The Quadrant

A marvellously striking object; the whole field strewn with sparkling stars

Image credit Robert Gendler

RA: 05h 33m 16s Dec: -66° 59′ 06″

Diameter: 980 light-years

OB Associations: LH 65, 77, 84

NGC Objects: NGC 2002, 2006, 2027, 2034

The mysterious… the striking… the unique Quadrant

A perfect quarter of a circle

The Quadrant is one of a pair of unique large arcs of young stars and clusters. The stars within each arc are practically coeval, and the arcs are parts of perfect circles, hence their names – the Quadrant’s stellar associations form a perfect quarter part of a circle; the Sextant, a sixth (so named by Efremov & Elmegreen, 1998).

The Quadrant lies at the southern end of the gigantic supergiant shell, LMC 4, and it spans an incredible ~980 light-years, and the ages of the stars and clusters within it are within 12–20 Myr (Efremov, 2012). It consists of the huge OB association LH 77 (which has an older stellar population as it has already expelled its ambient gas, and astronomers believe it has most likely contributed to the formation of LMC 4). The smaller LH 65 and LH 84 lie within the arc’s western and eastern ends respectively.

The Quadrant was first noted by Westerlund and Mathewson (1966), who recorded on a UV plate “the great arc of the bright blue stars,” saying that “Shapley called this arc Constellation III”. However, this was incorrect, as McKibben Nail & Shapley (1953), designated LH 63 = NGC 1974, which is located within the Sextant arc, as the identifier of Constellation III. They also noted that Constellation III is a triple cluster.

There is a great mystery to ponder while looking at this great arc… the origin of the two arcs. Many hypotheses have been advanced to explain them, but their origin remains both a controversial issue, and an important unsolved problem.

Observing the Quadrant

So gigantic and star-filled that it is clearly visible as a shimmering arc in my 10×50 binoculars, it is difficult to fathom the sheer immensity of this spectacular star cloud. It is well worth examining it at every magnification because, like all star clouds, it gives up its stellar secrets in stunning fashion with increasing magnification! At 90x it spans the field of view (all those stars!) and its quadrant-shape is superbly evident; its perfect curve filled with beautiful geometric and haphazard patterns that beguile the eye and enchant the imagination.

It is a lovely observing experience to roam randomly around this vast (60ʹ x 5ʹ) star cloud at medium magnification, just sightseeing… the bright stars blazing, the smaller stars gleaming like diamond chips, the gorgeous haze of countless unresolved stars, LH 84’s beautiful complimentary arc, smaller and less obvious LH 65, and the nine open clusters lying like tiny jewels among its multitude of stars and begging for higher magnification in order to show off their stellar attractions. At high magnification, the quadrant loses its arc shape and becomes field after field full of stars! Certainly, star clouds are amongst the most gorgeous objects!

LH 65 + SL 516 (OB Association + Open Cluster)

RA 05 30 05.0 Dec -66 56 36 Mag – Size 9′ x 5′

16″ at 228x: If any of the Cloud’s stellar associations deserves the moniker “the wallflowerʺ, it is LH 65! This small association is so overshadowed by the rest of the magnificent showy Quadrant that one doesn’t really notice it… until you do! Then, like any wallflower, it reveals itself to be a lovely little object; a collection of seven mag 11-12-13 stars set against a rich 2′ haze of unresolved stars.

Lying at the southern end of LH 65, SL 516 appears as a fairly faint, round cluster, ~20″ in diameter, and lying among a dazzling number of stars and the rich background glow of unresolved stars. No stars are resolved.

LH 65, with SL 516 at its southern end

NGC 2002 (Open Cluster)

RA 05 30 21.0 Dec -66 53 06 Mag 10.8 Size 1.9′ x 1.7′

16″ at 228x: Anchoring the north-western end of the Quadrant, the small but very bright NGC 2002 has a superbly bright core, ~15″ in diameter, and surrounded by a much fainter and hazy 30″ halo. No stars are resolved. It lies in a superbly rich, almost circular field of tiny stars that showcase it beautifully. Cluster KMHK 987 lies 1.5′ SE.

NGC 2002

KMHK 987 (Open Cluster)

RA 05 30 32.4 Dec -66 54 07 Mag 12.3 Size 1.0′ x 0.9′

16″ at 228x: Lying a mere 1.5′ SE of NGC 2002, KMHK 987 appears as a fairly faint glow, elongated N-S ~15″. No stars are resolved.

NGC 2002

NGC 2006 + SL 538 (Binary Cluster)

NGC 2006: RA 05 31 19.0 Dec -66 58 24 Mag 10.9 Size 1.6′ x 1.4′ Sep 0.9′ PA 172°

SL 538: RA 05 31 19.6 Dec -66 57 50 Mag 11.3 Size 1.4′

16″ at 228x: This is a real treat – a beautiful binary cluster pair lying among the Quadrant’s multitude of stars! The two clusters are separated by a mere 0.9′ and they actually look remarkably similar. NGC 2006 is the southern cluster, and appears as a relatively bright, round and smooth glow, ~30″ in diameter, brightening to a small but bright core. No stars are resolved. A mag 13 star lies on its east side. SL 538, the northern cluster, appears fairly bright, round and smooth, ~25″ diameter, with a slightly brighter centre. No stars are resolved.

NGC 2006 + SL 538

KMHK 1019 (Open Cluster)

RA 05 31 50.0 Dec -66 59 48 Mag – Size 0.7′ x 0.6′

16″ at 228x: This cluster appears as a faint, round and even glow, ~15″ in diameter. No stars are resolved.

KMHK 1019

KMHK 1074 (Open Cluster)

RA 05 33 58.5 Dec -67 00 34 Mag 12.6 Size 0.7′ x 0.6′

16″ at 228x: This tiny little cluster is lost amongst the glittering spectacle of stars, appearing as nothing more than a bright, and slightly fuzzy, stellar spark.

KMHK 1074

SL 586 (Open Cluster)

RA 05 34 43.3 Dec -66 57 46 Mag 11.17 Size 1.5′

16″ at 228x: SL 586 appears as a bright, round, knot, ~25″ in diameter, and with a very thin and faint halo. No stars are resolved. It is prettily paired up with white mag 11 HD 269787 star lying 1′ SW.

SL 586

HJ 3790 (Double Star)

RA 05 34 55.8 Dec -66 53 55 Mag 1 10.5 Mag 2 14.0 Sep 10.4″ PA 154°

16″ at 228x: This Quadrant truly is a glorious object for among its plethora of stars is this pretty double with its nicely contrasting magnitude!

HJ 3790

LH 84 = NGC 2034 + NGC 2027 (OB Assoc. + Open Cluster)

NGC 2034: RA 05 35 49.0 Dec -66 53 24 Mag 9.3 Size 8′ x 4′

NGC 2027: RA 05 35 00.3 Dec -66 55 05 Mag 10.9 Size 1.1′ x 0.9′

16″ at 228x: Gorgeous! LH 84 = NGC 2034 lies at the eastern end of the Quadrant, and it is composed of a graceful and beautifully star-rich crescent extending ~4′ x 2′ and open to the northwest. It contains two dozen bright mag 12 and fainter stars embedded in the gorgeous glow of unresolved stars. Mag 10 HD 269855, with a close mag 13 companion lies off the arc’s northeast end, and off the western end lie a lovely wide pair of stars – mag 9.8 HD 269796 beign the western star of the pair, and mag 10.9 HD 269806 the eastern.

The beautiful knot of stars, NGC 2027, lies at the western end, and at 333x, it appears as a beautiful little knot of 8 mag 13.5 and fainter stars embedded in a rich glow of unresolved stars, ~40″x 20″, elongated N-S.

The small cluster BSDL 2450 lies ~40″ east of NGC 2027 and appears as a tiny knot of faint, condensed stellar light.