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Deep sky catalogues of the southern skies

In this section: Lacaille, James Dunlop, John Herschel, J.L.E. Dreyer, Royal Observatory (Cape of Good Hope), Jack Bennett.

Messier's list isn't the only one, you know. Nor the oldest, for that matter. Messier published the first version of his catalogue in 1771; twenty years before him, another Frenchman, Nicholas-Louis de la Caille, was observing the southern sky from Cape Town and compiled the first southern list of non-stellar objects.

Abbe Nicholas Louis de la Caille

One of the earliest catalogues of deep sky objects is the one prepared in the 1750's by Abbe Nicholas Louis de la Caille (1713–1762), who has the distinct honour of being the first person to systematically observe the whole sky. During his stay at the Cape of Good Hope, Lacaille drew up a short list of the most remarkable objects he had come across.

Lacaille was at the Cape primarily to compile a star catalogue. During his short visit (1751–1752) he accurately recorded positions and magnitudes of no less than 10 000 stars. He also published a list of 42 southern nebulae, which he divided into three types: 'Nebulosities not accompanied by stars', 'Nebulosities due to clusters', and 'Stars accompanied by nebulosity. In his report to the French Academy of Sciences, Lacaille wrote:

"The so-called nebulous stars offer to the eyes of the observers a spectacle so varied that their exact and detailed description can occupy astronomers for a long time and give rise to a great number of curious reflections on the part of philosophers. As singular as those nebulae are which can be seen from Europe, those which lie in the vicinity of the south pole concede to them nothing, either in number or appearance. I am sketching out this description and list to serve as a guide for those with the equipment and leisure to study them with larger telescopes. I would have greatly desired to present something more detailed and instructive in this article, but with ordinary refractors of 15 to 18 inches [in length] such as I had at the Cape of Good Hope, I had neither adequate nor convenient enough instruments for this kind of research. Those who do take the trouble to see what has occupied me during my foreign sojourn will see well enough that I did not have time to make that sort of observation."

"I have found a great number of the three types of nebulosities in the southern part of the sky, but I do not flatter myself to think that I have noticed them all, especially those of the first and third types, because they can only be perceived after twilight and in the absence of the moon. However, I do hope that the list is passably complete in regard to the most remarkable of the three types." (Quoted from Gingerich)

Evans (1990) notes: "Considering that this list is based on the data from [the star catalogue], that is, almost always one shot of the passing sky in a very small telescope, certainly very inferior to modern binoculars, it is as good as might be expected. One can also reflect that Lacaille seems to have been the first person ever to observe systematically the whole sky, an honor which the present author once ascribed to Sir John Herschel, who used much the same technique with a telescope of some ten times larger diameter from a site some 6 miles south of Lacaille's. He, of course, did very much better and is the principal source of the southern data in the NGC."

Lacaille also studied the Magellanic Clouds:

"As a result of examining several times with a telescope . . . those parts of the Milky Way where the whiteness is most remarkable and comparing them with the two clouds common called the Magellanic Clouds, which the Dutch and Danes call the Cape Clouds, I saw that the white parts of the sky were similar in nature, or that the clouds are detached parts of the Milky Way, which itself is often made of separated bits. It is not certain that the whiteness of these parts is caused, according to received wisdom, by clusters of faint stars more closely packed than in other parts of the sky, whether of the Milky Way or of the Clouds, I never saw with the ... telescope anything but a whiteness of the sky and no more stars than elsewhere where the sky is dark. I think I may speculate that the nebulosities of the first kind are nothing more than bits of the Milky Way spread round the sky, and that those of the third kind are stars, which by accident are in front of luminous patches." (Quoted from Evans)

His record of the Coalsack, the prominent dark nebula in Crux, is one of the first on record:

"One can add among the phenomena which strike the eye of anyone looking at the southern sky, a space of about 3 degrees in every direction which seems intensely black in the eastern part of the Southern Cross. This is caused by the contrast with the brightness of the Milky Way which surrounds this space on all sides."

James Dunlop

The first "proper" southern deep sky catalogue was drawn up by a former factory hand who had taught himself astronomy – James Dunlop (1795-1848). Dunlop arrived in Australia in 1821 and observed the southern skies with a 9-inch speculum mirror from Paramatta ('Place of Eels'), New South Wales.

He drew up a list of about 600 deep sky objects, for which he was awarded the Gold Medal of the Royal Astronomical Society. In his 1827 article, presented to the Royal Society by Sir John Herschel, Dunlop writes:

"The following nebulae and clusters of stars in the southern hemisphere were observed by me at my house in Paramatta, situated about 6" of a degree south and about 1s.78 of time east of the Brisbane Observatory. The observations were made in the open air, with an excellent 9feet reflecting telescope, the clear aperture of the large mirror being nine inches. This telescope was occasionally fitted up as a meridian telescope ...

"... the eye end of the telescope was raised or lowered by a cord over a pulley attached to a strong wooden post let into the ground about two feet: with this apparatus I have observed a sweep of eight or ten degrees in breadth with very little deviation ... and the tremor was very little even with a considerable magnifying power.

"I made drawings or representations of a great number of the nebulae and clusters at the time of observation ... and also very correct drawings of the Nebulae major and minor, together with a representation of the milky nebulosity surrounding the star Eta Robur Caroli. ...

"The reductions and arrangement have been principally made since my return to Europe; and I trust this catalogue of the nebulae will be found an acceptable addition to that knowledge which the Brisbane observatory has been the means of putting the world in possession of, respecting that important and hitherto but little known portion of the heavens."

For various reasons, a great many entries in Dunlop's catalogue are suspect. Some are badly described and are difficult to verify, while others simply do not exist. John Herschel was the first astronomer to try and locate Dunlop's objects and he had modest success (see his comments in the next section). This suggests a great project for a modern-day observer: what happened to Dunlop's objects?

Further reading

1. Cozens G & White G L (2001) James Dunlop: Messier of the southern sky. Sky & Telescope, June, 112–116.
2. Cozens G (1987) James Dunlop – Pioneer of the Southern Skies. Universe, February, 6–7.

John Herschel

Shortly after Dunlop's work appeared, the renowned observer John Frederick William Herschel arrived in Cape Town to continue his survey of the northern skies. In 1847 his lengthy "Cape Results" was published, containing the first thorough deep sky catalogue of southern objects. Evans writes:

"On January 15, 1834 there arrived at the Cape a young English scientist and philosopher who for sheer drive, intellectual capacity and versatility has had few equals in the long history of callers at Table Bay.

"John Frederick William Herschel, Bart., K.H. was born at Slough, near Windsor, on the 7th March, 1792 and died at Collingwood, Kent on the 11th May, 1871, in his eightieth year. He is buried in Westminster Abbey not far from Newton. John was the only child of Sir Frederick William Herschel and, like his father, was one of the greatest astronomers of all ages."

"In 1820, assisted by his father, he completed a mirror of 18 inches diameter and 20 feet focal length [f/13] which became the heart of the telescope with which much of his later observing was done."

"...there was an air of dilettantism about Herschel. His mind ranged over a vast number of subjects ... but when it came to physical hard work his enthusiasm would often flag ... Professor Pritchard in his biographical notice in the Encyclopaedia Britannica wrote 'Herschel had become an astronomer from a sense of duty, just as his father had become one by fascination and fixed resolve: hence it was by filial loyalty to his father's memory that he was now impelled to undertake the completion of that work which at Slough had been so grandly commended. William Herschel had explored the northern heavens; John Herschel determined to explore the heavens of the south, as well as re-explore the north.' "

Evans continues:

"We need now to envisage Herschel some time in 1832: He is one of the most celebrated scientists in Europe, a knight, honored by numerous scientific bodies, happily married, acquainted with all the most distinguished scientists of Europe, a polyglot, with German, French, Italian, Latin and Greek at his command. He has made a name for himself in mathematics, astronomy, chemistry and several other fields. Now he thinks of observing the southern sky just as he has done the northern, and he seeks for a place to carry out his ambition. The choice is very limited . . . South America, South Africa, and Australia." For obvious reasons he chose South Africa; it offered, amongst others, "an astronomical tradition and an active observatory, a healthy climate, and a convenient longitude."

"To achieve this object, he and his family set sail from England late in 1833 aboard the East Indiaman, the 'Mountstuart Elphinstone'. After a voyage of two months they came ashore, Herschel himself, his wife, the four children Caroline, Emilia Mary, William James, and Isabella. Then there was the mechanic, John Stone, who had in his charge the 20-foot reflector of 18 inches aperture (with classical pedantry the mirrors provided for it were often referred to as 'sesquipedalian') and an equatorially mounted refractor of 5 inches diameter and 7 feet focus [f/17]."

For the next four years, Herschel swept the southern skies, producing a detailed catalogue of non-stellar objects, published in 1847 as Results of Astronomical Observations made during the years 1834, 5, 6, 7, 8, at the Cape of Good Hope; Being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825.

This southern catalogue gives positions and descriptions for 1708 objects, of which 98 also appear in his northern catalogue. Of these objects, Herschel notes:

"206 have also been identified, with more or less certainty . . . with objects observed by Mr Dunlop, and described in his Catalogue of Nebulae. The rest of the 629 objects, comprised in that catalogue, have escaped my observation; and as I am not conscious of any such negligence in the act of sweeping as could give rise to so large a defalcation, but, on the contrary, by entering them on my working lists . . . took the usual precautions to ensure their rediscovery; and as I am, moreover, of opinion that my examination of the southern circumpolar region will be found, on the whole, to have been an effective one, I cannot help concluding that, at least in the majority of those cases, a want of sufficient light or defining power in the instrument used by Mr Dunlop, has been the cause of his setting down objects as nebulae where none really exist. That this is the case, in many instances, I have convinced myself by careful and persevering search over and around the places indicated in his catalogue."

John Herschel's diaries during his 19th century stay at the Cape (extracts of which were published by Warner & Warner (1984) and Evans et al (1969)) give a fascinating insight into the life of this active observer.

J. L. E. Dreyer

Harold G. Corwin, Jr., of the NGC/IC Project, writes:

Johann Dreyer's 1888 update of John Herschel's 'General Catalogue' has become the most widely-used deep sky catalogue of all times, listing 8,000 objects of all types excepting dark nebulae.

Dreyer was born in Denmark, but emigrated to Ireland in 1874 to work at Lord Rosse's great observatory in Parsonstown. Though an amateur, the third Earl of Rosse had built successively larger speculum mirror reflecting telescopes through the late 1830's and early 1840's. The series culminated in a massive 72-inch telescope, the largest in the world from the date of its completion in 1845 until its dismantling just before the first World War. Rosse, his son (the fourth earl), and his observers (Dreyer was one of these) spent years examining and measuring the known nebulae in the northern sky with the famous "Leviathan of Parsonstown," and discovered many more fainter nebulae themselves.

During the observations, it became clear to Dreyer that it was time to update Sir John Herschel's so-called GC (General Catalogue) of nebulae and star clusters, published in 1864. Just a decade later, there were simply too many new nebulae being discovered and too many different lists to consult for previous discoveries. Preparing observing lists or simply finding if a nebula had been previously found by another observer had become a time-consuming chore. Thus, Dreyer published a supplement to the GC of about 1000 new objects in 1878, and - having suggested yet another supplement in 1886 - was instead asked by the Royal Astronomical Society to assemble a "new general catalogue" of non-stellar objects. Dreyer, who had in the meanwhile been appointed Director of Armagh Observatory in Northern Ireland in 1882, then added the latest 1500 objects to the previous lists, combined them all in Right Ascension (for 1860) order, and the "New General Catalogue of Nebulae and Star Clusters" appeared as Volume 49, Number 1 of the Memoirs of the RAS in 1888.

Assembling the NGC, Dreyer had to contend with conflicting claims of position and description for what he often suspected to be the same nebula. Some of these he could check himself; most - because of the overwhelming numbers of objects - he simply had to accept as published. Fortunately, he was an excellent transcriber - very few of the errors in the NGC can be traced to carelessness on his part (his most common error, or that of his uncredited assistants, was to apply precession with the wrong sign to declinations). Most of the problems in the NGC are with the original positions and descriptions, coming as they did from many different observers using telescopes ranging in size from 2 inches to 72 inches, and relying on auxiliary instrumentation that ranged from nonexistent to state of the art.

At one extreme were the careful observations of the astrometrists working at professional observatories. These observers were actively involved not only in nebular astronomy - indeed, with one or two exceptions, the nebulae consumed very little of their time and attention - but in setting up the fundamental coordinate reference system which we still use today in a form little different than it was a century ago. These observations, made with precision optical micrometers, tie the positions of the nebulae into the fundamental system with random errors on the order of a few tenths of an arc-second. Only in the past two decades have better positions for a significant number of galaxies been determined from plates taken with astrographic telescopes.

On the other hand, there were many amateur and semi-professional astronomers actively making discoveries. Some of these observers were well-equipped with large, solidly-mounted telescopes fitted out with modern instrumentation. Some, like Isaac Roberts, were even at the forefront of astronomical photography. Others were neither as fortunate in their observing gear, nor as careful in their observing habits. Many scanned the skies with nothing more than an eyepiece and setting circles at their disposal. Yet, they published their discoveries along side those from the professional observatories. Dreyer had to handle them all.

Thus, the NGC is a compendium of the good and the not so good. Through long experience and comparison of lists of positions from various observers, Dreyer had a good idea whose observations were trustworthy, and whose were not. He also realized the importance of not letting this information be lost (as well as simply giving credit where it's due!), so he listed the discoverers of each nebula and star cluster, and gave fairly complete references to the discovery publications.

In 1895 and 1908, he published supplements to the NGC which he called the Index Catalogues. Nearly all of the bright, large, nearby non-stellar celestial objects have entries in one of these three catalogues. Thus, the catalogue numbers - preceded by the catalogue acronyms, NGC and IC - are still frequently used by astronomers to refer to these objects.

Further reading

1. Gingerich O (1988) J L E Dreyer and his NGC. Sky & Telescope, December, 621.
2. Sinnott R W (1988) NGC 2000.0, The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer. Sky Publishing Corporation and Cambridge University Press.

Royal Observatory, Cape of Good Hope

Several deep sky objects were discovered at the Royal Observatory, Cape of Good Hope, around the turn of the 20th century. R T A Innes and J Lunt both recorded 7 new IC objects, while W H Finlay discovered three. Most of the discoveries were made with the 6- and 7-inch refractors, although the 18- and 24-inch telescopes were also used.

Notes to the table:
The description is taken from the published article, as referenced in the last column: (1) Monthly Notices of the Royal Astronomical Society, Vol 62, April 1902, p 468. "Notes on Nebulae observed at the Royal Observatory, Cape of Good Hope." (communicated by Sir David Gill, KCB, FRS, HM Astronomer). (2) MNRAS, Vol 59, March 1899, p 339. "Nebulae observed at the Royal Observatory, Cape of Good Hope, in 1898." (communicated by David Gill, CB, FRS, &c, HM Astronomer). (3) MNRAS, Vol 58, March 1898, p 329. "Nebulae observed at the Royal Observatory, Cape of Good Hope." (communicated by Dr David Gill, CB, &c, HM Astronomer).
Entries marked with an asterisk are flagged as non-existent in the RNGC.

Jack Bennett

For two decades, starting in the late 1960's, the southern sky was patrolled by a dedicated South African comet-hunter named Jack Bennett. He observed from his urban backyard with a 5-inch low-power refractor. Not only did he discover two comets, he also picked up a 9th magnitude supernova in NGC 5236 (M83), becoming the first person ever to visually discover a supernova since the invention of the telescope.

Bennett was born on April 6th, 1914 and passed away on May 30th, 1990. A long-standing member of the Astronomical Society of Southern Africa (ASSA), he was elected President in 1969. The Society awarded him the prestigious Gill Medal for services to astronomy in 1970 and in 1986 he received an Honorary Degree of Master of Science from the University of Witwatersrand. In 1989, at the recommendation of Rob McNaught of Siding Springs Observatory, the asteroid VD 4093 was named after him.

Bennett was a skilled observer and in the spirit of Charles Messier drew up two lists of southern objects that appeared comet-like in his telescope. His first list (Bennett, 1969) was published four months before he discovered his first comet. The supplementary list (Bennett, 1974) was followed three months later by his second discovery.

In his 1969 Presidential Address to the ASSA Bennett said: "As an aid to the recognition of comet-like objects in the Southern sky, and to help observers to eliminate them in comet searches, I have over the past five years compiled a list of 130 such objects visible south of the celestial equator. Nearly a hundred of these have been encountered under varying conditions in comet sweeps using a 5-inch short-focus refractor with a magnification of 21 diameters. The rest have been added, and duly observed with the same telescope, after consulting various sources, notably E. J. Hartung's first-rate book Astronomical Objects for Southern Telescopes which includes details of the appearance in telescopes of various apertures of all but 16 of the 130 objects."

Bennett's 1974 article "Some objects of interest in the southern sky" introduced 22 new comet-like objects "which had been observed (many of them repeatedly) in comet sweeps" since his first list was published.

These two lists have been combined to form the Bennett Catalogue. Bennett's list reads like the "Who's Who of the Deep Sky". Among the 152 objects are the Tarantula, Omega Centauri, 47 Tucanae, Sombrero and the Silver Coin. Twenty-six of Messier's objects are listed. Bennett noted that including such bright objects may be unnecessary, but added: "it is surprising how easily even these can be mistaken for comets when seen at low altitudes and poor conditions."

Almost half the objects in Bennett's list are globular clusters, which makes sense since these bear a striking resemblance to comets. The constellation richest in Bennett objects is Sagittarius, followed by Ophiuchus. Bennett wrote that "the constellations Scorpio, Ophiuchus and Sagittarius… contain a bewildering variety of comet-like objects. These are mostly globular clusters and all except the largest defy attempts to distinguish them from tailless comets. This relatively small area of sky contains about a third of all the comet-like objects visible with small telescopes south of the equator."

Dorado also contains many Bennett's – five galaxies and six clusters and nebulae. The latter lie within the Large Magellanic Cloud which, according to Bennett, "should normally be avoided like the plague by anyone looking for comets. There are, however, a few objects on the outskirts of the Clouds which are regularly encountered in comet sweeps, and these have been included in the list, if only as a warning to the observer of the perils that lie ahead of him!"

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