Costellazione Sagittario, stelle principali e mitologia
Il Sagittario, in latino Sagittarius, è una costellazione che giace per la maggior parte nel piano della Via Lattea, proprio al centro della nostra Galassia, dove è presente una fonte di onde radio chiamata dagli astronomi "Sagittarius A". È situato tra lo Scorpione, a ovest, e il Capricorno, a est. Viene rappresentato come un centauro che tende un arco, ma non bisogna aspettarsi di trovarsi davanti una figura metà uomo e metà cavallo: le sue stelle più luminose formano, infatti, un asterismo che, per la sua configurazione, viene detto la Teiera.
In the sky of the Northern Hemisphere, the best period to observe the constellation of Sagittarius is between June and September.
Main stars of Sagittarius
The brightest star in the constellation of Sagittarius is ε Sagittarii (Epsilon), a blue-white giant with magnitude 1.79, known as Kaus Australis, as it represents the lower part of the bow in the hand of Sagittarius (kaus derives from the Arabic al-qaus, “the bow”, and australis is a Latin word meaning “southern”). The second brightest star in this constellation is Sigma Sagittarii, called Nunki, an Assyrian or Babylonian name which in Mesopotomy meant “star of the proclamation of the sea”. It is a very powerful blue giant (magnitude 2.05) but also quite distant, which is why it does not seem very bright to us. Zeta Sagittarii, traditionally known as Axilla, Latin for “armpit”, is the third brightest star in Sagittarius (2.6 magnitude) and is located under the archer’s left armpit.
Followed, in order of magnitude, δ Sagittarii, or Kaus Media, and λ Sagittarii, or Kaus Borealis, two orange giants which respectively represent the central and upper part of the bow in the centaur’s hand. The sixth brightest star is Pi Sagittarii (magnitude 2.88), also known by its Arabic name Albaldah.
Gamma Sagittarii is a yellow giant of magnitude 3 also known as Al Nasl, “the tip” in Arabic, and represents the tip of the archer’s arrow. β Sagittarii is a double star made up of two whitish stars and is known as Arkab, from the Arabic “archer’s Achilles tendon”.
Finally, Alpha Sagittarii, also called Rukbat, from the Arabic rukbat al-rami, “archer’s knee”, is far from being the brightest star, having a magnitude of only 3.96.
Other celestial objects
The constellation of Sagittarius does not have particularly bright stars, but, being crossed by the halo of the Galaxy, inside it, several notable objects are visible to the naked eye, including nebulae, open clusters and globular clusters. In all there are 15 Messier objects: 3 famous nebulae (Laguna, Omega and Trifid), 5 open clusters and 7 globular clusters.
The Lagoon Nebula (M8), observable with the naked eye on particularly clear nights, is located about 7° north of the star Al Nasl and is home to various interesting objects, such as open clusters and young stars; with binoculars, it appears as an opaque spot surrounded by stars.
Omega (M17) is a very bright emission nebula (i.e. it emits light due to ionized gas), easily observable in the dark and clear evening skies in the period between June and October. Finally, the third famous nebula in the constellation of Sagittarius is the Trifid Nebula (M20), so called because part of the dust that composes it absorbs light, creating dark lines that divide it into three. It is located just north of n. Lagoon and in binoculars it appears as a circular, opaque spot.
Among the globular clusters, we remember M 22, the fifth of the entire celestial vault in order of brightness and one of the closest to the Earth (about 10 thousand light years away). On clear nights, it is possible to distinguish a very small star even with the naked eye.
The celestial object that catches the eye most of all is the open cluster known as the Sagittarius Star Cloud (M24), a region with a very high density of stars.
The constellation of Sagittarius is depicted as an archer with horse legs and a man’s bust, with a bow stretched in his hands in the act of shooting an arrow towards nearby Scorpio, to avenge the death of Orion, caused by the animal’s sting . Sagittarius has sometimes been mistakenly identified with Chiron, causing confusion with the southern constellation Centaurus. The two celestial centaurs, in reality, have a very different nature: while Chiron, inventor of medicine, is famous for his wisdom and knowledge, Sagittarius is a skilled hunter.
The myth of Sagittarius was already known to the ancient Sumerians, who associated the god of war Ninurta with it, but it was subsequently adopted by the Greeks, who distorted it. Eratosthenes, in the Catatherisms, described him not as a centaur but as a satyr, an ancient hybrid with a human torso and a goat’s tail and legs, and associated him with the legendary Crotus, son of the satyr Pan, god of shepherds and countryside dwellers, and of Eupheme, the nurse of the Muses.
Crotus, inventor of the art of archery, grew up on Mount Helicon together with the Muses, enjoying their company and witnessing the wonderful manifestations of their arts. To express admiration towards his 9 half-sisters, Croto invented applause. The Muses were so grateful to him that they begged Zeus to put him in the sky and transform him into a constellation suitable for him. So it was that the king of Olympus decided to give him the body of a horse, since he was a skilled horseman, and a bow and arrow to celebrate his skill as an archer.