It is a constellation particularly difficult to see in the city sky, now more and more polluted by lights: Cancer does not present particularly bright stars (the brightest, β Cnc, Al Tarf, is of magnitude 3.5) and it can be found, however, very easily halfway between Castor and Pollux on one side and Regulus on the other. Already with a good pair of binoculars we could see, almost at the center of the constellation, the open cluster called Nativity, of which we will see a nice picture later on.
The name, the story, the myth of Cancer Constellation
The fixed stars of Cancer once marked the position of the Sun at the summer solstice, coinciding for Mesopotamians with the gateway for the descent of souls to the incarnation.
His Sumerian name was Al. Lul, which means "crab"...
In the Egyptian tradition, consistently, the group of stars of Cancer was the sunrise god Khepre, heavenly personification of the humble dung beetle, symbolizing fertility, life and rebirth.
Mythologically, Cancer represents the second effort of Hercules who, during the killing of the monster of the swamp of Lerna, the Hydra (another constellation, was a monster with several heads of which only the beheading of one of the many could finally determine the death) also killed a crab of considerable size that Hera, the queen of Olympus sent against the hero. The crab pinched Heracles' foot with force and, enraged, crushed it violently with its heel. Moved to compassion, Hera decided to reward the animal's effort by reserving a place in the sky.
Cancer is a zodiacal constellation, therefore it is crossed every year by the Sun in its apparent path among the constellations: our yellow dwarf crosses its boundaries on July 20th and August 10th, stopping therefore a little less than a month, since the constellation is quite extensive.
Its Latin name, Cancer, actually means Crab and this is what it should be called: it is however customary to call it vice versa with this nickname, which however evokes not very pleasant situations, but unfortunately very present today.
As for the other constellations that have in common the fact of being crossed by the ecliptic, a stamp has been dedicated to Cancer by S.Marino, for the series dedicated to the Zodiac Constellations. We know that this nice philatelic issue was created in 1970 and this explains why the value of the stamp is expressed in lire: in this case L. 4, which at the time had a certain value.
Already from this image, which shows the brightest star, you can see that a crab has little to do with it: not even H.A.Rey was able to join the dots to get something more recognizable.
It's a constellation not really flashy, but inside it presents a handful of stars quite close to us, always on a cosmic scale: two stars are below 20 al, another two straddling this value and another one at 40 al (and it's the only one represented and representable in the 3D diagram, since the first 4 are very weak).
The nearest star to us is the variable DX Cnc, also known as GJ 1111, a red dwarf of class M6 and 15a, placed at a distance of just 11.8 al: this fact implies that our Sun, seen from the proximity of the star, appears of 2.6, therefore potentially visible to the naked eye, if one day we could go in person with a (for now) science fiction spaceship. In the meantime we can once again take advantage of our inseparable virtual spaceship, the well known program Celestia, to see in this picture that the Sun appears in a zone of sky poor of bright stars, where the most known is Rigil Kentaurus A, no less than Alpha Centauri, but now of 2.6, just like our Sun.
the triple system GJ 3522 from 9 UA distance
The second star for distance is GJ 1116, at the century LP426-40 or better EI Cnc, an anonymous variable star of class M5 placed at 17 from the Sun, which in the photo of Celestia appears to be 3a, not far from the usual Alpha Centauri. The appearance of the Sun does not change much if we move on the other two stars at 20 al: the third in the ranking is another dwarf star of class M6, LHS2090 or LP368-128, located at 20.8 al, while the fourth is GJ3522, a triple star system whose center of gravity is placed at 22.1 al from the Sun. I don't propose you the photos that can be easily realized with Celestia because the star field is particularly insignificant, while on the side you can see the beautiful situation of the star system of GJ3522 from the distance of 9 UA.
Vice versa you can see the photo of the Sun seen from the fifth star in order of distance, ρ1 Cnc or 55 Cnc, a star of G8 class (next relative of our Sun) placed at 40 al from us and from which our star appears at the limit of visibility to the naked eye in a zone of sky where there are illustrious colleagues such as Sirius, Raccoon, Altair and Fomalhaut, all not particularly bright.
A few stars big enough
As we can see in the usual diagram of comparison of Cancer stars with other star monsters encountered previously, the Crab presents four M-class stars with a diameter greater than 70 times that of our Sun: they are 27 Cnc, 53 Cnc, μ1 Cnc and 21 Cnc, with diameters respectively equal to 117, 84, 81 and 76 times that of our yellow dwarf. I have added also two stars of the same spectral class of Aldebaran just because they are a little bigger than the most famous star of Taurus: in this way I can do justice to two stars, β Cnc (already mentioned several times before) and 60 Cnc little known only because they are less bright, because of their greater distance, respectively 290 and 582 al, compared to just 65 al of Aldebaran.
In order not to lose the habit, I went to my friends Twenty-seventhseventeen to see what their monster star looked like from the not so small distance of 10 UA (Saturn's distance from the Sun, just to be clear): in the picture we can see that a red giant with a radius equal to 117 times that of the Sun appears decidedly threatening. My friends are lucky enough to live in a rocky planet at 117 UA from their star and therefore they don't even notice that their star is so threatening: are you able to calculate how big it appears to their eyes (which for the record are four)?
Deep sky objects
Inside this constellation there are some very interesting objects: I chose three of them, very beautiful. The first one is the very famous open star cluster called Crib, contemplated in the Messier catalogue as M44, which for English-speakers is Beehive, the hive.
the open cluster M44, Nativity scene
The second is another open star cluster from the Messier catalogue, M67
the open cluster M67
Moving on to the galaxies and the photos of the unparalleled HST, we see first a fantastic spiral galaxy, visible almost in front of it, very rich in textures full of details, NGC 2775
the spiral galaxy NGC 2775
then we see a pair of spiral galaxies interacting with each other, NGC 2535 and NGC 2536.
the two galaxies NGC 2535 and 2536 that are interacting
Finally, here are two spiral galaxies, both barred, the first of which is NGC 2500
the galaxy NGC 2500
while the second is NGC 2608
the galaxy NGC 2608
Let's have a crab...
...and let's represent him. That's what Bayer did in Uranometry...
Hevelius in his work of sky mapping
and the programmers of Stellarium, in this more choreographic than scientific functionality
Now let's see what Silvia Pascucci tells us about the Crab and its vicissitudes.
The names of the stars
Here is the usual overview of names of Cancer stars, some of them found on the internet:
- Acubens (α Cnc): the claws
- At Tarf (β Cnc): the end
- Asellus Borealis (γ Cnc): the northern donkey
- Asellus Australis (δ Cnc): the southern donkey
- Alamaf (ε Cnc): the stable
- Tegmine (ζ1 Cnc): under cover
- Zubanah (ι1 Cnc): the claws
- Piautos (λ Cnc): the eye
- Nahn (ξ Cnc): the nose
Where and when to see Cancer
For the convenient time that I have always considered in this series of articles, 9 p.m., Cancer begins to rise in the Northeast in the first days of December, culminates in the South at about seventy degrees on the horizon in the first days of April (of the following year), while finally it is low on the northwest horizon in the first half of July, ready to set with the passage of time.