Canis Major

Lore & Background

In Western astronomy, ancient Mesopotamia saw Sirius as an arrow aiming towards Orion, while the southern stars of Canis Major and part of Puppis were viewed as a bow, named BAN in the Three Stars Each tablets dating to around 1100 BC. In the later compendium MUL.APIN, the arrow (Sirius) was linked with the warrior Ninurta, and the bow with Ishtar. The Ancient Greeks replaced the bow and arrow depiction with that of a dog. In Greek mythology, Canis Major represented the dog Laelaps, a gift from Zeus to Europa; or sometimes the hound of Procris; or the one given by Aurora to Cephalus. It was also considered to represent one of Orion's hunting dogs. The ancient Greeks refer only to one dog, but by Roman times, Canis Minor appears as Orion's second dog. The Roman myth refers to Canis Major as Custos Europae, the dog guarding Europa but failing to prevent her abduction by Jupiter. In medieval Arab astronomy, the constellation became al-Kalb al-Akbar, 'the Greater Dog'. Among the Merazig of Tunisia, shepherds note six constellations that mark the passage of the dry, hot season; one called Merzem includes the stars of Canis Major and Canis Minor.

In non-Western astronomy, the modern constellation of Canis Major is located in the Vermilion Bird in Chinese astronomy, where the stars were classified in several separate asterisms. Sirius was Tiānláng, the Celestial Wolf, denoting invasion and plunder. Both the Māori people and the people of the Tuamotus recognized the figure of Canis Major as a distinct entity. The Tharumba people of the Shoalhaven River saw three stars of Canis Major as Wunbula (Bat) and his two wives. To the Boorong people of Victoria, Sigma Canis Majoris was Unurgunite, and its flanking stars Delta and Epsilon were his two wives.

Canis Major is a constellation in the Southern Hemisphere's summer (or northern hemisphere's winter) sky, bordered by Monoceros to the north, Puppis to the east and southeast, Columba to the southwest, and Lepus to the west. Its official constellation boundaries, set by Belgian astronomer Eugène Delporte in 1930, are defined by a quadrilateral. Covering 380 square degrees, it ranks 43rd of the 88 currently-recognized constellations in size.

Reader's Guide

Canis Major is notable primarily for containing Sirius, the brightest star in the night sky, which is very bright because of its proximity to the Solar System and its intrinsic brightness. The constellation also includes several other bright stars: Epsilon Canis Majoris (Adhara) is the second-brightest and the brightest source of extreme ultraviolet radiation in the night sky; Delta (Wezen) at magnitude 1.8; Beta (Mirzam) at 2.0; Eta (Aludra) at 2.4; Omicron2 at 3.0; and Zeta (Furud) at 3.0. The red hypergiant VY CMa is one of the largest stars known, while the neutron star RX J0720.4-3125 has a radius of a mere 5 km. Two other stars are thought to have previously outshone all others in the night sky: Adhara shone at -3.99 around 4.7 million years ago, and Mirzam peaked at -3.65 around 4.42 million years ago. Another, NR Canis Majoris, will be brightest at magnitude -0.88 in about 2.87 million years' time. The constellation was included in Ptolemy's 48 constellations in the second century and is counted among the 88 modern constellations. Its three-letter abbreviation, adopted by the International Astronomical Union in 1922, is 'CMa'.

Did You Know?

• Sirius is a binary star; its companion Sirius B is a white dwarf with a magnitude of 8.4, 10,000 times fainter than Sirius A.
• The neutron star RX J0720.4-3125 in Canis Major has a radius of only 5 km.