Triangulum Australe
“The Southern Triangle” · Southern · best around July evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
The southern triangle — smaller, brighter and neater than its northern cousin, beside the Pointers.
How to find it
Just south of Alpha Centauri.
✦ What lives inside it
The deep dive
Researched for the Atlas from Wikipedia — Triangulum Australe (10,758 characters read) · updated Sep 20, 2026
01 A triangle brighter than its northern twin
Despite being a small constellation, Triangulum Australe punches well above its weight in stellar brightness. Its three defining stars — Alpha, Beta, and Gamma — are all noticeably brighter than their counterparts in the northern Triangulum, even though the article notes those stars are individually smaller. The brightest of the trio, Atria (Alpha Trianguli Australis), shines at apparent magnitude +1.91, placing it 42nd among all stars visible from Earth. Beta and Gamma follow closely at magnitudes 2.85 and 2.87 respectively, making all three corners of the triangle comfortably naked-eye objects even under moderately light-polluted skies. The near-equilateral shape of the pattern is almost immediately obvious once a southern-sky observer locates it: the article specifically describes it as a roughly equilateral triangle that is easily identifiable. This compactness — combined with the uniform brightness of its vertices — gives the figure a neat, geometric quality that northern constellations rarely achieve so cleanly.
02 Atria: a star that nearly swallows Venus Deeper
Atria is classified as a bright giant of spectral type K2 IIb-IIIa, an orange star sitting at a distance of 424 light-years (130 parsecs) from Earth. Its absolute magnitude of −3.68 reveals just how intrinsically powerful it is: the star is 5,500 times more luminous than the Sun. Even more striking is its sheer physical scale. Atria carries a diameter 130 times that of the Sun, which is so enormous that if it were placed at the centre of our Solar System it would almost reach the orbit of Venus. That orbit sits roughly 108 million kilometres from the Sun, so Atria's radius would extend to around 90 million kilometres in every direction. The name Atria itself is simply a contraction of its Bayer designation Alpha Trianguli Australis, compressed into a convenient proper name — an economy of language typical of how later astronomers tidied up the labelling of bright southern stars. Lacaille formally assigned its Bayer letter in 1756.
03 Beta's mystery companion and Gamma's simplicity Deeper
Beta Trianguli Australis, the second corner of the triangle, is a double star whose primary is an F1V main-sequence star shining at apparent magnitude 2.85 and lying only 40 light-years (12 parsecs) away — making it by far the nearest of the three triangle stars. Its absolute magnitude of 2.38 confirms it is a fairly ordinary star by cosmic standards. A companion star sits almost 3 arcminutes away and registers at 13th magnitude, but its physical relationship to the primary remains uncertain; the article states plainly that it may or may not be in orbit around Beta. Gamma Trianguli Australis, the third corner, is considerably more straightforward: a white A1 V main-sequence star at apparent magnitude 2.87, sitting 180 light-years (55 parsecs) away. Unlike its two triangle partners it carries no companion controversy, no complex classification, and no unusual behaviour — it is simply a hot, young star holding its place at the apex with quiet reliability.
04 Hidden variables and a carbon oddity Deeper
Beneath the three bright cornerstone stars lies a collection of variable stars that reward patient observers with modest equipment. Three Cepheid variables — R, S, and U Trianguli Australis — pulse through brightness cycles of 3.389, 6.323, and 2.568 days respectively, ranging across roughly half a magnitude each time. All three are yellow-white F-type giants. Far more unusual is RT Trianguli Australis, which the article classifies as a carbon Cepheid of spectral type R, displaying strong absorption bands from molecular fragments of C₂, CH, and CN. It varies between magnitudes 9.2 and 9.97 over just 1.95 days. Carbon Cepheids form a rare and debated subclass; the presence of such molecular bands makes RT stand out even among variable stars. Nearby Gamma, the carbon star X Trianguli Australis adds further complexity: it averages magnitude 5.63 but oscillates on two simultaneous periods of around 385 and 455 days, a dual periodicity that signals a particularly evolved, thermally pulsing stellar interior.
05 Dwarf novae, a slow nova, and two exoplanets Deeper
The constellation harbours some dramatically energetic stellar systems. EK Trianguli Australis, first noticed in 1978 and formally described in 1980, is a dwarf nova of the SU Ursae Majoris type: a white dwarf locked in an orbit with a donor star so tight that the pair circle each other every 1.5 hours. Material siphoned from the donor builds on an accretion disc until the system erupts — reaching magnitude 11.2 in superoutbursts and 12.1 in normal outbursts, before settling back to a quiet magnitude of 16.7. NR Trianguli Australis offered a different kind of drama when it flared as a slow nova, peaking at magnitude 8.4 in April 2008 before fading to magnitude 12.4 by September of the same year. On a quieter note, the Sun-like star HD 147018 — apparent magnitude 8.3, spectral type G9V — was found in 2009 to host two exoplanets, HD 147018 b and HD 147018 c, adding a planetary system to the constellation's inventory.
06 Zeta and Iota: two systems hiding depths Deeper
Several of the fainter lettered stars in Triangulum Australe conceal unexpected complexity. Zeta Trianguli Australis looks like a single F9V star at magnitude +4.91, but spectroscopic analysis reveals it as a binary pair, with a probable red dwarf companion orbiting every 13 days. Its proper motion links it to the Ursa Major moving group — a stream of stars sharing a common origin that stretches far across the sky. The article describes Zeta as a young star, making its membership in this kinematic group a clue to its birthplace. Iota Trianguli Australis reveals itself as a multiple system when viewed through a 7.5 cm telescope: the brighter component has a spectral type of F4IV and is itself a spectroscopic binary, with two yellow-white stars orbiting every 39.88 days. The primary of that inner pair is also a Gamma Doradus variable, pulsating over a period of 1.45 days — a type of pulsation driven by convective processes near the stellar surface. The fainter visual companion is not physically associated with the system, making the whole arrangement an optical double layered over a genuine spectroscopic binary.
07 From Vespucci's lost catalogue to Bayer's atlas
The recorded history of Triangulum Australe begins at the very edge of the European Age of Exploration. Amerigo Vespucci, navigating the New World at the start of the 16th century, catalogued southern stars for King Manuel I of Portugal, a document now lost. In written descriptions sent to his Florentine patron Lorenzo di Pierfrancesco de' Medici and published as Mundus Novus in 1504, Vespucci described a triangle that may be either Triangulum Australe or the neighbouring Apus — the identification is uncertain. The first definite depiction came in 1589, when Flemish astronomer-clergyman Petrus Plancius placed it on a 32.5 cm diameter celestial globe published in Amsterdam by Jacob van Langren, calling it Triangulus Antarcticus. Plancius placed it incorrectly, south of Argo Navis. Petrus Keyzer and Frederick de Houtman then coined the Dutch name Den Zuyden Trianghel. It was left to Johann Bayer's 1603 Uranometria to depict the constellation accurately and introduce its modern Latin name.
08 Draughtsman's tools and rival name Libella
When Nicolas Louis de Lacaille remapped the southern sky in 1756 he brought a craftsman's imagination to the constellations around Triangulum Australe. He grouped Norma, Circinus, and Triangulum Australe together as a set square and ruler, a compass, and a surveyor's level respectively — a coherent set of draughtsman's instruments scattered across that region of sky. For Triangulum Australe, the surveyor's level interpretation was striking enough that German astronomer Johann Bode adopted it independently, giving the constellation the alternate name Libella — Latin for a spirit level — in his Uranographia. That name never achieved broad acceptance, and Triangulum Australe held its ground. Lacaille's 1756 work also mattered practically: he assigned Bayer Greek-letter designations to twelve stars in the constellation, running from Alpha through Lambda, with two close stars both called Eta. Lambda was eventually dropped due to faintness, and one of the two Eta stars was later reclassified under its Henry Draper catalogue number, HD 150550.
09 Three patriarchs and a Rainbow Serpent
European and Indigenous Australian traditions each found entirely different meanings in the same triangle of stars. German poet and author Philippus Caesius read the three principal stars as the Three Patriarchs of the Hebrew Bible — Abraham, Isaac, and Jacob — with the brightest star, Atria, representing Abraham. This reflects the common early modern European habit of mapping biblical figures onto southern constellations newly charted by explorers. A completely different tradition comes from the Wardaman people of the Northern Territory in Australia, who saw the stars of Triangulum Australe as the tail of the Rainbow Serpent, a figure of enormous importance in many Australian Indigenous cosmologies. In the Wardaman view, this serpentine figure stretched from near Crux across to Scorpius. When the constellation rides overhead in October, the Rainbow Serpent is said to give Lightning a nudge, triggering the wet season rains that arrive in November — a seasonal calendar encoded directly in the night sky.
10 The Great Attractor sits on its border
One of the most consequential structures in the observable universe quietly straddles the boundary between Triangulum Australe and the neighbouring constellation Norma. This is the Great Attractor — the gravitational centre of the Laniakea Supercluster, the vast supercluster of galaxies that includes our own Milky Way. The Milky Way, along with hundreds of thousands of other galaxies, is flowing toward this enormous concentration of mass. Because the Great Attractor lies almost exactly in the plane of the Milky Way as seen from Earth, the dense foreground of stars and interstellar dust makes direct observation extremely difficult, which is part of why its precise nature took astronomers so long to characterise. The article does not give a distance or mass figure for the Great Attractor itself, but the sheer scale of influence implied — governing the motion of a supercluster — makes its quiet placement at the edge of this small southern constellation one of the more remarkable coincidences of celestial cartography.
11 Flags, secession, and three southern states
Triangulum Australe carries real political meaning in South America. It appears on the flag of Brazil, where it symbolises the three states of the South Region — Paraná, Santa Catarina, and Rio Grande do Sul. This makes it one of a select group of constellations given direct national symbolism on a major country's flag, reflecting Brazil's long tradition of incorporating specific stars and star patterns into its flag design based on the sky as seen from Brazilian territory on a particular historical date. The constellation also appears as the sole astronomical symbol on the flag of The South Is My Country, a Brazilian secessionist movement, demonstrating how the same astronomical figure can be appropriated across very different political contexts. In both cases it is the distinctively compact triangular shape — easily read at a glance — that makes the constellation serviceable as a graphic emblem, a quality that sets it apart from most other constellations.
12 NGC 6025: the constellation's showpiece cluster
For observers with binoculars or a small telescope, NGC 6025 is the most rewarding target Triangulum Australe offers. It contains about 30 stars ranging from 7th to 9th magnitude, spread across a physical diameter of about 11 light-years (3.4 parsecs). The cluster lies approximately 2,500 light-years (770 parsecs) from Earth and sits 3 degrees north and 1 degree east of Beta Trianguli Australis — close enough to Beta that Beta itself serves as a natural pointer. The brightest individual star in the cluster is MQ Trianguli Australis at apparent magnitude 7.1. Beyond NGC 6025, the constellation's deep-sky offerings thin out considerably. The planetary nebula NGC 5979 glows at magnitude 12.3 and takes on a blue-green hue at higher magnifications. Henize 2-138 is a smaller planetary nebula at magnitude 11.0. Two galaxies round out the list: spiral galaxy NGC 5938 roughly 300 million light-years away, and the merging galaxy pair ESO 69-6 at about 600 million light-years, whose gravitational interaction has drawn long tidal tails from each galaxy.