Canis Minor - the Lesser Dog

Canis Minor - the Lesser Dog
Constellations Monoceros, Canis Minor and Canis Major, based on the star atlas of Johannes Hevelius []

Canis Minor is one of Orion's hunting dogs, trotting along behind its master and unperturbed by the unicorn Monoceros. It's a small constellation with little to offer other than one bright star. Yet it has quite a long history.

The constellations we know as Canis Major and Canis Minor have been recognized in some way for three thousand years. In Ptolemy's second-century Almagest, Canis Major was listed as Kyon and Canis Minor as Prokyon. Kyon was “the dog” and Prokyon, which rises earlier than Kyon, was “before the dog”.

Though Ptolemy listed Prokyon as a constellation, it wasn't much of a constellation, having only two stars. One of the stars, now called Procyon, is a very bright star and is ten times brighter than its companion Gomeisa [go-MAY-suh]. Oddly, three millennia ago in Mesopotamia, the two stars were known as ‘the Twins’, at one time representing twin deities.

In ancient Egypt, Canis Minor was associated with Anubis, the god represented as a jackal-headed man. He dealt with mummification and the afterlife. In mythology Anubis was also linked with Sirius, the sacred star of the goddess Isis who reared Anubis.

The Romans gave the constellations the Latin names still in use. Kyon became Canis Major (the greater dog) and Prokyon became Canis Minor (the lesser dog).

Medieval Arab astronomers followed the depiction of 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.

Although we now often think of Canis Major and Canis Minor as Orion's dogs, there are many other stories attached to them. My favorite is one in which Canis Major represented the dog Laelaps who always caught his prey. And Canis Minor was the Teumessian fox which couldn't be caught. When the hunt was on, the resulting paradox gave Zeus such a headache that he finally just turned them both into stone.

Canis Minor
The modern constellation is no longer just two stars, but only Procyon and Gomeisa are notable.

Procyon is a first magnitude star. However, its brightness is due to its being one of our nearest stellar neighbor 11.5 light years away. Interestingly, it's turned out to be a binary star. In the mid-nineteenth century, wobbles in Procyon's orbit showed there was a companion. Only about fifty years later was someone finally able to see the companion. We now know that it's a white dwarf, the remnant of sunlike star that used up its nuclear fuel and collapsed into a small superdense object.

Gomeisa, which appears to us so much fainter than Procyon, is actually twice as massive and much brighter. However it's also 14 times further away.

A planet, a neighbor, a cannonball
The orange giant star HD 66141 is over twenty times the size of the Sun and 170 times more luminous. This sounds impressive, but its interesting feature is that it's the only star in the constellation known to have a planet. The planet HD 66141b is several times more massive than Jupiter and orbits the star every 480 days.

At the opposite extreme to the giant stars are red dwarfs, which are around 10% of the Sun's size. One of them, Luyten's Star, is 12 light years away. It was named for the Dutch-American astronomer who first determined its proper motion, i.e., its motion through space relative to the Sun. Despite the star's closeness to us, red dwarfs are too faint to be seen without a telescope.

A quite different red dwarf (PSS 544-7) is about 1500 light years away. Observational measurements strongly suggest that it's headed at high speed away from the disk of the Galaxy. It may be a cannonball star, a star that's been ejected from its star cluster.

Deep sky objects
The Milky Way, full of stars, goes through Canis Minor. Yet 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”.

Winter Triangle
The star Procyon, along with its bright neighbors Sirius and Betelgeuse, forms the Winter Triangle.

You Should Also Read:
Do Red Dwarfs Live Forever
White Dwarfs
Winter Hexagon

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