The Sextant is one of two large arcs of young stars and clusters prominently situated in the northeast corner of the Cloud. The enormous arcs are unique: not only are the stars within each arc practically coeval, but the arcs are also parts of perfect circles, hence the names they were given by Efremov & Elmegreen (1998) – the Quadrant whose stellar associations form a perfect quarter part of a circle, and the Sextant whose stellar associations form a perfect sixth part of a circle. The Quadrant lies in the supergiant shell LMC 4 and spans an incredible ~980 light-years. The Sextant lies on LMC 4’s southern periphery and spans 650 light-years. In 1953, McKibben Nail & Shapley designated NGC 1974 as the identifier of Constellation III, a massive 26′ x 28′ in size.

The Sextant is unutterably beautiful at all magnifications, beginning with 10×50 binoculars in which it is plainly visible as a small, glowing arc! At 90x, and without a filter, the glittering stars of the four stellar associations clearly delineate the arc’s sextant-shape. The number of resolved stars along the arc is surprising; the unresolved ones showing as that gorgeous frostiness of the finest star dust that simply begs for increased magnification. The four clusters associated with the stellar associations are all embedded in small and varying pockets of unresolved starlight and delicate wisps of faint nebulosity.

The arc of stars responds to increases in magnification in a dramatic fashion – at each increase in magnification there is a startling increase in the number of stars. One bandies around the word “diamonds” when describing stars, but these stars really do look like a collection of diamonds of varying carats swooshed out in a beautiful arc across a piece of tatty and irregularly faded grey silk.

The Sextant responds like magic to the UHC filter! At 90x, the arc is swathed in a delicate and complex tapestry of nebulosity; a beautiful silky smokiness that, albeit faint, is beautifully complex in the way it sweeps in and out of pockets of brightness. Best of all, the two superbubbles really snap into view. N51D is very evident as a broken ring of roundness – faint, smoky, but obvious. N51A is less evident. Its northeastern side has a beautiful arc-shaped glow (open to the southwest) glow and an exceedingly faint curve of nebulosity curves away from it to the south, and although it fades very rapidly it certainly gives one the idea of its superbubble-shape.

16″ at 228x: The superbubble contains two OB associations – LH 54 = NGC 1955 which lies on the superbubble’s eastern side, and LH 51 = SL 456 which lies on the western rim of the superbubble. LH 54 is a beautiful congregation of a couple of dozen stars that glint brilliantly against the lovely glow of unresolved stars, and which stands out beautifully against the soft folds of nebulosity in which it is embedded. About half a dozen brighter mags 11.5 – 12.5 stars form a small E-W arc, open to the north. The arc is intensified by mag 11.6 Brey 31, a Wolf-Rayet star, and mag 11.8 HD 269525, one of the two ionizing stars. The complex’s other ionising star, 11.5 mag HD 269540, with a 12.3 mag star south of it, lies just east of the visible nebulosity.

LH 51 = SL 456 appears as a pretty gathering of half a dozen or so small, but bright, stars elongated in a gentle E-W arc, open to the south, and glinting against the western rim’s faintish, pearly nebulosity. Averted vision reveals tiny pinprick stars that pop in and out of view and surround the arc.

Even without a filter the superbubble’s beautiful circular shape is evident. It has what looks like a pair of brighter nebulous “brackets” on the east and west sides. The superbubble certainly responds to the UHC filter! The western “bracket” is smaller and less bright than the eastern side. It is an off-round patch of soft and relatively smooth nebulosity elongated N-S. It has no edges, it simply dissolves away into nothingness. The eastern bracket is brighter and more complex – a large, lovely, irregular loop that extends N-S with some subtle irregular definition and unevenness. The southern end of the loop ends at mag 9.5 star, HD 269523. The northern end fades away more rapidly. The arc of nebulosity bulges out to the east, and the edges along this section are quite nicely defined; the only part of the superbubble’s edges that are defined. There is a very faint and short smudge of nebulosity along the southwestern edge of the superbubble. And the whole of the interior of the superbubble has very faint and very tenuous filaments of nebulosity – it looks like the very faint smudges left on a blackboard after it has been wiped. All in all, a beautiful and intriguing superbubble! (Interestingly, the superbubble contains the first ever extragalactic Herbig-Haro object to have been discovered! It was first detected in 2005 by Chu, Chen, Gruendl & Williams. Obviously, we can’t see it in our telescopes, but knowing it is there certainly adds to N51D’s observing experience!)

16″ at 228x: The OB association LH 60 = NGC 1968 continues the Sextant’s famous arc from the superbubble N51D eastwards towards the superbubble N51A, appearing as a light dusting of small, faint stars on N51D’s west side that lead the eye straight to the open cluster NGC 1968, a beautifully bright little cluster embedded in a lovely cloud of patchy nebulosity. It is elongated ~2.5′ x 45″ E-W with 12 resolved stars, including a couple of mag 12-13 stars, set against the bright glow of unresolved starlight.

N51C responds beautifully to the UHC filter. The nebulosity surrounds the cluster in a beautiful soft glow that brightens on the east end in a large, bright, round knot, and then sweeps in a southwest direction in the direction of the superbubble to the west. It eventually just fades away into the surrounding sky, and indeed the whole of N51C’s nebulosity fades away at the edges, apart from the bright knot on the east side which has lovely defined edges.

NGC 1968 appears as if it is almost connected to superbubble N51A’s NGC 1974 by a few faint stars that serve as tiny bright stepping-stones between the two! Lovely!

16″ at 228x: LH 63 = NGC 1974 is gorgeous! A roundish group, ~2.5′ in diameter, of 20 or so resolved stars, including a couple of mag 12-13 stars, set against the soft background glow of faint, misty nebulosity. NGC 1974 is involved in the superbubble’s eastern side’s nebulous haze, and without a filter the soft pearly haze with the tiny sparklers studding it is just gorgeous.

Without the UHC filter, there isn’t much to be seen of the superbubble and even with it, there isn’t much to be seen, except for the arc of nebulosity to the northeast around NGC 1974, which then extends in a very, very faint and short, hazy strand of nebulosity that curves to the southeast and fades rapidly. The nebulosity has no edges, it simply dissolves into the background. It’s not much, but seeing any part of a superbubble is a special observation, and it’s very nice to have seen a part of this superbubble’s rim!

The radius of the Sextant

In their 1998 paper Efremov & Elmegreen stated that “a remarkable feature of the two arcs is their circularity”. They determined that the radius of the Sextant arc is ~554 light-years, and at its centre there is the small cluster HS 288, and that at 228 light years to the north there is a larger, dispersed cluster, HS 287, surrounded by the H II region N50.

16″ at 228x: HS 288 appears as a very small, but relatively bright, glow of unresolved starlight with no resolved stars. At 228x, it is impossible to miss N50 – it is a soft, round glow, 2′ in diameter, with the cluster HS 287 showing as a small faint pool of unresolved starlight with a couple of the tiniest pinpricks of stars busily popping in and out of view. N50 has a good response to the UHC filter, appearing as a lovely mistiness with edges that simply melt away into the sky.

Although not a part of the Sextant, this nebulous patch offers a nice little observation

And before leaving… it is impossible to explore the Sextant without a visit to the lovely nebulous patch that lies directly south of it and contains the stellar association LH 55, although it isn’t part of the Sextant.

16″ at 228x: Three stars in descending brightness lead like another set of small stepping-stones from the south edge of N51D to the nebulosity associated with LH 55, which contains three open clusters. Even without a filter the nebulosity appears as a large patch of relatively bright nebulosity that brightens beautifully along the northwestern edge (N51B) and looks like a ship’s bow wave. A fairly bright, but very small and tight cluster, KMHK 894, lies within the brightest part of the bow wave at the west end of a very short and faint string of stars. The small and faint clusters, KMHK 905 in the centre and KMHK 900 to its south-southwest, appear as if they are trailing along behind the bow wave; both of them small smudges of brightish light, no resolved stars. The nebulosity has a good response to the UHC filter; its bow wave appearance enhanced with a relatively well-defined edge to its leading edge and the rest of the nebulosity (N51E) slowly fading away behind it into the dark sky.