Canis Major - The Great Dog

Canis Major is best seen from January through March. It lies southeast of Orion and represents one of Orion's hunting dogs. Canis Major is located just south of the celestial equator, and therefore is well seen by observers in southern latitudes.

History and Mythology

In ancient times, the star Sirius was used as a time marker for harvesting and celebration in many cultures. Its name, Sirius, comes from a Greek word "seirios", which means "scorcher". "Dog days", a common expression still used today, refers to the hot period during the summer months when Sirius, high in the daytime sky, is supposedly adding to the Sun's heat.

The Egyptians discovered that when Sirius rose just before dawn, the Nile would begin its yearly flood. This flooding of their fields represented the beginning of the new year. It was an event associated with the god Osiris' return from the dead, and it was central to the Egyptian calendar.

Canis Major has always been associated with Orion. Along with Canis Minor, it is one of the hunting dogs of Orion. Sirius is known as the Dog star.

In Indian mythology the stars of Canis Major formed a deer hunter, and the stars we call Orion were his prey. The Indians named the brightest star Tishtrya, controller of the rain and all waters on Earth.

The Polynesians thought another star near Taurus was the brightest star in the sky, and not Sirius. Sirius was angry and convinced the god Tane to hurl the star Aldebaran at this bright star, shattering it into a group of faint stars, which we now call the Pleiades.

Notable Stars

Sirius is the brightest star in the sky. Only the Sun, Moon, Venus, Jupiter and Mars are brighter. It is a white star of spectral type A, only 8.7 light-years from Earth. Sirius is twice the diameter of our Sun, and over 23 times as luminous.

Sirius has a famous companion called Sirius B. It is a white dwarf star, as massive as the Sun, but only the size of the Earth. This companion orbits Sirius every 50 years. Sirius B is very difficult to see because of its proximity to the brilliant neighbor. Under favorable conditions, it can be seen in an 8 inch telescope.

Ancient records often describe Sirius is being red in color; while today it is clearly white. There is no likely astrophysical explanation for such a major change in either star over a relatively short period of a few thousand years, so this mystery remains unresolved.

The 2nd-magnitude star Mirzam is located west of Sirius. When Mirzam appears above the horizon, Sirius is not far behind. Three stars form the dog's hindquarters: the supergiant stars Aludra and Wezen, and the blue-white giant Adhara. Adhara is the second brightest star in this constellation.

Clusters, Nebulae, and Galaxies

The splendid open cluster M 41 lies only four degrees south of Sirius. It contains some 80 stars of 7th magnitude and fainter. Visible in binoculars, it is an impressive object in an 8 inch telescope.

NGC 2362 is a fainter open cluster containing several dozen stars close to the 4th magnitude star Tau Canis Majoris. Tau CMa itself is an interesting triple star system.

IC 2177, also known as the Seagull Nebula, is a broad region of nebulosity located on the border between Monoceros and Canis Major. It spans a 120 x 40 arcminute area, about 7.5 degrees northeast of Sirius. This picturesque nebula is thought to be a distance of 3800 light-years from Earth.

For larger telescopes, NGC 2359 is a bright emission and/or reflection nebula in Canis Major, nicknamed the Duck Nebula or (more popularly) Thor's Helmet. Thor's Helmet is an interstellar bubble, blown as a fast wind from its bright central star sweeps through a surrounding molecular cloud, similar in nature to the Bubble Nebula (NGC 7635) in Cassiopeia. NGC 2359 is approximately 15,000 light-years away and 30 light years in size.