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Canis Minor – the Lesser Dog
Canis Minor is one of Orion's hunting dogs. It trots along behind its master unperturbed by the unicorn (Monoceros), and leaving the hare (Lepus) to the greater dog (Canis Major) to chase. It's a small constellation with not much more to offer than one bright star, but it has a long history.
Ptolemy's second-century Almagest shows both Canis Major, which he called Kyon, and Canis Minor, which he listed as Prokyon, the same as its brighter star. (It's now called Procyon [PRO.see.on].) Kyon was ‘the dog’ and Prokyon, which rises earlier than the much brighter Sirius, meant ‘before the dog’.
Although Ptolemy listed Prokyon as a constellation, with just two stars it wasn't much of a constellation. The second star was Gomeisa [go-MAY-suh] which is near to Procyon, but about ten times fainter. Nonetheless three millennia ago in Mesopotamia, the two stars were known as ‘the Twins’, at one stage representing twin deities.
In ancient Egypt, Canis Minor was associated with Anubis. He was the god represented as a jackal-headed man, who dealt with mummification and the after-life. In mythology Anubis was also linked with Sirius, the sacred star of the goddess Isis who reared Anubis.
For the Romans, Kyon was the Latin Canis. Procyon was still noted as rising before the dog in the name Antecanis. But when Procyon was also identified as as dog, Canis became Canis Major, and Procyon was Canis Minor, the lesser dog.
The medieval Arabic astronomers followed the Greeks in depicting Canis Minor as a dog. In the Book of the Fixed Stars al-Sufi showed Procyon and Gomeisa superimposed on a drawing of a dog.
In general, we identify Canis Major and Minor as Orion's dogs, but there are other stories attached to them. In one myth Canis Major was the dog Laelaps who always caught his prey, chasing Canis Minor, the uncatchable Teumessian fox. The resulting paradox of this race gave Zeus such a headache he turned them both into stone.
However Canis Major was definitely a dog in the story told by Hyginus (c. 64 BCE-17 CE). Icarius was the first mortal whom the god Dionysus taught to make wine. Sadly, when he shared it out for tasting, the recipients, unaccustomed to such a heady beverage, thought he'd poisoned them, and they killed him. His faithful dog Maera rushed to Erigone, Icarius's daughter, and brought her back to where her father's lifeless body lay. Both the girl and the dog took their own lives, and Zeus put the trio into the sky with Maera as Canis Minor.
Stars and planets
The modern constellation is no longer just two stars, but only Procyon and Gomeisa are notable.
Procyon and Gomeisa
We see Procyon as a first magnitude star, but that's only because at 11.5 light years away, it's one of our near stellar neighbors. Interestingly, it's a binary system, though no one realized this until the mid-eighteenth century. Even then no one could see the companion star, but deduced its presence from small wobbles in Procyon's orbit. The companion, finally spotted decades later, is 15,000 times fainter than the primary star. It's a white dwarf, the remnant of sunlike star that used up its nuclear fuel and collapsed into a small superdense object. By a strange coincidence, Sirius also has a white dwarf companion.
Gomeisa, which appears fainter than Procyon, is actually twice as massive and much brighter. However it's 14 times farther away than Procyon. ‘Gomeisa’ is derived from the Arabic meaning “the bleary-eyed one”.
A planet, a neighbor, a cannonball
HD 66141 is an orange giant, over twenty times the size of the Sun and 170 times more luminous. Its interesting feature is its one known planet. HD 66141b is several times more massive than Jupiter and orbits the star every 480 days.
At the opposite extreme in size to the giants are red dwarfs, which are around 10% of the Sun's size. One of them, Luyten's Star (GJ 273), was named after Willem Jacob Luyten, the Dutch-American astronomer who first determined the star’s proper motion, i.e., its motion through space relative to the Sun. It's one of the nearest stars to us, but still too faint to see without a telescope.
An even fainter red dwarf is PSS 544-7, 685 light years away. Measurements strongly suggest that it's headed at high speed away from the disk of the Galaxy. It may be a cannonball star, one that's been ejected from its star cluster.
Deep sky objects
The Milky Way, full of stars, goes through Canis Minor. Nonetheless the constellation offers little in the way of deep-sky objects. There are no Messier objects, and the brightest deep-sky object is spiral galaxy NGC 2485 which needs a reasonably large telescope to see. Even the great 18th-century deep-sky observer William Herschel could only manage to find “two faint galaxies and two nondescript asterisms”.
Procyon, along with its bright neighbors Sirius and Betelgeuse, forms the Winter Triangle.
(1) A cannonball star candidate in Canis Minor, https://doi.org/10.1016/j.newast.2005.04.001
(2) Bratton, M., The Complete Guide to the Herschel Objects
Sir William Herschel's Star Clusters, Nebulae and Galaxies
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