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|Citation||Burton, C. E. (1875) "On the Southern Nebulae 30 (Bode) Doradus, and the Nebula about eta Argus." MNRAS, 36, 69.|
By C. E. Burton, Esq.
In the month of November 1874, while serving on the Transit of Venus Expedition at the island of Rodriquez, several opportunities occurred of studying the nebulae referred to above with a silver-on-glass reflector of 12 inches aperture. The results of my work in that direction are embodied in the subsequent portion of this Note, and in two of the figures which accompany it.
30 Doradus coming to the meridian at an hour which did not interfere with other work, received most of my attention. Two careful eye-draughts of this nebula, and of the stars involved in it, together with two or three other noticeable stars, were made on the nights of 1874, November 9 and 13. These sketches were combined to form the sketch laid before the Society. There was no time to spend in forming a skeleton chart of the stars upon which the nebula might be laid down, an operation which I was prepared to perform had the opportunity offered itself.
The recent sketch is placed beside a copy of Sir John Herschel's Cape drawing enlarged to nearly the same scale, and semi-reversed as regards the East and West points, I having used a Newtonian reflector.
Change in Nebula.– (1) In the Cape drawing the streak of nebulosity between the North and South loops is highly curved, and concave toward the South. In the 1874 sketch the same streak bends northward at its West extremity, from which it springs a very distinct loop curving southward, which does not seem to be indicated in the earlier representation.
(2) In the Cape drawing the bright star involved in the South loop seems to be situate in a very faint region. In 1874 that knot of the nebula was sufficiently bright to render it difficult to distinguish the fainter stars involved in it. See the
Argo nebula for a proof that the bright knot just mentioned was not due to instrumental glare in the faint region around eta.
The distant stars and branches of 30 Doradus were hastily put in from rough eye estiamtes of their position referred to the bright central region. The bright South loop (Herschel) appeared very faint in 1874. There does not seem to have been much change amongst the involved stars, if allowance be made for eye determinations of position as compared with micrometric places, especially places obtained by Sir J. Herschel.
A pocket spectroscope inserted in the place of the eyepiece of the 12-inch reflector showed a strong continuous spectrum crossed by a bright line, estimated to be the nitrogen line of the nebular spectra. The continous spectrum appeared to be too bright and free from longitudinal streaks to be attributed, even mainly, to the minute stars involved in the nebula.
The Nebula of eta Argus
The sketch must speak for itself, as I was not fortunate enough to obtain main notes on the nebula. Those which were obtained are as follows: eta was considered to be 9 mag. (Herschel's scale) and orange red. It was well outside the bright nebulosity East of the "Lemniscate." (N.B. In the copy of Sir J. Herschel's drawing of the region, eta has been placed somewhat too near the vacuity.) The stars in Sir John's drawing have in most cases their counterparts shown in the sketch of 1874. I am disposed to doubt whether there has been any great change in the region depicted during the forty years separating the epochs of the observations. eta I thought had not so "hard" a disk as that of the 8.5m ruby near beta Crucis, with which it was compared udner the same power, 400 linear, which was used in making all the sketches of the nebulae described.
Mr. Abbott's drawing of the Argo nebula was at once recognised on making use of an achromatic of 2.75 inches aperture, power 100, with which the "Lemniscate" was utterly invisible.
1875, Nov. 12
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Document type: Journal article.
Document source: SAAO Library
Scope: Complete article.
Keyed in: AS
Proof reading: AS
Online version: 2010 August 19.
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