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Observations by John Herschel ( Sculptor Galaxy, NGC 253 )

 

Observations of the Sculptor Galaxy ( NGC 253 ) by William and John Herschel

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Part 2.  Observations of "Caroline's Galaxy" by Sir John Herschel,  1830's

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Sir John Herschel, the only child of Mary Baldwin and Sir William Herschel, was born in 1792 when his father was in middle age and already famous as one of world's leading astronomers.  Having excelled in school, and no doubt inspired by his famous elders, John Herschel decided upon a career as a 'man of science' and set out to pursue a wide range of interests; with one particular focus being a continuation of the study of the heavens commenced by his father and aunt, Caroline Herschel.   

In 1820, with the assistance of his father, John Herschel supervised the construction of a new telescope at Slough in England.  As described in the extract below ( from a paper presented to the Royal Society in 1826, titled "Account of some observations made with a 20-feet reflecting telescope ... " ), the telescope had a polished metal mirror with clear aperture of 18 inches, focal length of 20 feet and was modelled on the same design created by his father.

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It is this telescope, in the 1820’s and early 30’s, following the death of his father and the return of his aunt Caroline to Hanover, that John Herschel used to 'sweep' the night sky and extend the catalogue of nebulae and clusters of stars that was published by his father ( see W. Herschel's  Catalogue of One Thousand new Nebulae and Clusters of Stars ).

On the 1st of July 1833, having complied sufficient observations, John Herschel presented to the Royal Society an updated list of the positions and descriptions of the Nebulae and Clusters of Stars that he had thus far observed.  As noted in the introduction to the paper published in the Philosophical Transactions, he had planned to wait before publishing until he had complied a fully comprehensive general catalogue of objects visible from the south of England.  However, due to his expectation of “several more more years additional work” needed to complete the task and his assessment that he now was in a position to address, at least in part, the then current “... want of an extensive list of nebulae arranged in order of right ascension ...”, he elected to present his list, “ ... simply stating the individual results of such observations as I have hitherto made ... “.  It was not until October 16, 1863, some thirty years later, that Sir John would deliver to the Royal Society his General Catalogue of Nebulae and Clusters of Stars.

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As well as introducing many objects that had not previously been recorded, Sir John’s list of 1833 included a re-examination of, and in some cases a small correction to, the positions of many of the deep sky objects observed by his father and noted down by his aunt.   One of these re-visited objects was, unsurprisingly, the large and bright nebula discovered by Caroline Herschel in 1783 and recorded in Sir Williams’s catalogue as V.1 / CH 10 ( object number one, of class five ( very large nebulae ) / Caroline Herschel #10 ).

In total, John Herschel records around 2500 observations of nebulae and clusters of stars in his 1833 paper; with observation #61 being V.1, the “ Sculptor Galaxy “

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The measured position of V.1is given in RA and the angle from the north celestial pole ( all reduced to epoch 1830.0 ).  The description can be interpreted by reference to the legend in the paper.

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Thus, “ A vL mE vB neb “ becomes “ A very large, much extended, elliptic or elongated, very bright nebula “.  He also notes that in addition to this observation, #61, noted down from sweep #306, V.1 was also observed in sweep #292, “but no place was taken”.  

The figure to which he refers , figure 52, is included towards the back of his paper and is a sketch he made of the Sculptor Galaxy.

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to be continued ...

 


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