Constellations: Triangulum Australe 'the Southern Triangle' Content from the guide to life, the universe and everything

Constellations: Triangulum Australe 'the Southern Triangle'

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There fluttered round the spring
a fly of filmy wing
Libella, lightly ranging;
Long had she pleased my sight
From dark to lovely bright

- Johann Wolfgang von Goethe, from Our Joys

The Constellation Triangulum Australe

Name:Triangulum Australe (Latin: 'southern triangle')
Genitive:Trianguli Australis
Meaning:Southern Triangle
Short form:TrA
Area:110 square degrees
Co-ordinates1:16h, −65°

Triangulum Australe, with the alternative name Libella 'the Level', was among the constellations identified during the 16th Century by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman (although the constellation was probably first identified on a star chart by Amerigo Vespucci in 1503). A plumb bob was drawn into the almost perfect isosceles triangle made up by its three brightest stars, giving it the look of a surveyor's level. Along with the geometer's compass (Circinus) and a rule and set square (Norma), it formed a group of constellations akin to surveying instruments.

The 'Southern Triangle' was later shown south of Argo Navis on a globe of 1589 by the Dutchman Petrus Plancius, along with a Southern Cross, and the constellation was re-introduced in 1603 (exactly a hundred years after Vespucci) by Johann Bayer, as a southern equivalent of the northern constellation Triangulum.


Triangulum Australe is an easily found constellation near the Southern Cross, defined by three relatively bright stars (alpha TrA, beta TrA and gamma TrA) in the same general region as alpha Centauri, bordered by the constellations Norma, Ara, Apus and Circinus.

Navigators have named its brightest star alpha Trianguli Australis (an orange giant) Atria, a contraction of its scientific name. Of interest in the constellation is iota Trianguli Australis, a multiple system with a binary pair of yellow-white dwarfs, one of which is a gamma Doradus-type variable (a more distant companion star encompasses them both, making the iota Trianguli Australis a triple star system).

Variable Stars in Triangulum Australe

Newly-discovered variable stars are given an upper case English letter, and those in the Southern Triangle (at time of writing) are:

  • R Trianguli Australis (2,000 light years2) and S Trianguli Australis (2,700 ly) are Cepheid variables: their prototype is a cosmic yardstick.

  • X Trianguli Australis is an irregular variable carbon star3 (1,500 ly) which varies between 6th and 7th magnitude.

The following Star Table lists some of the stars to be seen in the constellation:

Star Table

StarDesignationBrightness (m)Distance
(light years)
α TrAalpha TrA+1.92415K2IIb-IIIaOrange giant, Atria
β TrAbeta TrA+2.8540F2IIIYellow-white giant
γ TrAgamma TrA+2.89183A1VWhite giant
δ TrAdelta TrA+3.85621G5IIYellow supergiant
ε TrAepsilon TrA+4.11216K0IIIOrange giant
ζ TrAzeta TrA+4.9139F9VSpectroscopic binary
ι TrAiota TrA+5.28 var132Multipleγ Dor variable
triple star system
HD 141913Sellors 11+6.4/+8.8VariableB9IIBinary

New General Catalogue (NGC) and Index Catalogue (IC)

The New General Catalogue (NGC) is a list of interesting deep-space objects (that is, objects outside our solar system). It was compiled by John Louis Emil Dreyer at Armagh Observatory, based on the observations of Sir William Herschel. Since the NGC was created improved detection methods have uncovered many other deep-space objects, and these are now also registered in the Index Catalogue (IC). Some of those identified in Triangulum Australe are listed below:

Deep Space Objects

CatalogueTypeBrightness (m)Distance
(light years)
NGC 5844Planetary nebula+12.12,900No central star
NGC 5938Spiral galaxy+11.7UnknownDreyer's object
NGC 5979Planetary nebula+11.211,700Remnants of a red giant
NGC 6025Open cluster+5.12,00030+ stars
NGC 6156Spiral galaxy+11.62,400None
IC 4584/85Spiral galaxiesVariableUnknownInteracting
IC 4595Spiral galaxy+12UnknownEdge-on

NGC 5979 is a planetary nebula that had its origins in the death of a red giant star, and now resembles a dramatic cloudburst.

NGC 6025 was identified by French astronomer Nicolas Louis de Lacaille in 1751 - 52 from South Africa, and is included in Sir Patrick Moore's Caldwell Catalogue as 'Caldwell 95'. It can be found about three degrees NNE of beta TrA, and is an excellent cluster to view with binoculars, although a telescope will enable you to pick out some of the larger stars even better.

IC 4584 and IC 4585 are interacting spiral galaxies, which mean they dance a wonderful pas de deux together.

Extrasolar Planets in Triangulum Australe

In 2009 the yellow dwarf star HD 147018 was discovered to have two planets in orbit. Planets HD 147018 b and HD 147018 c are both superjovian worlds.

Triangulum Australe Trivia

  • The stars of Triangulum Australe are actually used in the flag of Brazil, and represent the three states of Southern Brazil; Rio Grande do Sul (alpha TrA), Santa Catarina (beta TrA) and Paraná (gamma TrA).

  • 'Southern Triangle Island' can be on the Great Sea in the game The Legend of Zelda: The Wind Waker. It is the southernmost island possessing a statue to position Nayru's Pearl within, to enable the Tower of the Gods to rise from the ocean floor.

1Current International Astronomical Union (IAU) guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2A light year is the distance light travels in one year, roughly roughly 5.88 trillion miles or 9.46 trillion km.3Carbon stars are a major contributor to the carbon in the Universe.

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