Canis Minor

FIELDS OF STUDY: Stellar Astronomy; Astronomy

ABSTRACT: Canis Minor is a small constellation made up primarily of two stars. In Western astronomy it is commonly interpreted as a small dog. The constellation’s most notable star is Procyon, one of the brightest stars in the night sky. Canis Minor is a member of the Orion constellation family, which includes some of the most famous classical constellations. Canis Minor was known to a number of ancient peoples and has been featured in several cultural myths.

The Small Dog Constellation

Canis Minor is a constellation, a segment of the night sky as designated by the International Astronomical Union (IAU), the main international organization of professional astronomers. The eighty-eight official constellations are based around asterisms, or imagined patterns of stars that are interpreted as images. Historians have found astronomical studies recognizing asterisms dating back at least as far as the ancient Babylonians, around 4000 BCE. Although virtually all cultures developed asterisms, it was the ancient Greeks who established many of the star patterns used by modern astronomers. The astronomer Ptolemy (ca. 100–ca. 170 CE) compiled forty-eight classical constellations in the second century, including Canis Minor.

Ptolemy studied astronomy, astrology, and mathematics. Some of his conclusions, such as his theory that Earth was the center of the solar system, were completely incorrect. However, his star charts were incredibly accurate for their time. His astronomical work The Almagest (ca. 150) contains twenty-one northern constellations, twelve zodiacal constellations, and fifteen southern constellations. He based them on historical records as well as his own observations. All of Ptolemy’s constellations are officially recognized by the IAU. Ptolemy gave Canis Minor its Latin name, meaning "lesser dog" or "small dog."

Constellations were commonly used for navigation by many cultures. This is because stars always appear to move in a consistent pattern. Once these patterns were mapped, people could use the stars to get a sense of direction, even at sea when land was not in sight. Bright, easily visible stars were most useful for navigation. Canis Minor’s main star, Procyon, forms the highly visible Winter Triangle asterism along with fellow bright stars Sirius and Betelgeuse.

In the ancient world, people associated constellations with myths, gods, and religious ideals. Some civilizations, such as the ancient Greeks, believed that the gods placed the stars in the sky as reminders of important people or stories. For this reason, the Greeks named the constellations after their mythical heroes and monsters.

Canis Minor in Myth and Legend

Canis Minor is an ancient constellation. A number of ancient cultures interpreted its main pair of visible stars in various different ways. Usually it is linked to the nearby Canis Major, or "greater dog." In fact, the ancient Greeks only recognized Canis Major, which was often considered to represent one of the dogs of the hunter Orion, another nearby constellation. The stars now known as Canis Minor were sometimes seen as a fox being chased by Canis Major. Alternatively, the star Procyon (Greek for "before the dog") was viewed separately, signaling the rise of Sirius, the Dog Star. By Roman times, Canis Minor was recognized as a second dog of Orion.

Canis Minor is the smallest member of the Orion family of constellations. This family includes Orion the hunter, which is known for containing the three bright stars that form the asterism Orion’s Belt. Orion also contains the red supergiant alpha Orionis, better known as Betelgeuse. Canis Major is another member of the Orion family, known for the extremely bright star alpha Canis Majoris, more commonly called Sirius. The other constellations in this family are Lepus, the hare, and Monoceros, the unicorn.

According to Greek and Roman mythology, Orion was a great hunter. He boasted that he could defeat any animal on Earth. After his death, the gods placed Orion and his faithful dogs in the heavens as stars. Various versions of the myth depict Orion’s dogs as chasing Lepus or attacking Taurus, the bull.

Another Greek legend associates Canis Minor with Maera, the dog of Icarius. The god Dionysus taught Icarius to make wine, and Icarius gave some of his wine to a group of local shepherds to taste. The shepherds had never tasted alcohol before; when they became drunk, they believed that Icarius had poisoned them, so they killed him. When Maera saw his dead owner, he ran to Icarius’s daughter, Erigone, and led her back to her father’s body. In response to Icarius’s death, Maera and Erigone took their own lives. So the tragedy would not be forgotten, the Greek gods placed Maera, Erigone, and Icarius in the sky as the constellations Canis Minor, Virgo, and Boötes.

The stars of Canis Minor were referenced in the myths of other cultures as well. Procyon and Sirius are separated by the visible path of the Milky Way galaxy. In one Arabian myth, the Milky Way represents a great river, and Procyon and Sirius represent two sisters who tried to cross it. Sirius, the older sister, was strong enough to cross the river, but Procyon, the younger sister, was unable to make it across. This is why Sirius is on one side of the Milky Way and Procyon is on the other. It is also why Procyon is sometimes referred to as the "weeping one." Early Chinese astronomers also interpreted Procyon as part of a river, or alternately as a gate or guard.

Notable Features of Canis Minor

Procyon, also known as Antecanis or alpha Canis Minoris, is the brighter of the two visible stars in Canis Minor. Though not as bright as Sirius, Procyon is one of the ten brightest stars in the sky. This is because, at about 11.4 light-years away, it is relatively close to Earth. Though it looks like a single star to the naked eye, Procyon is actually a binary system. Binary stars are two stars in close proximity that orbit a common center of mass. The larger star in the system is known as Procyon A, and the smaller star is Procyon B.

Procyon A is a main-sequence star, meaning it is in the same stage of its life cycle as Earth’s sun. It is 1.4 times the mass of Earth’s sun and more than seven times as bright. Procyon B is a white dwarf, the degenerate core of a collapsed star that is no longer producing heat and is slowly cooling over billions of years. Procyon B is only 60 percent of the mass of the sun. It was first observed in 1896, but its existence was speculated in 1844 due to the wobbling of Procyon A.

Canis Minor’s other main star is beta Canis Minoris, also known as Gomeisa. It is also a main-sequence star, classified as a gamma Cassiopeiae variable star. This means it has a fast rotation and its luminosity varies irregularly. This behavior is caused by the ejection of material from the star.

Though there are few other easily visible stars in Canis Minor, the section of the Milky Way it contains is of great interest to astronomers. Many nebulae and star clusters can be seen in this region of the sky using binoculars or telescopes. The constellation is also linked to the Canis Minorids meteor shower.

Locating Canis Minor

The Orion family of constellations is visible from most parts of the world. Canis Minor can be seen anywhere from the North Pole to 75 degrees south latitude. It is most readily visible during March. While special equipment is not necessary for viewing Procyon and Gomeisa, Procyon B cannot be seen without a telescope.

Canis Minor is found in the second quadrant of the northern celestial hemisphere. It is bordered by the constellations Monoceros, Cancer, Gemini, and Hydra. It can commonly be located by finding the easily recognizable Orion’s Belt; Canis Minor is on the same level as Orion’s shoulders.

PRINCIPAL TERMS

• constellation: a section of the night sky officially recognized by the International Astronomical Union and based on patterns of stars as seen from Earth.
• International Astronomical Union: the authoritative international organization of professional astronomers.

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