The Constellation Centaur (Centaurus-Cen)

The name, the history, the myth


For the Greek mythographers this constellation represented the most illustrious of the centaurs, Chiron, illegitimate son of Cronus (Saturn) and Filyra, a nymph of the sea. The centaurs, creatures half man and half horse, enjoyed a very bad reputation because of their wild nature and brutal customs, but Chiron was an exception: wise and good, friend of men, it is said he taught them various arts and he is attributed the idea of grouping the stars in constellations.


There has been some confusion between this "southern" centaur and the Sagittarius of the zodiac, which represents the aggressive nature of the centaur. And the result of the confusion was sometimes the exchange of the name.

According to a myth, the centaurs were created by Chronos, to avoid that his wife Rhea saw him flirting with Filira, daughter of Ocean, but surprised by Rhea, Chronos escaped taking the shape of a horse with a flowing mane. Philira hid for shame in the mountains, where she gave birth to a being, a centaur, immortal as a god and half horse.

Another story tells of the treacherous Greek king Ission, invited to a banquet by Zeus and Hera. Issione longed for Hera, who in turn was happy to take revenge for her husband's repeated betrayals. Realizing what was happening, Zeus, however, gave Hera the consistency of a cloud, and it was from this that Issione mistakenly took pleasure. The father of the gods surprised him in the spasms of passion and tied him to a flaming wheel, which rotates in the sky for eternity (the Sun, according to some myths). The fedifraga Hera took the name of Nephele ("cloud"), and from the seed of Issione gave birth to the first centaur.


The most popular myth of Chiron tells of the tragic wound he suffered when, running to the aid of his friend Heracles (Hercules) against the other centaurs, he was accidentally hit on one knee by an arrow shot by Heracles himself, who had been poisoned with the blood of the Hydra of Lerna. Despite his limbs and magical herbs the wound did not heal and began a terrible agony for him, made infinity by immortality.

He escaped from his unhappy fate by saving Prometheus (see myth linked to the constellation of the Eagle), whose suffering would have ceased when another immortal had renounced immortality for him. Chiron accepted the exchange to secure the sweet relief of death and Zeus accepted through Heracles' intercession, and finally placed his image in heaven.


The constellation that is the subject of this article has always been associated to its main star, Alpha, which gave it its notoriety: over the centuries, what seemed a single star, has instead turned out to be a multiple system. Even better, it was discovered at the beginning of the 20th century that the smallest of these components is no less than the nearest star to the Sun.

α Cen is a triple system, with two components A and B (a yellow dwarf and an orange dwarf, of spectral classes G2 and K1) very bright and very close, while the third is a red dwarf (of spectral class M5) decidedly more detached from the others and much weaker: the center of gravity of the system is located at a distance from the Sun of a little more than 4 light years, while the third component (C) is absolutely the nearest star to the Sun, so much so that it has received since its discovery in 1915 the Latin appellative of "Proxima", the closest.


When in a while we go to see how it appears (once I would have written "would appear", but now the conditional is superfluous...) the Sun, if observed from a hypothetical planet in orbit around the trio of wonders, we will realize that finally the Sun appears with a decent brightness and moreover placed in a nice constellation, a fact we have already talked about before.

The three stars that make up the system are the three stars closest to the Sun and this characteristic makes all three have their own important motion: I remember that this term always means the motion that all the stars have because they orbit around the center of the Milky Way. Depending on the physical position within the galaxy and depending on the distance, the farthest stars usually have a very small motion of their own, but as always there are many exceptions of distant but fast stars or near but slow. In the case of A, B and C, these three stars move in the sky in a remarkable way, creating very interesting situations.

For once, to make these trips in the past and in the future I didn't use Stellarium, just because by choice of the programmers doesn't manage its own motion or at least it doesn't do it correctly: this time I went to another free program (SkyChart), born with the first objective to prepare star maps with the characteristics we decide: it manages well the motion of the stars and this fact will allow us to follow the movements occurred by fixed stars (which are not so fixed) over the millennia.


In particular in 5000 BC α Cen was inside the Southern Triangle (which was then a "Southern Trapeze"!), while in 2000 BC it was in the constellation Circinus (Compass).

The strange thing is that Alfa Centauri entered inside its constellation, that is the boundaries that were established at the beginning of '900 by the UAI, only around 1750 and only since then it could be officially called "α Cen". Going on in the future, we can already imagine what will happen: in the year 10000, the two components A and B will always be inside the constellation of Centaur, but on the other side of β Cen.

the Centaur at the time of the founding of Rome

Going back to Stellarium, I verified that from Rome, in the year of its foundation, 753 BC, the constellation was almost all visible above the horizon, but just the α, which is in its southern part, at most touched the horizon and therefore could not be seen. But it was enough to sail already inside the Mediterranean and reach the delta of the Nile or Phoenicia, to see the star appear above the horizon. All the more reason the star was well visible from the southern areas of Egypt, easily reachable going up the course of the Nile: in fact, in mid-May, in the evening, our star appeared very bright at about 20° of , looking southwards.

The distances, these unknowns...

The alpha Centauri A, B and C system

A little while ago we have analyzed in time and space the system of Alpha Centauri, a double one accompanied from afar by component C: let's open for a moment the discourse on the proximity of the stars.

My 25 readers know that I am very keen to point out that a distance of just 4 light years is an incredible and disproportionate distance, well beyond imagination: many people delude themselves by saying that a star so close is "around the corner". Well, not really! And I'll never tire of remembering that expressed in km this distance is worth just 38,000,000,000,000,000,000 (38 trillion km!)... Are you still convinced that it's really just around the corner?!

With all my strength, however, I would like to be denied by the facts and be able to say tomorrow instead that with modern interspace means this star can be reached in just 10 years of travel. But this is not the case: don't get your hopes up. The technological leap required is not improvised from today to tomorrow and the costs of such a project are absolutely prohibitive: I dare not imagine the right criticism regarding possible investments in this sense, by those who would point out that world hunger is a much more important problem, but that it would require less money to be solved... Not to mention that we still have gasoline cars, while we could already hijack investments on hydrogen...

I'm not pessimistic, nor refractory to news: I try to be realistic at least on this subject, since as you know I like to fantasize a lot, sitting on my desk and using Celestia.

Four steps nearby

Proxima: first stop towards the stars

We finally arrived at the Star closest to the Sun, Proxima Cen, which with its distance of 4.24 al is currently our next door neighbour: a pity that it is very weak, invisible to the naked eye, precisely because it is a red dwarf with a ray equal to 0.15 times that of our yellow dwarf. Its radius is practically equal to the diameter of Saturn (without rings, however) and this says a lot about its size: much bigger than a gas planet, but much smaller than a star.

This time a trip to visit my friends Approximati was really necessary: to deny their name, I'll tell you that they are precise, fussy characters, so much so that if they tell a joke, they feel obliged to explain it to you... Just to keep the condominium relations good one makes a good face to bad game by closing one eye, and then reopen it looking out of their windows to finally see a brilliant Sun (of magnitude 0.4) positioned inside the constellation of Cassiopeia, which is not a "W" but a zigzag ("/W"). The same thing happens going on Alpha Centauri (both component A and B): the Sun is always beautifully bright (even if a little weaker than Proxima) and is in the same point of the sky.

where is Proxima?

Going back to Proxima, just for its small diameter, it has a very low brightness ( 11), that makes it visible only with important instruments: the strange fact is that even if it is part of the triple system of Alfa Centauri, it is very detached, so much that to find it in the sky, we have to move with respect to α Cen more than 2°, as to say that between the two stars there could be comfortably 4 lunar disks! The fact is that physically the red dwarf is at a considerable distance of 12500 UA from the barycentre of the system formed by the other two main stars, which orbit one around the other according to an elliptical orbit with a distance that goes from 11 UA to 36 UA, that is from a little farther than Saturn up to Pluto's orbit.

comparison between the dimensions of A,B and C and other known objects

This unbalance between the distances, from 11 to 36 UA between A and B and 12500 UA towards C, makes it practically impossible to draw an intelligible diagram of the situation or to activate our Celestia to see the three components appear as a diskette in the same frame: we can only imagine the beauty of really being on a planet of the system and being able to see two very bright suns alternating and chasing each other very slowly in the sky, with Proxima very weak, but still visible to the naked eye ( 4.5). In the figure we see the comparison between the three components, the Sun and the two gas giants of the Solar System.

the Sun from HIP 57433

Earlier I had mentioned that the star HIP 57443 is only 30 al from the Sun: while we are at it, let's head with our spaceship Celestia to find my friends Hippy57, strange beings with a guitar-shaped body, so hardened smokers of strange herbs that their planet is always wrapped in a cloud of smoke. For this reason they don't know the Sun, which anyway has a 4.65 from there and therefore still visible to the naked eye, if only they had the opportunity.



Audio Video The Constellation Centaur (Centaurus-Cen)
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