John Herschel's Southern Nonexistent rngc objects




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John Herschel's Southern Nonexistent RNGC Objects
Abstract. Sir John Herschel’s observation of deepsky objects from the Cape of Good Hope produced a catalogue of 1708 objects, later included in the New General Catalogue (NGC). Sulentic and Tifft’s 1973 version of the NGC, the Revised New General Catalogue (RNGC), lists 89 of Herschel’s Cape objects as “nonexistent”.

It is shown in this article that 12 of these are true objects, 33 have equivalent identities and 9 are nonexistent as nonstellar objects. The remaining 34 doubtful objects refer to milky way clouds.

Furthermore, of the 109 objects listed in the RNGC as “unverified southern objects”, all but three have been confirmed.

During the course of the work a further 50 equivalent identities were uncovered that are not listed in the RNGC.

All results have been combined into one comprehensive table of corrections to the NGC and IC catalogues.

Auke Slotegraaf

auke@psychohistorian.org

Director: Deepsky Observing Section,

Astronomical Society of Southern Africa

1. Development Of The NGC
1.1 The Northern Catalogue

John Herschel's career in observational astronomy started in 1821 at his parent's home in Slough, England. Two years later he began a systematic survey of the northern heavens, completing the work his father had begun in 1783. The method of observing was called sweeping. The sky was divided into zones of declination three degrees wide and each zone carefully examined for nebulae, clusters and double stars. The telescope was held fixed at a certain inclination on the meridian. “In this way, as the earth rotated, all the objects at a certain declination would be carried successively into the field and could be noted. Their positions could be derived either from the inclination of the telescope and the time shown on a sidereal clock, or from the positional coordinates of a number of standard or 'zero' stars of which the positions were known by other means.” (Evans et al., 1969:xvi) By 1832, Herschel had carried out over 400 sweeps and recorded 2307 objects. The results were published in the Philosophical Transactions of the Royal Society for 1833 as a “Catalogue of Northern Nebulae and Clusters.”



1.2 The Southern Catalogue

Herschel was determined to extend his northern catalogue to cover the southern skies, and set out in November 1833 to carry out his ambition. He arrived at the Cape of Good Hope in January 1834 and less than two months later he started his systematic survey of the southern sky. When he returned to England in 1838, he took with him an extensive catalogue of southern nebulae and clusters observed in 380 sweeps.

After some delay, his southern catalogue was published in 1847 as the “Results of Astronomical Observations made during the years 1834-1838 at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825” (hereafter referred to as the Cape Observations). Herschel wrote: “The number of nebulae and clusters comprised in the catalogue here presented, is 1708. Of these eighty-nine are identical with objects previously observed by myself at Slough, and which occur, in consequence, in my Northern Catalogue. . . Of the objects remaining, 135 are nebulae and clusters of my Father's catalogues, now, for the first time, reobserved; 9 are Messier's, 5 of which are identical with objects catalogued by Mr. Dunlop.” (Herschel, 1847:3).

1.3 The General Catalogue

With his Cape Observations published, Herschel found it “extremely desirable to have presented in one work, without the necessity of turning over many volumes, a general catalogue of all the nebulae and clusters of stars actually known, both northern and southern, arranged in order of right ascension and reduced to a common and sufficiently advanced epoch which may serve as a general index to them, and enable an observer at once to turn his instrument on any one of them, as well as to put it in his power immediately to ascertain whether any object of this nature which he may encounter in his observations is new, or should be set down as one previously observed.” (Herschel, 1864) Herschel combined his own and his father's catalogues (along with about 450 objects from other observers) into one comprehensive work, which was published in the Philosophical Transactions for 1864 as a “General Catalogue of Nebulae and Clusters of Stars.”



1.4 The New General Catalogue

By this time, several other observers were involved in measuring the positions of non-stellar objects, but their results were not available when the General Catalogue was being compiled. Work in this field continued steadily, and in 1878 J. L. E. Dreyer published a list of corrections to the General Catalogue as well as a catalogue of new nebulae. Several lists continued to appear and were combined by Dreyer in 1888 into “A New General Catalogue of Nebulae and Clusters of Stars, being the Catalogue of the late Sir John F. W. Herschel, Bart., revised, corrected and enlarged.”

This important work, containing 7840 objects, is popularly referred to as the NGC. In the introduction to the NGC, Dreyer wrote: “Before the appearance of this valuable work [Herschel's General Catalogue], several astronomers had commenced determining accurate positions of nebulae . . . There are many cases where the General Catalogue, although evincing the most scrupulous care both in observing and reducing, is not in accordance with the heavens.” (Dreyer, 1888)
1.5 The Revised New General Catalogue

The first major revision of the NGC appeared in 1973 as the “Revised New General Catalogue of Nonstellar Astronomical Objects” (RNGC). Authored by J. W. Sulentic and W. G. Tifft, it contains 8163 objects. The revision started in 1964 when Sulentic began compilation of a file for NGC objects, listing references in the literature and standard catalogues for about 5000 objects. All objects north of -45°, unidentified in the literature or listed as nonexistent, were examined on prints of the Palomar Observatory Sky Survey. An NGC entry was accepted if an appropriate object was found to lie within a radius of 5' of the original published position. If a relatively bright object, which matched closely the Dreyer description, fell outside the 5' radius, it was usually accepted.

The lack of a suitable photographic sky survey south of -45° declination resulted in difficulties in the identification of southern objects. After employing available literature sources, there remained about 150 objects for which no reference could be found except in the original NGC. Fifty southern objects were identified on plates specially obtained using the 24-inch Michigan Curtis-Schmidt telescope at Cerro Tololo Inter-American Observatory. An additional ten objects were identified with the aid of plates from the Cordoba and Radcliffe observatories. Approximately ninety southern objects remained for which no verification was possible. The authors conclude: “it is apparent that most [of these] objects do in fact exist and will lie within 5' of their NGC positions.” Regarding objects classified as nonexistent, the authors note: “In the course of this work, explanations for many nonexistent objects became apparent. It was found that, when plotted, NGC number frequently appear clustered in pairs or groups; different sources were generally given for each group member. In reality, only one object is probably involved, and the differing positions apparently represent differences between observers. In many cases, an NGC number was assigned to a close pair or group of faint stars which appeared nebulous to an early observer. Sometimes, there is an NGC object described as a cluster which is now considered to be only a field irregularity or very loose grouping not generally accepted as a cluster.” (Sulentic & Tifft 1973:xvii)
2. Sir John Herschel's Cape Observations And The RNGC
2.1 Nonexistent objects

Of particular interest to southern observers are nonexistent RNGC objects which were observed by Herschel at the Cape of Good Hope. Eighty-nine such objects are listed in the RNGC. For this study, these objects were divided into six groups based on the RNGC reason for their nonexistence. Each object was examined by comparing the RNGC information with the original Cape Observations and various modern sources, several of which were unavailable at the time the RNGC was compiled. This resulted in six annotated tables of nonexistent RNGC objects - see Table 1.



Table 1. Number Of Herschel's Objects Called Nonexistent In The RNGC




RNGC reason for nonexistence

No. of cases in Cape Obs.

Table 2.

Equivalent identities given

8

Table 3.

No reason given for nonexistence

11

Table 4.

Not Found or Missing

31

Table 5.

No nebulosity

1

Table 6.

Stars; Single, Double, Groups

4

Table 7.

No cluster

34

The information gathered from these tables was combined into four new lists, giving Equivalent identities and 2000.0 Coordinates (Table 8), Nonexistent as nonstellar objects (Table 9), Existing objects called nonexistent in RNGC (Table 10) and Doubtful objects (Table 11).

The first three tables are additions and corrections to the RNGC. Table 11 lists “doubtful” objects that will need further examination. Four of these objects (NGC 3505, NGC 4776, NGC 4778 & NGC 6082) may be nonexistent as nonstellar objects. The remaining 30 objects are either large clusters of stars or nebulous milky way patches. While these objects may not be true physical entities, they are no less real to the visual observer. A good example of this is M24, the Small Sagittarius Star Cloud. Although only a milky way star cloud, it nevertheless forms a gestalt in the observer's mind. Conversely, many of Collinder's open clusters appear in the telescope as little more than a handful of scattered stars, bearing only slight resemblance to the traditional view of a cluster. When observing the 30 “clusters and nebulae”, it should become clear that their visual appearance should dictate their inclusion in future observing guides.



2.2 Unverified Southern Objects

In addition to the 89 “nonexistent” objects, the RNGC lists a further 109 entries as unverified southern objects. Of these, only one (NGC 1957) was not recorded by Herschel. Table 12 lists these objects with their 2000.0 coordinates taken from the ESO/Upps survey and includes comments. Only three objects on the list are doubtful: two scattered clusters (NGC 1252 & NGC 2348) and one possible galaxy (NGC 3283).



2.3 Equivalent identities for Herschel's objects not stated in the RNGC

In the course of compiling these tables, it became clear that a number of Herschel's Cape objects were also recorded in the Index Catalogues. Furthermore, equivalent identities (within the NGC) not resulting from a RNGC nonexistent code were found. These new identities are presented in Appendix A.

For convenience, Appendix A is merged with Table 8 (Equivalent identities and 2000.0 Coordinates) to produce, as Appendix B, a comprehensive table of corrections to the NGC and IC.

3. Concluding Remarks

I would appreciate comments from observers who would like to check up on my suggestions presented in Tables 8 through 11. Several of the doubtful objects listed in Table 11 should be within reach of binoculars – all observers are encouraged to try and find these “nonexistent” objects and send their observations to me.


4. Acknowledgements

I would like to thank Mrs Ethleen Lastovica, SAAO librarian, for her knowledgeable and patient assistance, and Dr Harold G. Corwin Jr. for supplying a copy of the machine-readable version of the “Southern Galaxy Catalogue”.



Table 2. Equivalent Identities Given In RNGC

NGC

RNGC

NGC

NGC 2000.0

ESO/Upps

SGC

Morel

Notes

241

= 242




= 242?

= 242, 029-SC006




= 242

1

724

= 723

? = 723

= 723

= 723, 477- G013

= 723




2

1340

= 1344

= 1344

= 1344

= 1344, 418-G005

= 1344




3

1457

= 1448

= 1448

= 1448

= 1448, 249-G016

= 1448




4

1649

= 1652




= 1652?

= 1652, 055-SC032




= 1652

5

1855

= 1854




= 1854?

= 1854, 056-SC072




= 1854

6

2442

= 2443







= 2443; 059-G008

= 2443




7

6216

= 6222







= 6222, 277-SC014







8
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