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|Citation||Dunlop, James (1828) A Catalogue of Nebulae and Clusters of Stars in the Southern Hemisphere, Observed at Paramatta in New South Wales. RSPT, 118, 113.|
Read December 20, 1827.
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 9-feet reflecting telescope, the clear aperture of the large mirror being nine inches. This telescope was occasionally fitted up as a meridian telescope, with a strong iron axis firmly attached to the lower side of the tube nearly opposite the cell of the large mirror, and the end of the axis rested in brass Y's, which were screwed to blocks of wood let into the ground about 18 inches, and projecting about 4 inches above the ground; one end of the axis carried a brass semicircle divided into half degrees and read off by a vernier to minutes. The position and index error of the instrument were ascertained by the passage of known stars. 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 degree in breadth with very little deviation of the instrument from the plane of the meridian, 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, several of which are annexed to this paper; and also very correct drawings of the Nebulae major and minor, together with a representation of the milky nebulosity surrounding the star η Robur Caroli. The places of the
small stars in the Nebulae major and minor, and also those accompanying the η Robur Caroli, I ascertained by the mural circle in the year 1825, at which time I was preparing to commence a general survey of the southern hemisphere. These stars being laid down upon the chart, enable me to delineate the nebulosity very accurately.
The nebulae are arranged in the order of their south polar distance to the nearest minute for 1827, and in zones for each degree in the order of their right ascension. The column on the right hand shows the number of times the object has been observed.
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 know portion of the heavens.
The Nebula Minor, to the naked eye, has very much the appearance of a small cirrus-cloud; and through the telescope, it has very much the appearance of one of the brighter portions of the milky way, although it is not so rich in stars of all the variety of small magnitudes, with which the brighter parts of the milky in general abound, and therefore it is probably a beautiful specimen of the nebulosity of which the remote portion of that magnificent zone is composed.
Plate IV. is a very correct drawing of the nebula, which if faithfully represented by the engraver, will convey a better idea of it than I could possibly hope to do by words.
Its situation in the heavens is between 0h 27′ and 1h 6′ or 7′ in right ascension, and between 74° 30′ and 72° 53′ in south declination. Its position is oblique to the equator, south preceding and north following; and its form is nearly that of a parallelogram about two degrees long and fully one degree broad, and may be arranged according to its natural general appearance, into bright, faint, and very faint nebulosity. The bright nebula forms the south extremity and the preceding side, and is equal to the breadth of the nebula at the south end, and gradually diminishing in breadth and brightness till it terminates in an accumulation of the nebulous matter in the north extremity. The bright portion of the nebulous matter is not uniformly bright, but has something the appearance of small cumular clouds, although not very decidedly marked, and which I cannot well delineate. The faint nebula which is on the following side, is broad at the north extremity and gradually diminishing in breadth to where, with the other faint shade, it joins the following side of the brighter portion of the nebula, near the south extremity. The very faint shade is also on the following side, and extends from the northern to the southern extremity of the nebula, and is rather more strongly marked at what I would call its terminating border, than where it joins or blends with the faint shade; and I suspect it is faint connected with a patch of faint nebula which follows at a little distance, and is represented in the figure.
There are two pretty bright small nebulae situated in the following margin of the bright shade, and a considerable number of faint nebulae and accumulations of the nebulous matter variously situated throughout, and also in the patch which follows; but they are described in the general catalogue.
The figure of the Nebula Major is so irregular, and divided into so many parcels, that without the assistance of letters of reference it will be impossible for me to attempt a description. However, the appearance and construction of the different nebulae which compose it, are more minutely described in the general catalogue. I will here only attempt to describe the apparent connection of one portion or branch of the nebulous matter with another. I find the existence of extensively diffused faint nebulosity throughout a great portion of this quarter of the heavens, from the Robur Caroli to the Nebula Major, and I can even trace its existence in the vicinity of the Nebula Minor.
The Nebula Major is situated between 4h 46′ and 6h 3′ in right ascension, and between 66° 30′ and 71° 30′ of south declination; but the body or principal portion of the nebula is situated between 5h 7′ and 5h 40′ in right ascension, and between 69° and 71° of south declination, and is composed of very strong bright nebula, very rich in small nebulae and clustering stars of all the variety of small magnitudes: I compared this portion of the nebula with Sobieski's Shield, which in this latitude is near the zenith. The observation says, "The Nebula Major very much resembles the brightness in Sobieski's Shield; it is scarcely so large, but I think it is equally bright." Another observation says, "The ridge or brighter portion of Nebula Major is more condensed than the Shield." Plate V. is a correct representation of Nebula Major.
The bright ridge or body of the nebula is extended obliquely to the equator, north preceding and south following, and the following extremity breaks off rather suddenly, faint, decreasing in brightness in a south following direction to the distance of fully a degree and a half towards the star β, which is slightly involved in the narrow extremity: preceding the star marked γ, a considerable increase of the brightness of the nebulous matter takes place; another accumulation takes place at δ about 15′ diameter. There is a small star north with a small nebula preceding, but neither of them are involved in the accumulation of the nebulous matter. δ and ε are connected by streams of unequal brightness, ε is pretty large and is rich in small stars and nebulae: opposite δ and ε, towards the principal body of the nebula, the nebulous matter is very faint and of unequal brightness; ε is south following a beautiful group of nebulae of various forms and magnitudes, on a ground of strong nebulosity common to all, with the 30 Doradus (Bode) in the centre.
South of the 30 Doradus a pretty bright accumulation of the nebulous matter takes place, extended, preceding and following, and is joined by pretty strong nebula to the arm κ, which proceeds in a northerly direction form the body of the nebula; the bright star near the north extremity of the arm is not involved in the bright nebula. Between the arms κ and λ the nebula is very faint, and the bright accumulations of the nebulous matter on the north side are all connected together by nebulosity of various brightness, and are connected to the main body by the arms κ and λ; and I strongly suspect the nebula at θ is connected by very faint nebula with the group surrounding the 30 Doradus. The accumulation of the nebulous matter at ξ is connected with the preceding extremity of the body of the nebula, by nebula increasing in brightness towards the neck of the body, but I cannot say that the ψ is connected with the ξ. Two arms proceed from the neck towards the south, which are connected by faint nebula between them, which gradually increases in brightness towards the junction of the arms; between the arm η and the body the nebulosity is faint, of various shades of brightness, and from the arms η and ν, to the head ξ, the nebulosity is of various degrees of brightness.
I have made a very good general representation of the various appearances of the milky way, from the Robur Caroli to where it crosses the zenith in Scorpio. Plates VI. VII. and VIII. This was generally made by the naked eye, except in particular places where I suspected an opening or separation of the nebulous matter, when I applied the telescope. However, the dark space on the east side of the Cross, or the black cloud as it is called, is very accurately laid down by the telescope: the darkness in this space is occasioned by a vacancy or want of stars; it contains only two or three of the 7th magnitude, and very few of the 8th or 9th magnitude. I may here remark that the Nebula Minor is not so bright as the Nebula Major.
Neither of the two nebulae, Major and Minor, are at present in the place assigned to them by LACAILLE; and it has been suspected that nebulous appearances change their form and also their situation. Yet, although the situation of these nebulae, as given by LACAILLE and compared with their present situation, would be favourable to such a surmise, still we must consider the dimensions of the instruments with which he made his observations, and make a reasonable allowance.
However, the 30 Doradus is at present involved in pretty strong and pretty bright nebula, and is also situated very near the brightest part of the Nebula Major; and it would be singular if its relative situation was the same when LACAILLE observed it as it as present is; that he should have assigned to it a place in the Dorado and not in the Nebula Major, to which, from its nature, it was not only nearly allied, but in which is was actually involved. This circumstance, it must be confessed, is favourable to the conjecture; and the 47 Toucani is similarly situated, with respect to distance, from the Nebula Minor, although it is not involved in nebulosity or connected with the nebula.
When reflecting on these circumstances, I was led to examine the present state of these nebulae, and find that scarcely any nebulae exist in a high state of condensation, and very few in a state of moderate condensations towards the centre. A considerable number appear a little brighter towards the centre, and several have minute bright points immediately at the centre. Others have small or very minute stars variously situated in them, but many of the bright points in, or near, the centre may be stars, for the Nebula Major in particular is very rich in small stars. But the greater number of the nebulae appear only like condensations of the general nebulous matter, into faint nebulae of various forms and magnitudes, generally not well defined; and many of the larger nebulous appearances are resolvable into stars of mixed small magnitudes; and a great portion of the large cloud is resolvable into innumerable stars of all the variety of small magnitudes with strong nebula remaining, very similar to the brighter parts of the milky way. And whether the remaining nebulous appearance may not be occasioned by millions of stars disguised by their distance, is what I cannot say.
But a critical examination of these nebulae would not only be a valuable treasure for the present generation to possess, but an invaluable inheritance for them to transmit to posterity. For it must be by the comparison of observations, made at distance periods of time, that we can draw any satisfactory conclusions concerning the breaking up or the greater condensation of the nebulous matter. It seems beyond a doubt that stars must assume a nebulous appearance when situated at immense distances; but whether all nebulous appearances are occasioned by stars, is a problem beyond the reach of man to resolve without the assistance of analogy, which ought not to be
trusted too freely, especially with objects almost equally beyond the reach of our hands and telescopes. Several of the very faint and delicate nebulae can be resolved into stars, and also many of the brighter nebulae are composed of stars: but there are a greater number which have not yet been resolved or shown to consist of stars; and it is not improbably, that "shining matter may exist in a state different from that of the starry."
P.S. Plate IX. has been added, at the suggestion of Mr HERSCHEL, to illustrate the distribution of the Southern Nebulae. The nebulae are laid down without any regard to their form, magnitude, brightness, or nature; and but little to their strict places, only so far as to case every rectangular space on the map, which occupies one degree in Polar distance and one hour in Right ascension, to contain the same number of nebulae as actually occur in the heavens, according to the observations detailed in this paper; the object of the plate being solely to give an idea of their arrangement generally in the heavens.
Document type: Journal article.
Document source: SA National Archives
Keyed in: AS
Proof reading: AS
Online version: 2009 September 13.
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