Harold Corwin

NGC 6548 and NGC 6550 = NGC 6549. There are two galaxies in this field; the brighter, NGC 6548, was found by William Herschel in June of 1786, and given the number III 555 in his catalogue of nebulae and clusters. William Herschel's position reduces to 18 03 55, +18 33.5 (B1950.0), about 2.2 arcmin southeast of the modern position for the brighter object. The position in GC and NGC was either reduced with respect to a rather crude position for the comparison star (101 Her), or a (simple digit?) error of -20 seconds of time has crept into the RA.

The fainter galaxy was first seen by Marth in July 1864, and was rediscovered 18 years later by Stephan. Both noted the brighter object. Marth simply says "... near III 555" in his description, while Stephan says "Distinct from [GC] 4377 and [GC] 5892." Dreyer condensed that for the NGC by adding "... near m361" to Stephan's description. Since there are only the two nebulae here, and because Stephan did not measure the brighter objects he claims to have seen, we can only speculate on what his third object must have been. Perhaps it is the line of three stars east of Marth's galaxy.

In any event, it is clear that Stephan and Marth found the same galaxy. Stephan's accurately measured position precesses to 18 03 38.4, +18 31 48, while Marth's less accurately estimated position precesses to 18 03 36, +18 32.2, still quite close to the galaxy.

There matters would have stood had Lewis Swift not published a cryptic note in his 11th list of nebulae (AN 147, 210, 1898): "NGC 6550 = H III 555. 6550 must be struck out." The wording of Swift's original note in his "Catalogue No. II ..." (which appeared in PASP 9, 186, 1897, and in MNRAS 57, 629, 1897) makes better sense: "NGC 6550 must be struck out, as it is identical with H. III 555." Dreyer made what sense he could of all this, and has a Note in the second IC which reads "6548 = 6550, Swift in Cat. XI." (Dreyer also changed the NGC number to "6550" for H III 555 in his 1912 collection of William Herschel's papers.) Swift was apparently trying to tell us that there are only two galaxies here, too, but his wording in the AN list just made the cataloguing problem worse.

Enough people have read the IC Note that the modern identifications are thoroughly confused. An obvious predilection for the objects in RA order has also fed the confusion. In the end, though, it is clear that William Herschel found the brighter, northeastern galaxy, while Marth saw both objects -- and Stephan not only saw the two real galaxies, but (apparently) an asterism as well.

NGC 6549, by the way, has several stars superposed and is, itself, superposed on a much fainter galaxy. I've attempted to give the positions of the NGC object as well as the background galaxy.

― NGC Notes by Harold Corwin

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