which more hereafter.
From 1834 to 1838 John Herschel made his famous journey to the Cape of Good Hope, which resulted, among other things, in the discovery of about 1700 new southern nebulae (besides the clusters and nebulosities in the Magellanic Clouds). Back in England, Herschel some years later started to compile a general catalogue of all nebulae and star clusters discovered up to that time. The only "general catalogue" prepared so far was the old list by Bode dating from 1777. As several observers were now in possession of telescopes which allowed them to perceive fairly faint nebulae, a general catalogue was highly desirable, in order to avoid already known objects being reported as new discoveries; it also happened now and then that nebulae were mistaken for comets. When John Herschel started this work, he had at his disposal a manuscript catalogue prepared by Caroline Herschel during the first years of her retirement at Hanover after the death of her brother. This catalogue contained all the nebulae observed by William Herschel, with positions reduced to the common epoch 1800.0 and arranged in declination zones of 1° in breadth. The first step was to reduce these positions to 1830.0 and combine this catalogue and John Herschel's own from 1833 into one. While this work on the new catalogue was in progress, Arthur Auwers at Konigsberg published (in 1862) a sort of "general catalogue" which had been compiled quite independently of John Herschel's project: it was a list, including all of William Herschel's discoveries, arranged in right ascension and precessed to 1830.0, as well as similar lists for Messier's, Mechain's and Lacaille's objects and a list of 50 nebulae discovered by other observers. John Herschel was able to add some new nebulae to his catalogue from Auwer's lists; however, the data from Caroline Herschel's manuscript (in which all separate observations were listed) permitted a greater accuracy in the positions than Auwers had been able to achieve using William Herschel's printed catalogues. Moreover, Auwers did not list John Herschel's own observations. Thus, his ambitious work was bound to be overshadowed by John Herschel's, as A General Catalogue of Nebulae and Clusters of Stars was published by the latter in the Philosophical Transactions two years later, in 1864. It contained 5079 entries, arranged in right ascension and reduced to the epoch 1860.0. This catalogue, known among nebula observers as "GC", was an indispensable standard work for more than two decades, and it did not entirely lose this position even when more extensive catalogues were published. GC has been considered a more homogeneous work than its successors NGC and IC.
Three years after the publication of John Herschel's General Catalogue, D'Arrest published his Siderum Nebulosorum Observationes Havnienses, containing observations made with the 16-ft. Merz refractor at the Observatory in Copenhagen from 1861 to 1867. This work, written wholly in Latin, contains 4800 observations of 1942 objects, with a separate determination of the position on each occasion and careful descriptions of the appearances of the objects. (On the recommendation of Knut Lundmark, D'Arrest's Latin text was translated into English by Per Collinder in the 1930's; this translation arranged as a card catalogue is now preserved at the Uppsala Observatory.) D'Arrest's careful work revealed several corrections to the positions listed in GC. An analysis of errors in the catalogues by Lacaille, Messier and William Herschel is also to be found in D'Arrest's list of 1856 (Resultate aus Beobachtungen der Nebelflecken und Sternhaufen).
In these years, the determinations of accurate positions of nebulae attracted much attention, as the nature of the objects was unknown and the possibility of proper motion could not be excluded. Thus, the positions were usually determined with great care, and the natural way was to measure the positions of the nebulae in relation to nearby stars. The most extensive observation programs of this kind were undertaken by Schönfeld at Mannheim and by Schultz at Uppsala, who studied about 500 objects each. Dreyer, in his introduction to NGC, states that, next to D'Arrest's work, the publications by Schönfeld and Schultz furnished most corrections to John Herschel's General Catalogue when Dreyer's New General Catalogue was compiled. A discussion of several other lists of nebulae published up to the late 1880's is to be found in this introduction by Dreyer.
During the 1880's it became increasingly clear that a revised general catalogue of nebulae was needed, partly because more accurate positions were available for many objects in GC, and partly because a large number of new nebulae and clusters of stars had been discovered since 1864. The Danish-born John Louis Emil Dreyer had worked as an assistant to the Earl of Rosse at Parsontown from 1874 and made observations with the 72-in. reflector, the largest telescope in the world at that time. The power of this instrument caused him to focus his attention especially on the nebulae and star clusters, and he became very much aware of the errors in GC. During his time at Parsontown, he compiled for his own use a supplement to GC, published in 1878 in the Transactions of the Royal Irish Academy. Dreyer was able to add 1172 objects to those listed by John Herschel, besides corrections of errors. In 1879, Lord Rosse himself published a list of observations made from 1848 to 1878, together with observations made at several other places. Dreyer arranged a new supplemantary list, which he submitted to the Council of the Royal Astronomical Society. The Council, however, proposed that Dreyer should compile an entirely new general catalogue, combining all existing catalogues into one. This was the origin of Dreyer's famous work A New General Catalogue of Nebulae and Clusters of Stars (NGC), published in the Memoirs of the Royal Astronomical Society in 1888.
NGC contains positions to the epoch 1860.0 (with precessions for 1880.0), "summary descriptions", according to William Herschel's notation system, and references to GC, to John Herschel's catalogue from 1833, to William Herschel's catalogues and to lists by various other observers. It contains 7840 entries. Dreyer did a skilful job in comparing and weighing the observations, whose quality varied very much as between the different sources. In the introduction he states that "with regard to the very numerous new nebulae recorded in late years, it was frequently a matter of some difficulty to decide about the identity of objects announced by several observers, and differing little as regards place, but often much as to description". Later investigations have shown, of course, that several duplications exist in NGC, and that some position errors from the late 18th century have still survived in this catalogue. Among the objects in the northern sky, about 85 per cent have been identified as galaxies in the Catalogue of Galaxies and of Clusters of Galaxies by Zwicky et al. The identification of the faintest objects is, however, often difficult and sometimes impossible. Even for the brighter objects a few difficulties exist; the Virgo area, including NGC 4341, 4342 and 4343, is a well-known case.
Seven years later, Dreyer was ready to publish his Index Catalogue of Nebulae found in the Years 1888 to 1894 (IC I). He listed 1529 entries, together with notes and corrections to NGC. During the next few years, a rapidly growing number of nebulae were recorded photographically. About 1890, Max Wolf had already started a photographic "Durchmusterung" for nebulae at his private observatory at Heidelberg, and he continued this work on a larger scale when he became director of the Königstuhl-Sternwarte. The "Königstuhlnebellisten" (lists of nebulae from the Königstuhl) finally included about 6000 nebulae in selected fields, and only a small percentage of them are listed in NGC. Among the striking results of these surveys was the discovery of the Coma cluster of galaxies ("Wolfs Nebelneste") in 1901. Wolf also discovered the Perseus cluster (in 1905). The well-known classification system for nebulae, the Wolf code a-w with 23 standard objects, was presented for the first time in 1909.
Although Heidelberg may be regarded as the centre of the early photographic studies of the nebulae, similar observations were subsequently made at several other stations, especially at Harvard and its station at Arequipa in Peru (from about 1900). Moreover, several hundreds of nebulae were discovered by visual observers (Burnham, Finlay, Howe, Swiff and others). In 1908, Dreyer published the Second Index Catalogue of Nebulae and Clusters of Stars, containing objects found in the years 1895 to 1907 (IC II), with 3857 entries. About 1300 objects were taken from the "Königstuhlnebellisten" already published; however, the Coma cluster was not included, as Dreyer believed the nebulae there to be merely "conspicuous points of condensations or `knots' in one great mass of nebulosity". The notation spir was introduced in the description column to mark objects with spiral structure. Because of the selected use of photographic investigations, IC II is a very inhomogeneous catalogue, which for some areas lists objects down to a limiting magnitude of about 17.
Dreyer, an indefatigable compiler for more than three decades, then turned his attention to the history of astronomy and prepared his splendid editions of William Herschel's and Tycho Brahe's works, The Scientific Papers of Sir William Herschel and Tychonis Brahe Dani Opera Omnia; the later project occupied him for most of the remainder of his life. For the observers of "nebulae", NGC and IC have remained standard reference works up to the present time.
The growing importance of photographic methods during these years makes it easy to forget the enormous amount of work which was done visually at the Paris Observatory by M.G. Bigourdan and his assistants. The project consisted in the determination of very accurate positions in relation to nearby stars for all nebulae visible through the 12-in. equatorial telescope in the western tower of the Observatory. These observations commenced in 1884 and continued up to 1909; they included about 6600 separate objects, and the results - covering some 2600 pages appeared gradually in the Annales de l' Observatoire de Paris from 1886 to 1911. The nebulae themselves are carefully described, sometimes also with diameters and magnitudes estimated. The extensive introduction to Observations de Nébuleuses et d'Amas Stellaires contains, among many other things, a history of early nebula research and an elaborate bibliography of publications concerning nebulae and clusters of stars up to 1912. However, the work by Bigourdan appeared too late to influence the development of nebula research in any important way. On the other hand, Bigourdan's catalogue contained a vast amount of information, and it was frequently used as a standard reference work; it appears also to have been the most homogeneous observational work concerning nebulae which was done during the "visual" epoch.
In 1912, Max Wolf advised his assistant Adam Massinger to start a photographic survey of all the objects included in John Herschel's General Catalogue. Massinger began this work; however, at the outbreak of World War I he was called up for military service, and on 21 October 1914 he was killed at Ypern. The project was in abeyance during the war, but in 1919 the work was taken up by Karl Reinmuth, and it was decided that the study should cover all of Herschel's objects which could be reached from Heidelberg. In all, 4445 such objects had positions to the north of = -20°, and of those only 247 had not been recorded on plates taken at the Königstuhl-Sternwarte when the project was brought to an end in 1924. Plates taken as early as 1900 were used in this survey. In 1926, Reinmuth published the results in a catalogue called Die Herschel-Nebel nach Aufnahmen der Königstuhl-Sternwarte. This catalogue contains positions for the epoch 1875.0, galactic co-ordinates, classifications in the Wolf system, position angles, diameters, and descriptions (in English). Reinmuth's catalogue was the only one before the present Catalogue which contained position angles for a large number of galaxies in the northern skies. Magnitudes were estimated in the Herschel code only, but several attempts were later made to evaluate this code numerically.
Several other projects of a similar kind were undertaken during the first few decades of the century, although Reinmuth's catalogue was the largest and most important one. Among the many publications on nebulae from the Harvard College Observatory, the catalogue by Solon Baily may be mentioned as an example: it contained all NGC objects which could be seen on plates taken with a 1-in. lens with an exposure time of one hour. Baily used a fairly elaborate classification system with four main groups and several subgroups. Another similar project was started in England at approximately the same time as Massinger commenced his work on the Herschel nebulae: J.A. Hardcastle, on the advice of A.R. Hinks, surveyed the sky, using the Franklin-Adams plates. These forerunners of the Palomar Sky Survey, 206 in number, were exposed through an 10-in. telescope and the plate scale was about 180"/mm. Hardcastle's catalogue contained 785 NGC and IC objects classified as "spirals", "elongated", "diffused" or "small"; the number of galaxies among them was later estimated by Shapley and Ames at 682. This catalogue contains, besides positions and classifications, only NGC or IC numbers and position angles.
In fact, the number of catalogues or lists published during these years is astonishingly large. Several appeared from the Harvard Observatory under the directorships of Edward Pickering, Solon Baily and Harlow Shapley. Seven of these catalogues were published in the famous volume 88 of Annals of the Astronomical Observatory of Harvard College, in which the most important item is that by Harlow Shapley and Adelaide Ames: A Survey of the External Galaxies brighter than the thirteenth Magnitude (1932). This was the first catalogue which was fairly complete to a certain photographic magnitude limit, and, moreover, it was the first survey which intentionally listed galaxies only. It contained 1249 objects, of which 1025 were considered to brighter than the 13th magnitude. The magnitudes had been estimated on small-scale plates (usually 600"/mm), in such a way that the galaxies were compared with stars in standard magnitude sequences. Besides the magnitudes and the positions for the epoch 1950.0, this "Shapley-Ames Catalogue" contains diameters and classifications in the Hubble system.
Early in this century, it became clear that the number of nebulae recorded on photographic plates was extremely large. J.E. Keeler used the Crossley reflector at Mount Wilson for nebula photography from 1898 to 1900, and he estimated the total number of photographable nebulae at 120,000; he also supposed that most of them would show spiral form. It was no longer meaningful to compile "general" catalogues of the objects, if the term "general" meant that all known objects were recorded. However, Knut Lundmark hoped to compile a "general" catalogue, including all published data - apparently in the form of a "data bank" at the Lund Observatory - but this project presented unsurmountable difficulties and was never finished. A separate account of the Lund General Catalogue of Nebulae and Clusters of Stars (LGC) will be given elsewhere.
The first extensive counts of galaxies - to reveal the distribution of galaxies over the sky-were made in the early 1930's. (Earlier "counts" had included mainly objects listed in different catalogues or were made on a fairly small scale; cf. Fath 1914.) Hubble's counts in selected areas included 44,000 objects. On Harvard plates, about 106 galaxies had been recorded up to 1957, and magnitudes had been estimated for some 170,000 of them (Shapley 1957). The Lick counts (Shane and Wirtanen 1967) included about 800,000 separate galaxies. Finally, Zwicky states (1971 a) that he plotted the positions of approximately 15 × 106 galaxies in defining the clusters of galaxies in CGCG (see below).
The modern catalogues of galaxies will be treated briefly as they are assumed to be well known to all workers in this field. Four important catalogues of galaxies have been published since 1960. The first which should be mentioned is the Reference Catalogue of Bright Galaxies (usually called BG or RCBG) by Gérard and Antoinette de Vaucouleurs, published in 1964. Its aim is explained by the continuation of the title : "... being the Harvard Surbey of Galaxies brighter than the 13th magnitude of H. Shapley and A. Ames, revised, corrected, and enlarged, with Notes, Bibliography and APpendices". The project of compiling this catalogue started as early as 1949. A description of the compilation procedure, which included the comparison, weighing and checking of a vast amount of data, is described in the Introduction to BG. The catalogue contains 2599 entries (i.e. about twice as many as the Shapley-Ames catalogue). The authors expect a 50 per cent completeness level to be reached near B = -13.0. A second edition is announced but had not yet appeared when this survey was written in February, 1973.
The Palomar Sky Survey was completed in 1956. One of the first surveys of this material was Abell's search for clusters of galaxies, made on the original plates. His catalogue appeared in 1958 and contained 2512 entries. This was the first extensive catalogue of clusters only. Between 1964 and 1968 appeared the Morfologiceskij Katalog Galaktik (Morphological Catalogue of Galaxies, MGC) in four volumes, compiled in Moscow by B.E. Voroncov-Vel'jaminov and his co-workers. This catalogue contains in all about 29,000 galaxies (with additional information concerning some 5000 objects) for the sky to the north of = -33°. The limiting magnitude is approximately 15.0 (partly estimated magnitudes, partly from CGCG; see below), with the exception of the Virgo area, where the limiting magnitude is 15.2. Total diameters and diameters for "the bright inner part" are listed, also intensities for the outer and inner regions on a scale from 1 to 6, and estimated inclinations for spiral galaxies. No classifications are made (classifications by other observers are listed in the notes), but the objects are described by a detailed notation system from the centre outwards. This system is well suited for the appearance of the galaxies on the Sky Survey prints, and it is naturally adjustable to the varying information contents of their images. However, like other pure description systems, it is impossible to use for statistical investigations. Similar galaxies may be listed with different descriptions (owing to different orientation or apparent magnitude), and different types of galaxies may be listed with the same descriptions.
The Catalogue of Galaxies and of Clusters of Galaxies (CGCG; sometimes called CAT) by Fritz Zwicky and his co-workers was published in six volumes from 1961 to 1968. It includes 31,350 separate galaxies and 9700 clusters of galaxies. The catalogue is designed to be complete to the photographic magnitude 15.5, and the limiting magnitude is 15.7 (measured by the "jiggle-camera" technique), which allows for a possible error of ± 0.2 magnitudes. Besides positions, only magnitudes and sometimes short descriptions ("diffuse", "compact", "double system") are listed. No classifications are made; like Voroncov-Vel'jaminov, Zwicky has no confidence in the existing classification systems, which he regards as too simple to cover the large variety of extragalactic objects. The positions of the galaxies and the cluster's contours are recorded on maps covering the same fields as the Palomar Sky Survey.
Although some galaxies are listed as "compact" (or "very compact", "extremely compact") in CGCG, these remarks are used rather selectively and only, as Zwicky puts it, "if they [the galaxies] might easily be mistaken for stars". In 1964, Zwicky presented to the IAU Assembly at Hamburg his first list of 210 "compact" galaxies, including the Humason-Zwicky "star" HZ 46, which had been found in 1938 and was the first stellar object recognized as extragalactic. In the years 1963 to 1969, Zwicky circulated in all seven lists of "compact galaxies, compact parts of galaxies, eruptive and post-eruptive galaxies", including some 2300 objects. These galaxies were random discoveries made during the compilation of CGCG. A complete survey of twelve fields covering about 450 square degrees led Zwicky to draw the conclusion that some 200,000 - 300,000 such objects could be found with the Hale 48-in. Schmidt telescope. In 1971, Zwicky summed up his observations in the Catalogue of Selected Compact Galaxies and of Post-Eruptive Galaxies, including about 3700 entries, with descriptions and magnitudes, as well as radial velocities for 250 objects.
Finally, Robert S. Dixon, at the Ohio State University Radio Observatory, has announced the preparation of a "Master List" of all known non-stellar objects, similar to the list of radio sources published by the same Observatory 1970. Although the details of this project are not known at present, it may be expected that this "Master List" will be the first literally "general" catalogue of such objects prepared since Dreyer's time.
U 3528A + 3536A) VV 248; a) 0.9 × 0.8, distorted SB(s)...; b) 0.8 × 0.6, sep 1.5 - U 428OA) VII Zw 220; a) (0.8 × 0.5), E; b) 0.22 × 0.20, sep 0.35; "red spherical compact (possibly plus star), m = 15.6 northeast of E galaxy. Total brightn of the pair, m = 14.4" (Zw) - U 5854A) comp 2.0, 310, 0.3 × 0.2; 2.2, 259, 0.3: × 0.1; 2.9, 230, 0.3 × 0.1 - U 7064A) three compact conds, one v red, alm invis on blue print; dif jets and bridges; sev F comps - U 7085A) VV 13, Arp 97; An 1203 A, B in BG;a) v = 7010, v0 = 7002; b) v = 6894, v0 = 6886; see ApJ 132, 627 -
U 1371) 1II = - 54.65, not + 54.65 - Page 182) add "3640" to
NGC numbers at the bottom of the page
U 8511) position angle 171, not 191
U 12493) radial velocity + 8700 in column 11, line a, not line b -