Canis Major

FIELDS OF STUDY: Stellar Astronomy; Astronomy; Cosmology

ABSTRACT: Canis Major is a constellation that is visible from both the Northern and the Southern Hemispheres. In Western astronomy, it and nearby constellation Canis Minor are commonly interpreted as dogs following Orion, the hunter constellation. Canis Major has been studied by civilizations since ancient times. It is notable for containing Sirius, the brightest star in the night sky.

The Great Dog Constellation

Canis Major is a constellation, a section of the celestial sphere officially designated by the International Astronomical Union (IAU). Constellations are based on imagined patterns of stars called asterisms, which represent certain figures or shapes. Humans have noticed asterisms since at least 4000 BCE, when the earliest constellations were recorded in Mesopotamia. The stars of Canis Major have been part of the constellations and myths of civilizations around the world. Because Canis Major contains the brightest star in the night sky, it was observed by the ancient Egyptians, Chinese, Cherokees, Sumerians, and Greeks, among others.

The eighty-eight modern constellations recognized by the IAU are mostly derived from those of the ancient Greeks. Forty-eight of the constellations were collected by the astronomer Ptolemy (ca. 100–ca. 170 CE), who based them on historical sources and his own observations. While some of Ptolemy’s work would later be proved incorrect, such as his assertion that Earth was the center of the solar system, his star charts were incredibly accurate for their time. His work The Almagest (ca. 150 CE) contains twenty-one northern constellations, twelve zodiacal constellations, and fifteen southern constellations. All the constellations found in The Almagest are recognized by the IAU. Because of Ptolemy and The Almagest, many people still use the Greek names for constellations.

Canis Major is one of Ptolemy’s original constellations. In the ancient world, many stars and asterisms were given supernatural or religious meaning. Some people believed that the constellations were the gods’ way of telling stories. Canis Major means "greater dog" in Latin. Though there are many different myths regarding the constellation, it is often seen as a hunting dog, along with its smaller neighbor, Canis Minor ("lesser dog"). Some Greeks associated the constellation with the mythical hunting dog Laelaps. Stories often claimed the dog was chasing Lepus, the nearby hare constellation, or battling Taurus, the bull constellation. The Romans associated Canis Major with Custos Europae, Europa’s guard dog, or Janitor Lethaeus, the dog that guarded the entrance to the underworld. Many myths associated the dogs with the hunter Orion, another neighboring constellation. Because of this, Canis Major is included in the Orion family of constellations.

Constellations were commonly used for navigation. Stars appear to move in consistent patterns, and star charts showed where each constellation could be found at any time on any given day. This allowed people to judge their position based on the stars. Canis Major was particularly important for navigators because it contains several very bright and easily identifiable stars.

The Dog Star

The most noticeable star in the Canis Major constellation is Sirius, the brightest star in the night sky. Sirius is sometimes called the Dog Star or the Star in the Jaws of the Dog because of its position in Canis Major. It is more than twice as large as Earth’s sun, twenty times brighter, and less than nine light-years away from Earth. Sirius is one of the three stars that form the Winter Triangle asterism, the other two being Procyon and Betelgeuse. Procyon is the largest star in Canis Minor, while Betelgeuse is a red giant found in Orion.

Civilizations have watched the Dog Star since ancient times. The Greeks and Romans believed that it was powerful enough to heat Earth, just like the sun. Because Sirius is sometimes visible during the day in the summer, they thought that the combined heat from Sirius and the sun was what made summer hotter than the other seasons. This is the source of the expression "the dog days of summer." Sirius was worshipped by or incorporated into the myths of many other cultures as well. Its brightness made it especially useful for navigation.

When most people look at Sirius in the night sky, they see a single star. In reality, however, Sirius is a binary star, or a system of two stars that orbit a common center of mass. The larger star in the binary system, called Sirius A, is the star most people see when they look at Canis Major. It is much larger and brighter than its companion star, Sirius B, which is a white dwarf star roughly the size of Earth’s sun. While Sirius A is still in a stable period of its life cycle, Sirius B is slowly dying. The small star was once much larger than Sirius A. Over billions of years, however, Sirius B used all its internal fuel, shed its outer layers, and shrank to a fraction of its former size. It is no longer creating heat through chemical reactions and is slowly cooling down. Eventually, Sirius B will stop glowing entirely.

Other Features

The red hypergiant star VY Canis Majoris is another important star in the Canis Major constellation. VY Canis Majoris is one of the largest known stars in the Milky Way galaxy. It is located more about 4,900 light-years from Earth, and its diameter is nearly two thousand times that of Earth’s sun. For comparison, if the sun were replaced with VY Canis Majoris, then Mercury, Venus, Earth, Mars, and Jupiter would all be swallowed by the hypergiant. Hypergiants burn through their internal fuel extremely quickly. For this reason, they tend to have much shorter life cycles than other types of stars. They live for only a few million years before undergoing a massive explosion called a supernova. Living hypergiants are extremely rare. After going supernova, some hypergiants become black holes. Despite its size, VY Canis Majoris is not as bright as Sirius in the night sky because it is far away from Earth. In fact, it is one of the dimmest stars in Canis Major.

The constellation’s second-brightest star is epsilon Canis Majoris, also known as Adhara. It is the brightest known source of ultraviolet radiation in the night sky.

Another feature of Canis Major is the Messier 41 (M41) star cluster. This group of stars is around 2,300 light-years from Earth. It appears as a faint blotch of light to the naked eye, but binoculars reveal the individual stars.

Viewing Canis Major

Canis Major can be seen from any location between sixty degrees north latitude and the South Pole. It is most visible during February. No special equipment is necessary to view it. Canis Major can be found by first locating Orion’s Belt, three bright stars in the Orion constellation that form a straight line, and then following that line southeast. It will intersect with Sirius, which is much brighter than any other nearby star.

PRINCIPAL TERMS

• constellation: a section of the night sky officially designated by the International Astronomical Union and recognized by patterns of stars.
• International Astronomical Union: the authoritative international organization of professional astronomers.

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