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Discovery of the Dunlop Pairs

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When Dunlop started his own observations of doubles star in 1826-1828, little was know the extent nor numbers of southern systems. Comparatively speaking, in the northern skies the number and quality of observations made by similar observers had been improving significantly. You have only to compare the work of likely the greatest double star observe of all, Frederick Wilhelm Struve (1793-1864), who was observed about the same time as Dunlop. Struve’s brilliant double star career began in 1814, whose first published double star catalogue in 1822 contained some 795 new systems, Later, using the sizeable quality 24.5cm (9.6-inch) Fraunhofer refractor at the Russian observatory in Dorpat, he undertook the methodical discovery and started measuring double with a very high-quality filar micrometer. Struve found more than 3,000 ‘Σ’ pairs, with the main catalogue published in 1837. After this, Struve continued to repeatedly measured most of the other then known northern pairs.

In comparison to this story, observations made by James Dunlop just seem an incompetent work and quite amateurish.

During the 1820s, the southern skies below about −30° declination were literally the ‘virgin frontier’. In the annals of history the works of Dunlop and Rümker marks only the first initial survey. Historically we have held in the wider astronomical community the persistent story that has much maligned and berated James Dunlop. They openly criticise his first catalogues, claiming that most objects were imaginary or even optical mirages from his poorly made equipment. Another common slight is that the many poorly obtained positions of all his objects contained too many avoidable errors. In regards his double star measures, many still deem these observations of little consequence — mainly because they are generally wide and likely most are only optical pairs.

Although some of the negative views maybe true, the reasoning behind his presumed weaknesses could be simply argued because of Dunlop’s isolation away from his peers and the rest of the astronomical community in both England and Europe. With the new colony of New South Wales several months away by ship, communication was very difficult and slow. A simple question or advisement could take three to five months to answer. At first, the new discoveries received many accolades. Yet, when other observers re-examined his discoveries, their great elation soon turns to real disappointment and honest hostile criticism.

Importantly, the key issue with our judgements is that Dunlop never did claim that his observations were either perfect nor very exacting. Creating his new double star list was made under the very least favourable conditions, often when atmospheric seeing was poor or when the Moon was near full, whose general timing was when deep-sky observing or measuring stellar astrometric positions was nearly impossible. For Dunlop, there could be no opportunity of clear skies to be lost. Moreover, he allegedly did all this work in just fractional over a single year. (There remain some disagreement with this later point, because the list of doubles may span back to 1822 when the observervations of the Paramatta Star Catalogue was being made.)

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The Demeanours of Rümker and Dunlop

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At the very start, likely from their first meeting, Charles Rümker and James Dunlop were never friends. Whilst in Australia they were really acquaintances and barely tolerated each other — forced together as astronomical coworkers. When they spoke to other people of the other’s attributes or abilities, they were sometimes expressed in quite derogatory terms. At times their open hostility caused several problems with the Colonial Government, requiring in one or two cases, intervention to settle issues with withholding of vital observations. After the sudden departure of Sir Thomas Brisbane in December 1825, who had been the sole financier and subsequent driver of the observational projects at Paramatta Observatory, these issues came to a head — especially in who should be chosen to supervise and run the Observatory. Several times, they openly rebelled against the Governmental decisions, and they clearly refused to work together. Both were so forthright and stubborn, that they were quite unashamedly quite happy to sacrifice everything — so, at different times, they preferred to leave Paramatta Observatory altogether for several years instead of collaborating.

The origin for these dramas were probably their
quite different professions and personalities.

Consequences From Their Actions

For whatever reason, the personalities and portrayed behaviours between Dunlop and Rümker was to place them both diametrically on opposite sides. Most of the merge literature about them, seems to suggest this is likely scenario that explains their many astronomical shortcomings and failures in New South Wales. It is not unreasonable to conclude that Dunlop’s shared Scottish heritage with Brisbane, made Rümker to be the ‘odd man out’. I am still unsure if Rümker personally did feel indignation or bitterness against them, but he was certainly more resentful of the strong long-term friendship and open camaraderie between the two Scots. There is no doubt that much the friction was exacerbated by differences in their personalities or their differing cultures and heritage. It is quite reasonable to believe this to be true.

In the end, this clash meant they were not being able to working together to produce double star and deep-sky probably irreparably damaged both of them. Had they contributed their skills towards their common cause, the story of the astronomical exploration of the southern skies would probably have been somewhat different and even more complete. Unfortunately, this was not to be, and the kudos and honour for this was to be taken by John Herschel in South Africa almost ten years later.

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Comparing the Double Star Catalogues of Rümker and Dunlop

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As we have said previously, large proportions of the Dunlop pairs are usually wide to ultra-wide. As the probability that two randomly selected stars in proximity are associated decreases with apparent distance, therefore, the natural conclusion will deem such pairs as less important. Moreover, even if both stars were physically joined by gravitation, it would still take many centuries before this could be ascertained. Indeed many of the later criticisms levelled against reputation of Dunlop stem for the huge broadness of his selection of pairs. The other is the very high proportion of missing pairs. Here only 165 of 253 stars (65%) are real pairs, with three being duplicates of multiple stars. I.e. γ Velorum (Δ64,Δ65), u Car (Δ102,Δ103) α^1,2 Crucis (Δ122,Δ123). In the published catalogue, thirty-two (32) were rediscoveries of previously known pairs, thirty (30) being unknown or lost doubles, with twenty-two (22) remaining uncertain with their identification. One star was not even double star being the variable star, R Scl as Δ3 !

Rümker instead has the better reputation, due to many of his pairs are closer together in separation and as they have better chances of being binaries. Today, and perhaps likely unfairly, his double stars remains much higher in importance than those of Dunlop. Rümker found only twenty-eight (28) pairs with sixteen (16) (57%) donned as real discoveries by him. Only five (5) of these were discovered earlier by Dunlop. I.e. RMK 1, 5, 6, 7 and 21. Another seven (7) are either missing, wrongly identified, or are single stars.

It is interesting why people continue to have this wayward impression that Dunlop was a poor observer but Rümker was better. For me, after looking the latest Washington Double Star Catalogue (WDS11) data, we have observed many of his pairs though there are fewer measures — at least when compared other more southern ‘valued pairs’. Perhaps we have continued to wrongly interpret our perspective on the achievements of these two double star catalogues. Observationally, and in the light of what was to come, BOTH Dunlop and Rümker reputations should be somewhat tarnished!

No doubt, the start of degradation against these observers began when the eminent John Herschel started his southern observations that aimed to join his father’s earlier northern work into the first all-sky survey of double stars and deep-sky objects. Between 1834 and 1838, John Herschel found over two thousand new pairs, and measured many of them. In addition, John Herschel found nearly all of the errors in the Dunlop double star catalogue.

For me it all depends on how you look at his double star catalogue. If you look at it from the perspective of an initial survey then what he has produced really just lays down the groundwork for the other surveys. John Herschel found much promise in the observations of Dunlop, and began to advocate for Dunlop in his isolated position. While John Herschel was still in England, he keenly read Dunlop’s deep-sky and double star work with great interest and enthusiasm. Herschel, however, was concerned with Dunlop’s general presentation, and so he reorganised the observations for publication. This finished paper was soon completed and read on 9th May 1828 to the Royal Society. Entitled “XVII. Approximate Places of Double Stars in the Southern Hemisphere, observed at Paramatta in New South Wales.” Mem.Ast.Soc. London, 3, 257; (1829).

Dunlop himself places the assessment on the merits of his own manuscript;

“In presenting this list of double stars, it may be necessary for me to make some apology for its imperfect state, as regards the true and apparent distance and position of a great many double stars, the situation of which points out in the heavens.”

Dunlop is not really being just humble here. It is likely Dunlop honestly is concerned with the observations and clearly states the nature of his own programme. For Dunlop the statement “nebulae being a prime object to me”; proves his main observational goals towards his endeavours. He affirms this by saying the deep-sky observations were; “devoted the whole of the favourable weather.” It seems the collection of double stars were only of secondary importance and many of then were likely found while doing his deep-sky surveys. It is likely that he observed the pairs again when the next opportunity came along under these poorer conditions. I.e. Observing in moonlight or during poor seeing — the latter condition beings the worst time to observe pairs. By observing during bad seeing this might account for many of the missed close pairs.

In the scheme of things Dunlop and Rümker’s observations maybe of little consequence but they did lay the foundations of later surveys by other astronomical “heavyweights”. For me we should perceive Dunlop, and Rümker to a lesser extent, as the ones who opened the celestial door into the observational southern skies. It was everyone else who later found the nature of the treasures contained within and gathered its rich harvest. As for opening this part of the celestial vault — for this alone we should be eternally grateful.

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Dunlop’s Other Astronomical Works

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James Dunlop made observations of other astronomical phenomena. One of the most important was observing Encke’s Comet. He measured the position of the comet some thirty times between October 26 and December 26 1828, and reduced the cometary positions when he returning to Brisbane’s Observatory at Makerstown in Roxburghshire. (MNRAS, 1, 120 (1829))

Other than double stars, nebulae and clusters in the southern skies, Dunlop also produced a paper himself on stars entitled “IV. Observations of the Magnitude, Colour, and Brightness of Stars in the Southern Hemisphere.”; MNRAS, 2, 190 (1830) This little examined document contains information on some 400 southern stars under headings of magnitude, lustre and colour. Stellar magnitudes were measured by means of the ‘double eyepiece’, where relative telescopic magnitudes and colours are determined by sizes of artificial bright disks. The original submitted paper is now currently available in the R.A.S. Library in London.

Also closely examined were the comparisons between “neighbouring stars”, in the hope, in Dunlop’s words, of “...enabling observers to discover any changes that may hereafter occur.” This project was an additional to the observations for the main positional catalogue of southern stars in the ‘Paramatta Catalogue of Stars’.

The Paramatta Catalogue was the second specific dedicated positional star catalogue made from the southern hemisphere. French astronomer, (Abbe) Nicolas-Lois de a Lacaillé (1713-1762) created the first catalogue, making extensive observations between 1751 and 1753 from the Cape of Good Hope. This published work in its partial form appeared in ‘Histoire Céleste Française&rsquo as the ‘Caelum Australe Stelliferum’ or ‘Coe Australe’ catalogue in 1763. It contains in total some 9,776 stars made within the very short time of eleven (11) months using only a small equatorial mounted 1.3cm. (½-inch) refractor.

This star catalogue was mainly of the naked-eye stars and many 7th and 8th but the positions certainly were not sufficiently accurate for astronomical purpose such as proper motions. A further completed reduction appeared in 1847.

Dunlop used much of this catalogue that was going to become the basis of the Paramatta Catalogue, which has many of his stars appear listed through its pages. He also partial examined the errors of Lacaillé. Whilst doing this star catalogue, Lacaillé had discovered and catalogued forty-two (42) nebulae and clusters that was first published as “Sur les étoiles nébuleuses du ciel austral”; Mémoires de l’Académie Royale des Sciences (1755). Of these some thirty-four (34) are actual known objects, some twenty-six (26) being Lacaillé’s own discoveries.

According to the Paramatta Star Catalogue, observations began in November 1821 and finished on 2nd May 1822. After June 1823, Brisbane made only a few further observations, with the majority of visual observations and measurements now obtained by Dunlop, and to a lesser extent by Rümker. Other separate positional observations specifically made for the British Admiralty occurred between the 2nd May 1822 and 2nd March 1826, and the results returned to England during 1829 and also into 1830. The catalogue results were soon followed, where Mr. William Richardson was duly employed by the “Lord Commissions of the Admiralty”, who then was directed to re-reduce all the observations.

Mural Circle observations of all these visual stars were computed for the latitude of −33° 48′ 50.685″. This was before Rümker had formally determined and published the observatory’s longitude in “Philosophical Transactions” Hamburg (1832) for epoch 1829, as 10h 04m 06.25s E of Greenwich. The Observatory height he also estimated as 60-feet.

Richardson says that the Paramatta Catalogue is a “Catalogue of Southern Stars, as far as 8th magnitude.” He goes on to say;

“During the greater part of the time embraced by the Paramatta observations, Mr. Dunlop was the only observer; and with the point of view the complete observations for the formation of an extensive catalogue, he abandoned the transit instrument, and fixed the Mural Circle as nearly to the meridian as he could, he commenced observing every star that circumstances permitted as it passed the central wire, regarding the time of transit, and read off as many microscopes as the interval before another object came to the wire would allow.”

Richardson goes on to state;

“In a period of about two years and a quarter… he observed 7,000 stars, and made nearly 40,000 observations, besides as extensive series of observations upon double stars and nebulae.”

James Dunlop’s main problem was having a variety of astronomical equipment for use, which was only in reasonable condition. It seems probable that both the main measurement devices; the Transit Telescope and Mural Circle may have been permanently damaged during long transport by ship from England to Australia, while being delivered to the Paramatta site or when being set-up for use. We know this from the problems faced when determining observations and the general poorness of the data seen in Paramatta Star Catalogue — importantly as something that cannot be solely be blamed just on the observers! Both Dunlop and Rümker attempted several times to determined some kind of correction to the instrumental error, but we know it was only partially successful. The final catalogue data never implemented many of these corrections.

At Paramatta Observatory, Brisbane transported to Australia an accurate clock made by Hardy that showed local sidereal time. This first exclusively used in conjunction with the principle positional equipment being the 16-inch repeat circle, and both the Troughton 5½-foot transit telescope and 2-foot Mural Circle. The Transit circle when installed caused many serious positional problems in the PSC catalogue, so Dunlop alone made the critical decision to do star transit observations using the less precise Mural Circle. This choice was to have serious consequences on the questionable quality of the final star catalogue. All these instruments were place in solid masonry at the observatory site.

Once Paramatta Observatory was finally demolished, the the powers that be just stored all the astronomical equipment for safe keeping. Around this time, many of the smaller parts of the equipment went missing — being the important pieces making the instruments quite unusable. When Sydney Observatory was first finished in June 1858, the exhumed equipment went there, but never used really used again, except for the clocks. This equipment remained there until the closure of the functioning Observatory in 1983. After this, all the equipment was removed to be properly cleaned, restored or revamped by the Sydney Powerhouse Museum. Now most of it is on the Sydney Observatory site under glass displays as part of the interesting historical exhibits. Some still remains away from the public to be perhaps rotated over the years for public display.

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James Dunlop’s Catalogue of Deep-Sky Objects

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In the years 1823-1827, James Dunlop (1795-1848) observed the southern skies from the Brisbane observatory at Paramatta, New South Wales, Australia. He compiled several catalogues, among them the Brisbane Catalogue of over 7,000 southern stars, and “A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed in New South Wales of 629 entries for deep-sky objects” [Dunlop (1828)]. John Herschel, however, could only verify 211 of them, mainly due to wrong positions or poor contrived descriptions. Recent research, led by Glen Cozens, brought up the number of further ‘real’ objects, so that actually roughly over three hundred (300) (or about 50%) seem to belong to real deep-sky objects. It seems the other half found by Dunlop are asterisms and multiple stars, most found with the comparatively small instruments but he did not resolve them. His main telescope was homemade reflector, being the 9-inch speculum metal mirror of 9-foot focal length. This he used for all the other observations that were roughly mounted on an altazimuth mount aligned along the meridian. As a telescope, it was perhaps equivalent to some modern 6-inch reflector, though the images produced we quite poor. A small selection of eyepiece optics was crude at best, and it seems likely that the high-powered eyepieces were almost unusable.

Perhaps, the most spectacular original discovery he catalogued was the peculiar radio galaxy NGC 5128 or Centaurus A in Centaurus, (Δ482.) Original discoveries also included the Sculptor Group galaxies; NGC 55 or Δ507, NGC 300 or Δ530, and NGC 7793 or Δ608. Others included considerable numbers of other southern galaxies, open and globular clusters, and diffuse nebulae.

Four (4) discovered planetary nebulae included; NGC 2818 or Δ564, NGC 5189 or Δ252, NGC 5882 or Δ447, and NGC 6563 or Δ606.

Dunlop has included six Messier objects in his list. These include; M54 or Δ624, M55 or Δ620, M62 or Δ627, M69 or Δ613, M70 or Δ614, and M83 or Δ628.

Arrangement of the Dunlop catalogue is in order of increasing declination or south polar distance, from south to north. Objects nearest to the South Celestial Pole come first, and also explains the late numbering of the comparatively more northern Messier objects; ; M83 is the brightest and one of the northernmost of all the Dunlop objects. This endeavour was the second major effort of a deep-sky object survey of the southern skies, after Lacaillé’s list of 42, of 1751-1752.

First used by John Herschel was the exclusive symbol of ‘Δ’ for all the Dunlop deep-sky and double stars, whose use continues today. This can be sometimes confusing between the deep-sky objects and multiples. In the Washington Double Star Catalogue or WDS, the designation for Dunlop pairs is ‘DUN’. It is preferable to use these designations, though more often than not they are commonly interchangeable with authors and among amateurs.

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ASTRONOMICAL PAPERS of JAMES DUNLOP

1. Dunlop, J.; “A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere observed in New South Wales.” Phil.Trans.Royal.Soc., 118, 113-151 (1828)
2. Dunlop, J.; “Dunlops Verzeichniéon Doppelsternen AN., 7, 113 (1829)
3. Dunlop, J.; “Places of Encke’s comet, from 30 observations.” MNRAS., 1, 120 (1829)
4. Dunlop, J.; “XVII. Approximate Places of Double Stars in the Southern Hemisphere, observed at Paramatta in New South Wales.” Mem.Ast.Soc. London, 3, 257; (1829)
   [Ref 1.]
5. Dunlop, J.; “ Observation of a small comet at Paramatta.” MNRAS., 1, 130 (1829)
6. Dunlop, J.; “Observations of the Magnitude and Colour, and Brightness of the Stars in the Southern Hemisphere.”; MNRAS., 2, 190 (1833)
7. Dunlop, J.; “Observation of moon-culminating stars, eclipses of Jupiter’s satellites, and occultations of stars.”; MNRAS, 5, 8 (1839)

SHORT NOTES on PAPER No.6.: James Dunlop presented a short paper to the RAS in 1833, which estimated brightnesses of some 400 southern stars using a double-image eyepiece. This made artificial stars into disks where the magnitude was determined proportional to known magnitude stars. Dunlop also listed their lustre and colour. His aim in the presentation of data was to discover any changes in the future and the discovery of any secular variables. After the great brightness changes seen in the star of Eta Argûs, now called Eta Carinae, southern observers thought that several other star might be similarly variable. Unfortunately, this paper is not available and appears only as an abstract summary. It is likely still somewhere in the Royal Astronomical Library (RAS) in London, but based on the methodology this paper is certainly quite flawed. Some of these observations do appear within his unpublished manuscript.

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