Triangulum Australe is a small constellation in the far Southern Celestial Hemisphere. Its name is Latin for “the southern triangle,” which distinguishes it from Triangulum in the northern sky, and is derived from the almost equilateral pattern of its three brightest stars. It was first depicted on a celestial globe as Triangulus Antarcticus by Petrus Plancius in 1589, and later with more accuracy and its current name by Johann Bayer in his 1603 Uranometria. The French explorer and astronomer Nicolas Louis de Lacaille charted and gave the brighter stars their Bayer designations in 1756. Triangulum Australe is bordered by Norma to the north, Circinus to the west, Apus to the south and Ara to the east. It lies near the Pointers (Alpha and Beta Centauri), with only Circinus in between. The constellation is located within the Milky Way and hence has many stars. A roughly equilateral triangle, it is easily identifiable. Triangulum Australe lies too far south in the celestial southern hemisphere to be visible from Europe, yet is circumpolar from most of the southern hemisphere. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 18 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 14h 56.4m and 17h 13.5m, while the declination coordinates are between −60.26° and −70.51°. Triangulum Australe culminates each year at 9 p.m. on 23 August.
In defining the constellation, Lacaille gave twelve stars Bayer designations of Alpha through to Lambda. The three brightest stars, Alpha, Beta and Gamma, make up the triangle. Readily identified by its orange hue, Alpha Trianguli Australis is a bright giant star of spectral type K2 IIb-IIIa with an apparent magnitude of +1.91 that is the 42nd-brightest star in the night sky. It lies 424 light-years away and has an absolute magnitude of −3.68 and is 5,500 times more luminous than our Sun. With a diameter 130 times that of our Sun, it would almost reach the orbit of Venus if placed at the centre of the Solar System. The proper name Atria is a contraction of its Bayer designation. Beta Trianguli Australis is a double star, the primary being an F-type main-sequence star with a stellar classification of F1V and an apparent magnitude of 2.85. Lying only 40 light-years away, it has an absolute magnitude of 2.38. Its companion, almost 3 arcminutes away, is a 13th-magnitude star which may or may not be in orbit around Beta. The remaining member of the triangle is Gamma Trianguli Australis with an apparent magnitude of 2.87. It is an A-type main sequence star of spectral class A1 V, which lies 180 light-years (55 parsecs) away. Triangulum Australe contains several cepheid variables, all of which are too faint to be seen with the naked eye: R Trianguli Australis ranges from apparent magnitude 6.4 to 6.9 over a period of 3.389 days, S Trianguli Australis varies from magnitude 6.1 to 6.8 over 6.323 days, and U Trianguli Australis’ brightness changes from 7.5 to 8.3 over 2.568 days. All three are yellow-white giants.
Triangulum Australe has few deep-sky objects—one open cluster and a few planetary nebulae and faint galaxies. NGC 6025 is an open cluster with about 30 stars ranging from 7th to 9th magnitude. Located 3 degrees north and 1 east of Beta Trianguli Australis, it lies about 2,500 light-years away and is about 11 light-years in diameter. Its brightest star is MQ Trianguli Australis at apparent magnitude 7.1. NGC 5979, a planetary nebula of apparent magnitude 12.3, has a blue-green hue at higher magnifications, while Henize 2-138 is a smaller planetary nebula of magnitude 11.0. NGC 5938 is a remote spiral galaxy around 300 million light-years away. It is located 5 degrees south of Epsilon Trianguli Australis. ESO 69-6 is a pair of merging galaxies located about 600 million light-years away. Their contents have been dragged out in long tails by the interaction. The galaxy is notable for its long tail, stretching across 260,000 light years. ESO 137-001 is a barred spiral galaxy in Triangulum Australe. It is located in the Norma Cluster (Abell 3627) which lies in Triangulum Australe and Norma constellations. Henize 2-138 is another planetary nebula, smaller in size than NGC 5979. It has a visual magnitude of 11.0. Credits: Constellation Guide, Wikipedia.