CN FORUMS ARCHIVE Telescope Reviews: Observing the SINGS Galaxy Sample

Okay, last of this first set of observations...

NGC 7793 is the faintest of the five prominent members of the Sculptor Group of galaxies clustered around the south galactic pole. One of the nearest galaxy clusters beyond the Local Group, NGC 253, NGC 55, NGC 300, and NGC 247 are also counted as members. NGC 7793 is the prototypical Sd-class galaxy. This classification was created by astronomer Harlow Shapley specifically for this object with its very loose spiral arms and tiny nucleus. The brightest stars in this galaxy have a blue magnitude of about 18. It spiral pattern is very loose and disorganized, even more so than M-33 or NGC 300 which are otherwise quite similar, with massive arms littered with HII regions and OB associations. In the Carnegie Atlas of Galaxies, astronomer Alan Sandage points out one additional fact about the Sculptor Group: there is a direct correspondence between the luminosity of the brightest stars in the member galaxies and their measured redshifts. This indicates an extremely quiet environment where there is almost no relative motion within the cluster, unlike our own Local Group where the two largest members (M-31 and the Milky Way) are hurtling toward one another and will eventually merge into a single object.

NGC 7793 is located pretty much in the middle of nowhere, without bright stars nearby to guide you to its location. To find it, I centered by scope on the lone beacon of the south autumn sky, Fomalhaut (Alpha Piscis Austrini). With your lowest-power, richest-field eyepiece in place, drop 3° south to Delta PsA, and then begin a long eastward sweep, hitting first Gamma Scultoris (4.8° east from Delta PsA), then Mu Scl (4.6° east of Gamma), and finally to 6th-magnitude AL Scl (HD 224113) with its 7th magnitude companion HD 224112 located 2'14" to the north. With this pair centered in your low-power eyepiece, nudge you scope slightly toward the southeast, and you should find NGC 7793 about 52' away in that direction.

At 30x, the first thing that always strikes me about NGC 7793 is just how bright it is. One would expect a late-type spiral significantly tilted toward the face-on position to be an object of very low surface brightness (like M-33). Not so for NGC 7793 – this object stands out very nicely despite its low altitude, displaying a small, bright core, surrounded by a ragged elliptical haze. The field stars are not numerous but there is a nice arc of them extending away from the heart of the galaxy toward the north and west – it is easy to imagine the galaxy suspended in space from a gently curving wire highlighted by stars.

Increasing the magnification to 60x, makes the bright nucleus more apparent and begins to reveal patches of slightly brighter nebulosity in the diffuse outer halo. At 120x, these trends continue and the halo – though beginning to fade due to over-magnification – displays a chaotic, irregular structure of bright and dark patches. These subtle mottlings fail to evoke much in the way of a spiral pattern, but I was, nevertheless, left with the vague impression of a counterclockwise rotation in the system.

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Eric
6" f/6 Parks Newtonian
10x50 Bushnell Binocs

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