The constellation Big Dog

A Big dog

Probably the name Cane Maggiore is not very well known, just as those who are barely interested in Astronomy barely know that this constellation includes that beautiful, glittering star, Sirius, perennially attached to Orion's belt.
In ancient times Sirius did not enjoy a good reputation, since his first morning sighting took place during the hottest and torrid period of the year: his name comes from the Greek σειριος which means shining, but also burning. Another nickname for this star was the star of the dog, in Latin canicula: this term was then associated with the period of the year when Sirius was sighted, a period that thus became the time of the heatwave.

In the image of Stellarium we see the Greater Dog as it is commonly depicted: at the top right we see Saiph, a star of Orion, which makes us understand where it is located in the night sky Sirius with its nearby stars. Sirius is the brightest star in the sky, having a negative magnitude (-1.45) : in the sky it shows itself very bright and observing it from an unobstructed zone (for example an open $campo$) sometimes it can give also the optical illusion to move in the sky, almost as airplanes do. However, it is an optical illusion due to its great brightness: in fact, it is enough to simply move and observe it side by side with a tree, a light pole or a building, to realize that it obviously does not move, except like all the other stars in the sky.

The name, the story & the myth of the  Big Dog

The symbolism of the southern constellation of Canis Major, or Dog Minor, and its brightest star, Sirius, dates back at least to the III millennium before Christ. At that time, Sirius, known as Sothis, was the fundamental star of the Egyptian Sotiac calendar. His heliacal rising (i.e. his appearance just before sunrise, after a period of invisibility of several months) fell in mid-July and coincided with the annual flood of the River Nile. During the long Egyptian history Syrius/Sotis was variously interpreted and identified with the goddess Isis, sister and bride of her brother Osiris, linked to Orion; when the cult of Osiris extended to include that of the cow-goddess Hathor, Sirius became the star of Isis-Hathor, depicted with cow horns.

The oldest symbol, however, is that of the dog. Sirius was in fact identified with the god Anubis with the head of a jackal, who - like the Greek Hermes - was the guide of the dead; inventor of the art of embalming bodies and lord of funerary rites. Later Egyptian traditions associated Sirius with the "days of the Dog", an identification from which the name of the star appears to have originated as the star of the Dog or Canicola.

The days of the Dog were originally referred to the 40-day period at the beginning of each Sotiac year, when summer was at the height of its heat. Classical authors often identified the power of Sirius with that of the Sun, and the star was sometimes represented with a crown of rays. The name Sirius comes from the Greek, meaning a star that sparkles or burns, and the star was believed to cause a deadly fever, exemplified by dog rabies.

The Greeks, while adopting the old myths about Sirius, brought the constellation into the riverbed of their mythology. The Greater Dog and the Lesser Dog were seen as belonging to the hunter Orion. We find the same image of a dog at the heels of a giant, pointing the Hare at Orion's feet ready to jump on it, in Mesopotamian stellar mythology.

Many authors, among them Ovid, (43 B.C. - 17 A.D.) identified in one or the other dog Maera, the faithful dog of Icarius (see Boote) , whose name, among other things, means "shining".

However, the modern hunting dog constellation seems to be better suited to this faithful and loving dog figure.

Another tradition identifies the Greater Dog with the terrible Cerberus, the three-headed dog that in Greek myth is placed to guard the entrance to Hades, the underworld of the underworld. As often happens in the myth, an apparently confused mixture of images leads to linear, logical associations: Cerberus, guardian of Hades, the kingdom of the dead, brings back to Anubis (Sirius), guide of the dead and in this capacity enabled to enter the "forgotten kingdom";

Maera's story also fits, having the dog led Erigone to the buried body of her father Icarius (again, guarding or serving the dead is a dominant theme). Again in reference to Greek mythology, the whole constellation owes its name to one of the hunter's dogs Orion or to Cephalus' dog, Lealpe, a very fast dog, who always managed to capture his prey. Brought to Thebes to hunt a fox that terrorized the countryside of the city, and that until then had proved elusive. The stubborn struggle between the two beasts went on for a long time, without success, until Zeus turned them into stones and led Lealpe into the sky among the stars.

However, the most 'accredited' mythological story has it that the Greater Dog is the oldest dog among those of the hunter Orion, and precisely for this reason it can be found near the great constellation of Orion.

Another interesting cultural crossroads: for the Chinese Sirius was T'ien-Iang, the heavenly jackal. The southern star of the constellation of the Major Dog represented the bow and arrow used to kill T'ien-Iang, after he had plundered the Chinese king's crops.

A small curiosity: a hypothesis long discussed is that the Dogon people of Mali, in West Africa, attributed to Sirius a companion called Po and defined "the heaviest star", and calculated its ritual periods on the basis of 50 years, i.e. according to the period of the elliptical orbit of the star. Only in 1862, however, has it been scientifically proven that Sirius is a double star, with the small companion Sirius B (magnitude 8.5) orbiting Sirius A in 50 years. It remains a mystery how the Dogons could have discovered this many centuries earlier.

The representations of the Big Dog

Since the constellation represents a Dog, the representations are hardly different: the first one is the usual one of Beyer in his Uranometria.

here instead we see the image (mirroring as always) of Hevelius

and finally the modern version given by Stellarium

A few big stars

Clicking on the diagram opposite we can open the image in which I have collected the biggest stars of the Big Dog: we immediately notice Menkelb Prior (ο1 CMa) great beauty of 248 times the Sun, then σ CMa with 192x, 145 Cma with 131x and δ CMa with 174x. I also added Sirius, which however is really small: just 1.9 times our Sun.
But... wait! There's another star we hardly noticed. Do you see it? It's right down there with all the others, red, with a weird initials and a crazy width...

This is really monstrous

Gentlemen, we are in the presence of the star that is currently the largest in the known universe: it is the red hypergiant variable VY CMa, more than 5000 al our distance and (according to the latest estimates) with a diameter of 2100 times that of the Sun. A Godzilla among the stars: remember the star P Cyg that we met while analyzing the constellation of the Swan? That was just 1900 times our Sun.
Here we are talking about a star that, placed at the center of the Solar System, arrives safely to the orbit of Saturn: since it is a variable star its diameter varies (always according to the latest estimates, which perhaps in some time will need to be updated) between shortly before and shortly after the orbit of the Lord of the Rings. I also added a small box in which I compared the stellona with the orbits of the two remaining planets and three other dwarf planets...

Really monstrous: just think that to see this star with the same apparent diameter subtended by the Sun (which is 1 UA away from us), we obviously have to go as far as 2100 UA, a huge distance, but still just three cents of $ $ light!
Some time ago I saw a movie on YouTube (I don't put any links, you can find as many as you want!) in which at the end it said something quite disturbing that sounded like this: assuming you could (could!) fly over the surface of VY CMa with an airliner travelling at 900 km/h, it would take X years (I can't remember what they said!) to complete the circumnavigation. So I did a bit of math, which I save for the faint of heart, since I've calculated curved integrals of three-dimensional matrices... But no! Simple multiplications and divisions lead to a shocking result: it would take more than 1100 years to circumnavigate the stellona with an airliner jet, obviously with air conditioning at maximum! Well, it shouldn't come as a surprise to you, if you think how big the star is, how many years it takes to reach Saturn, with a space probe, while we're talking about an airplane travelling around the circumference, whose length is notoriously 6.28 times the radius...

click to watch the VY CMa overflight movie in high resolution
If you remember, in the last episode I added some movies and also in this case I wanted to make a small addon of Celestia placing an improbable aircraft (called I don't know why Enterprise) very close to the boiling surface of VY CMa: then I started to increase the speed of time and at a certain point I started to see the (fictitious!) rotation of the star. Only at huge values of time acceleration, our plane started circumnavigating first and then splashing off behind the right edge and then disappearing into view: have you noticed how fast time goes by? The Enterprise then re-emerged from the other side and after a while I slowed down the race to get the spacecraft back more or less to its starting point: did you see that in what year we arrived? It was crazy!

After this simulation I now present you a beautiful shot by the tireless and excellent Hubble Space Telescope of this stellar monster, where we can see that the star is surrounded by a gaseous shell formed by material emitted by the star itself. It must be added that despite the disproportionate size of this star, its distance from us (about 5000 al) still prevents us from discerning a disk. By the way, to see it you need a small telescope, since its $magnitude$ is almost 8, absolutely invisible to the naked eye, but it is practically as bright as the planet Neptune: while this one is blue and has a fixed light, like all planets, the star VY CMa is, as said, red.

Nebulae and galaxies in the Greater Dog

Among the open clusters of this constellation, the best known since Messier's time is M41, here photographed in all its splendor by HST.

This other image instead witnesses a clash between galaxies, the NGC2207 and IC2163, but don't think it's bloody or involving huge amounts of stars: given the unlikely distances between one star and its nearest, there's a very low probability that stars from the first galaxy can actually collide with stars from the other. These two masterpieces of nature will interpenetrate each other, shuffling a little bit the cards on the table, in absurdly long times that not even with Celestia we would be able to speed up. Finally, in a few billion years (mark it on the $calendar$!) the two galaxies will abandon their cosmic embrace to continue their journey: but for now let's enjoy the show.

It's now the turn of another open cluster, NGC 2354, very similar to the M41 we saw before, but equally beautiful and colorful.

Let's finish this little gallery of images with another really spectacular picture of a nebula, the NGC 2359, very imaginatively called "Thor's helmet".

I don't know Thor, I don't know what he does and if his helmet can be compared to this other jewel in the universe: I just know that he is absolutely fantastic.

The names of the stars

Let us now see the names that over the centuries have been assigned to the brightest stars of the Big Dog.

  • SiriusCMa): we have already seen the meaning at the beginning
  • MirzamCMa): from Arabic, the announcer (of Sirius)
  • MuliphenCMa): from Arabic, but of obscure meaning
  • WezenCMa): from Arabic, the weight
  • AdharaCMa): from Arabic, the virgins
  • FurudCMa) : from Arabic, the lonely ones
  • AludraCMa) : from Arabic, the virgin
  • Menkelb Prior and Menkelb Posterior (ο1 and ο2 CMa): of unknown origin
  • Thanih al Adzari (σ2 CMa) : from Arabic, another virgin

When to observe the Big Dog

This constellation begins to rise towards South-East, at 9 p.m., our convenient time, at Christmas time, and can be followed evening after evening, always at that time, until the end of April. Its culmination in the South takes place at the end of February, but the constellation never exceeds $300°.

Audio Video The constellation Big Dog
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