IAU Constellation (TrA) · Genitive: Trianguli Australis · Southern Sky

Triangulum Australe (Southern Triangle)

Triangulum Australe IAU constellation (110 sq deg, rank 83): July far-southern triangle east of Alpha Centauri, led by +1.91 Atria and cluster NGC 6025.

Solid-Angle Area110 sq deg (Rank #83 of 88)
Best Evening MonthJuly
Visibility Latitudes+20° to −90°
Brightest StarAtria (Alpha Trianguli Australis) (+1.91)

Sky Navigation & Astrophysical Context

Although it ranks 83rd in area at just 110 square degrees, Triangulum Australe is far brighter and more nearly equilateral than its northern namesake Triangulum, standing out immediately in the southern Milky Way just southeast of the Southern Pointers (Alpha and Beta Centauri). First depicted by Petrus Plancius in 1589 and systematically charted by Pieter Dirkszoon Keyser in 1595–1597 before appearing in Bayer's 1603 Uranometria, its three principal stars are honored on the national flag of Brazil.

The southeastern apex of the triangle is anchored by Atria (Alpha Trianguli Australis, a contraction of 'Alpha Trianguli Australis,' V = +1.91), a luminous K2 IIb-IIIa orange-red bright giant at 120 parsecs with a radius ~130 times that of the Sun and X-ray emission suggesting a hot magnetic corona or hidden main-sequence companion. The western base of the triangle is formed by two third-magnitude white stars: Beta Trianguli Australis (+2.85, an F1 V star at 12.4 parsecs) and Gamma Trianguli Australis (+2.87, a fast-rotating A1 V star).

Three degrees north of Beta Trianguli Australis on the Norma border glitters the bright open cluster NGC 6025 (Caldwell 95, V = +5.1), which packs thirty 7th- to 11th-magnitude stars into a 12-arcminute triangle easily visible in binoculars, while inside the Southern Triangle pulsate two naked-eye classical Cepheids (R and S Trianguli Australis) alongside the two-shell planetary nebula NGC 5979.

Bordering IAU Constellations: Norma, Ara, Apus, Circinus.

Equatorial Center Position & Transit Altitude

Triangulum Australe is centered near Right Ascension 16h 04m 48s, Declination −65° 23′ 24″ (The Southern Triangle).

J2000 Equatorial Sky Position (Triangulum Australe) +90° 0° −90° 24h 12h RA 0h Peak Meridian Altitude 0° 40°N Lat 55° 30°S Lat RA: 16h 04m 48s Dec: −65° 23′ 24″ Epoch: J2000.0 · 110 sq deg (Rank #83) First-party vector diagram derived from IAU / CDS SIMBAD / Gaia DR3 astrometry.

Practical Star-Hopping & Observing Tips

  • Look 12° southeast of Alpha Centauri (Rigil Kentaurus) to spot the vivid orange apex of Atria (+1.91) and the white base stars Beta (+2.85) and Gamma Trianguli Australis (+2.87).
  • Sweep 3° north of Beta Trianguli Australis with binoculars or a low-power telescope to view the sparkling 5th-magnitude open cluster NGC 6025 (Caldwell 95).
  • Track the 3.39-day Cepheid pulsations of R Trianguli Australis (between Beta and Gamma TrA, varying from +6.4 to +7.0) in binoculars.

Notable Celestial Objects in & Near Triangulum Australe

Cataloged Stars and Deep-Sky Objects Associated with Triangulum Australe
ObjectFamilyClassificationRA (J2000)Dec (J2000)Min. Instrument
Proxima Centauri (Nearest Star to Sun)starsRed dwarf flare star (nearest extrasolar star to the Sun)14h 29m 42.95s-62° 40′ 46.1″80–100 mm telescope with detailed finder chart (Southern Hemisphere / low northern latitudes)
Omega Centauri (NGC 5139 Cluster)globular-clustersGlobular cluster / stripped dwarf galaxy nucleus13h 26m 47.24s-47° 28′ 46.1″Naked eye (south of 35°N)

Scientific Sources Cited