Triangulum Australe (“the southern triangle”) is a small southern sky constellation that was one of 12 invented by astronomer Petrus Plancius around 1597 based upon the observations of Dutch explorer Pieter Dirkszoon Keyser. Its three brightest stars form an equilateral triangle, known as the “Three Patriarchs,” in honor of the biblical Abraham, Isaac and Jacob. The brightest of these stars, Atria, represents Abraham, and is an orange giant found 391 light years from Earth with a visual magnitude of +1.91.
Triangulum Australe is the 6th smallest constellation, taking up an area of just 110 square degrees of the southern celestial heavens. It can be seen by observers located between +25° and -90° of latitude, where it can be viewed from April to June. Its neighboring constellations include Ara to its east, Circinus to its west, Norma to its north, and Apus to its south.
Dutch astronomer and cartographer Petrus Plancius, a founder member of the Dutch East India Company, sent chief navigator Pieter Dirkszoon Keyser to the East Indies (South/Southeast Asia) to help fill in gaps in European astronomical maps of the southern sky. Keyser, together with fellow explorer Frederick de Houtman, subsequently catalogued 135 new stars which on their return were turned into 12 new southern constellations by Petrus Plancius, and appeared on his 35-cm celestial globe in 1597. One of these was Triangulum Australe, which is also the smallest of the constellations created by Plancius.
Hercules Constellation Family
Triangulum Australe is a member of the Hercules family of constellations, together with Aquila, Ara, Centaurus, Corona Australis, Corvus, Crater, Crux, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Sagitta, Scutum, Sextans, Serpens and Vulpecula.
– Atria (Alpha Trianguli Australis), the brightest star in the Triangulum Australe Constellation, is an orange giant (K2 IIb-IIa) situated 391 light years from our solar system that shines with an apparent magnitude of 1.91. This 48 million year old star is 130 times bigger than the Sun, with 7 times its mass, and 5,500 times its luminosity.
– Betria (Beta Trianguli Australis), the second brightest star in the constellation, is a yellow-white star (F1 V) found 40.37 light years distant of magnitude 2.85. It is about twice the size of the Sun, with 1.65 times its mass, and 8.5 times its luminosity. It also has a 14th magnitude visual companion separated by 155 arcseconds that is not physically related to the main star.
– Gatria (Gamma Trianguli Australis), the constellation’s third brightest star, is a white dwarf (A1 V) located 184 light years away with an apparent visual magnitude of 2.87. This 260 million years old star is around 5 times bigger than the Sun, with 3 times its mass, and 220 times its luminosity. Gamma Trianguli Australis has an extremely fast rotational velocity of 199 km/s, and therefore has a rotation period of roughly 1.2 days,
Other stars of interest in Triangulum Australe includes the binary stars Delta Trianguli Australis, Epsilon Trianguli Australis, and Zeta Trianguli Australis; the triple star system Iota Trianguli Australis; the blue-white subgiant Eta Trianguli Australis; the yellow-white giant HD 133683; the yellow giant stars Kappa Trianguli Australis and Theta Trianguli Australis; and the red carbon star X Trianguli Australis.
Notable Deep-Sky Objects
The constellation contains no Messier objects, and only a few faint deep-sky objects.
– ESO 69-6 is a beautiful pair of interacting galaxies situated around 650 million light-years away from our solar system. Their strong interaction has caused long tidal tails of gas and stars to sweep out from their galactic cores, with the visual effect resembling musical notes on a staff.
– ESO 137-001 is a barred spiral galaxy found about 220 million light-years from the Milky Way. It lies within the Norma Cluster of galaxies, and has a striking 260,000 light year long tail of bright blue stars caused by the ram pressure stripping effect as it passes through the Norma Cluster.
– NGC 6025 is an open cluster situated 2,700 light years from the Sun with an apparent magnitude of 5.1. It is around 80 million years old, 9 light years across, and contains about 80 stars, with an 8-inch (200mm) telescope able to reveal 40 or so of them.
Other objects of interest in Triangulum Australe includes the spiral galaxy NGC 5938; and the planetary nebulae NGC 5979 and Henize 2-138.