The constellation of the Southern Triangle
For a well-known principle of geometry, three points not aligned constitute the vertices of a triangle: in this case, as you can see from the star map made with Stellarium, the Triangle (Austral, because there is also the boreal version, simply called the Triangle) is as good as done! The constellation is located in the southern sky, further south of the pair α and β Centauri and very close to the South Celestial Pole: it should be well recognizable since its three main stars are between the 2nd and 3rd magnitude.
The name, history and myth of the Southern Triangle
The first to refer to the constellation seems to have been Amerigo Vespucci in 1503, although the Southern Triangle does not appear on any star atlas for over a century. Already the Spanish navigator Maître João in 1500 would have mapped its stars. However, it originated in the 16th century, but when it was spotted by Dutch navigators, it was made official. Plancius in 1589 introduced it, or more precisely, understood its stars but without naming them. In the 1600's Hundius globe is reported the complete asterism and the associated name of Triangulum Aust.
It was Johann Bayer, in 1603, who included it in his Uranometria catalogue.
In the meantime let's see the representations over time: first the one provided by Hevelius
and then Stellarium's
Big and near stars
In the comparison diagram between the stars of the Southern Triangle and other notes, we can see a big orange disc: it is the α TrA, called Atria, an orange giant of spectral class K2, with a diameter equal to 116 times that of the Sun and also three and a half times the usual and blasted younger sister Aldebaran, much better known.It's time to see the beautiful picture taken by my Atriani friends of their star, majestic and imposing from 10 UA away: it's certainly not a gossip (in fact all the galactic chronicles talk about it) that my friends have particularly complex love lives, where the couple has been supplanted (can you imagine?) by the triangle, but the situation is so common that now they live these experiences in the light of the Atria, without false modesty. But let's move on to the other big stars, which is better...
They are two stars, big sisters of our yellow dwarf, δ TrA and κ TrA, class G5 and G6 respectively, with a radius equal to 45 and 35 times that of the Sun: even if they seem so small compared to the supergiants in the diagram, in reality a diameter of 35 or 45 times that of the Sun is really impressive.
As far as the near stars are concerned, we have a couple of them placed practically at the same distance from us (39.5 and 40.2 al from us), ζ TrA and β TrA, respectively of spectral class F9 and F2 : we could think that they are a double physics if it were not that in our sky they are separated by an angular distance of about 7.5°, which physically are little more than 5 al, as Celestia informs us. We can therefore expect, going near the two stars from my friends Betatrani and Zetatrani, that the Sun appears projected by the three-dimensional geometry in the same area of the sky where there are stars of our northern sky, such as those of Perseus and the Bears (Major and Minor), where however there are three other very well known and very close stars (Raccoon, Capella and Sirius): these stars are located where one less expects it because of their position in the sky, a position to be considered in the three dimensions. The two races are very similar, so much so that they differ only for the initial letter: if you ever see two almost twin people, one of the one and one of the other ethnicity, be sure that if one is called Carlo, the other is called Marlo, or one Mario and the other Dario. That's why just one photo is enough to see the appearance of the sky where a 5a star, our Sun, dominates.
Deep sky objects
This small austral constellation of name and of fact presents a nice number of deep sky objects, of which I have chosen six: the first one is ESO 69-6 and is formed by a couple of galaxies interacting in a very spectacular cosmic embrace
then we have a barred spiral galaxy, the ESO 137-001, which like the previous one is part of the ESO catalogue (managed by the European Southern Observatory, in Chile).
the galaxy ESO 137-001
then we have the nice spiral galaxy NGC 5938.
the galaxy NGC 5938
The fourth object I show you is the planetary nebula NGC 5979
the planetary nebula NGC 5979
followed by the open cluster NGC 6025, a grouping formed by about thirty stars all between 7a and 9a .
the open cluster NGC 6025
Finally we see the planetary nebula called Henize 2-138, in its photo taken by the Faulkes Telescope South observatory in Australia, famous, I read, for having observed for the first time a mutual occultation between two satellites of Uranus, an event particularly rare and very difficult to observe.
the object Henize 2-138
Let's close the analysis of the Southern Triangle remembering that its star α is commonly called Atria and that the constellation is never visible at our latitudes.