BNGC 13. The A-B-C Of Galaxies

By Steve Mencinsky, 1993

The text of the revised NGC (RNGC) discusses at length the approach taken in that catalogue in the case of NGC galaxies which have companion galaxies that are not themselves NGC objects. Such galaxies are typically given trailing A, B, C, etc extensions. These galaxies have not been included in the RNGC.

At least one such galaxy pair, however, has already been mentioned in BNGC : NGC 4945 and NGC 4945A in the wonderful constellation of Centaurus. This month's article deals with several more. Obviously, there are many, many more of these NGC - ABC combinations. Any interesting ones will of course duly make their way into future columns.

The first of these is 7213 and 7213A. These galaxies are very easy to locate: they are within a fraction of a degree from the first magnitude star Alpha Gruis. When your correspondent observed this pair with Bob Evans some time ago, Bob made the comment that as the long axis of the second galaxy was less than 1' in size on the ESO plates, it therefore did not qualify for an ESO classification and that hence "7213A" was indeed the most correct name for it.

Through Johannes, the galaxy is indeed very small but not all that faint: it is easily visible from Mt. White. The diagram shows the location of the two galaxies s seen through the 32mm wide-field . eyepiece in Johannes, the combination giving a field of view of almost a degree.

The second showed a small cluster of two galaxies (two galaxies is widely believed to be the smallest possible cluster - your correspondent would be fascinated to hear of any counter-examples!) also in Grus. Note that 7232A appears slightly displaced from the position as shown in Uranometria II. It is perhaps a comment on the state of sophistication of amateur astronomy that position errors of only a few minutes. of arc for non-NGC galaxies are now perceived as 'significant" .

It is also perhaps - and for no doubt similar reasons - a reflection on the generally very high accuracy of Uranometria II itself that such small errors are conspicuous by their rarity. Interestingly, 7232B was not visible while observing with Johannes from Queensland (that's Johannes the telescope, not an ex-Premier of the same name), despite it being listed in your correspondent's personal notes as being successfully observed through the late Jules.

Lastly, as if to demonstrate that interesting galaxy groups can grace even the most unpromising sky, the final two maps show small clusters of galaxies on the Sagittarius - Telescopium border, with one cluster in each constellation, and only a little more than two degrees between them. All were observed successfully from Mt. White through Johannes with no difficulty; all were unexpected discoveries.

NGC 6878 in Sagittarius is large and bright, easily found in the one-degree eyepiece which from Mt. White gives a very bright sky. 6878A is very small and quite faint, but the anonymous companion is very faint but surprisingly large - almost as large as 6878 itself. The star field shown is not as crowded as the chart may indicate - the four close companion stars South and east of 6878 are in fact quite faint, much fainter than 10th magnitude.

NGC 6845 was initially observed for no better reason than that it was nearby [from 6878) and it seemed as if it would be easy to find (it was). The photo in the much-mentioned ESO book shows a large and perturbed central galaxy with one large and two small companions. The sketch shows in solid outlines what was successfully observed, and in dotted outline what is on the photo. The central galaxy appeared as a relatively featureless irregular oval, with the nucleus well away from the centre - certainly consistent with the long and almost detached arm shown in the photo. The large companion was very easy w see. In fact, it had a considerably higher surface brightness than the main galaxy.

Some observing nights, such as the one on which the above two clusters were first seen, stand out in one's memory much more so than others. As was mentioned in the very fiat BNGC, the thrill of "discovering" these uncharted stellar islands is a heady elixir indeed!