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Southern Cross - August 1996

• Constellation of the Month - Triangulum Australe

Constellation of the Month - Triangulum Australe

Albert Brakel

Triangulum Australe, the Southern Triangle, is located east of Alpha Centauri, beyond the narrow constellation of Circinus. Its Alpha star is sometimes known as Atria. Despite lying mostly in the Milky Way, it is remarkably impoverished in deep sky objects. It seems that when clusters and nebulae were being distributed, Norma to the north got most of the goodies while the Triangle was left with some crumbs.

One of these "crumbs" is NGC 6025 (RA 16h 03.7m, Dec. -60 30") , an open cluster right next to the northern border and visible in finderscopes. It is a loose but pretty group of some two dozen moderately bright stars, and some fainter ones, mostly white with some pale yellow. The two brightest stars are located on the southern margin. The cluster is about 15' across in a N-S direction, but only half as wide E-W. There is no area of concentration.

Also in the far north, and just missed out from being included in Norma, is the planetary nebula NGC 5979 (15h 47.7m, -61 13'). With a 20 cm SCT and a suburban sky, it was a faint and difficult-to-find star-like point. I could not distinguish it from other faint stars nearby until an OIII filter was used, so that its brightness was undiminished while the nearby faint stars were all but blocked out. Returning to the unfiltered view, it could on close inspection be seen to be a bit fuzzier than a normal star. It lies about 15' W of a mag. 7 star, and is the leading apex of a flat isoceles triangle with two other stars. Supposedly it has a 15" disk, but this must be in photos or in a dark sky.

I tried for the two brightest galaxies in the constellation, NGC 6156 (16h 34.7m, -59 36') and NGC 5938 (15h 36.6m, -66 50'), but they were not bright enough to see in my viewing conditions.

The remaining telescopic objects in Trianglum Australe are double stars. Sellors 11 (15h 54.9m, -60 45') is half-way between NGC 6025 and NGC 5979, and consists of mag. 6.5 and 8.8 components 1.2" apart. Although theoretically resolvable with 20 cm aperture, I could not split them in only average seeing. The white star did begin to appear elongate with 235x power. On either side to the E and W are two wide companion field stars.

Iota TrA (16h 28.0m, -64 03') was given as an optical pair by Hartung, but is a double on the Herald-Bobroff chart and in Sky Atlas 2000.0. It is comfortably split by 50x, but a higher power will enhance the faint companion. Magnitudes are 5.3 and 9.4, separation is 13".

Lacaille 6477 (Rumker 20) (15h 47.9m, -65 27'), between Beta and Epsilon TrA, is a fairly close (1.8") yellowish pair, mag. 6.2 and 6.4, elongate with 117x, and just split with 154x.

1.2 deg. N of Gamma TrA lies Innes 332 (15h 20.7m, -67 29'). Your first impression may well be "What a very wide pair", but the fainter star 1' away is not part of this system. The binary is in fact the brighter white star. At a separation of 0.8", I could not resolve the mag. 6.5 and 8.5 components on several nights that I tried.

X TrA (15h 14.3m, -70 05'), 1.5 deg S of Gamma, is worth looking at as a bright red, N-spectral type variable.

Give these a try and good luck with the difficult ones!