The Canis Major Constellation

The Canis Major Constellation.

Canis Major is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemy’s 48 constellations and is counted among the 88 modern constellations. Its name is Latin for “greater dog” in contrast to Canis Minor, the “lesser dog;” both figures are commonly represented as following the constellation of Orion, the hunter through the sky. Canis Major is bordered by Monoceros (which lies between it and Canis Minor) to the north, Puppis to the east and southeast, Columba to the southwest, and Lepus to the west. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a quadrilateral; in the equatorial coordinate system, the right ascension coordinates of these borders lie between 06h 12.5m and 07h 27.5m, while the declination coordinates are between −11.03° and −33.25°. Covering 380 square degrees or 0.921% of the sky, it ranks 43rd of the 88 currently-recognised constellations in size.

Canis Major is a prominent constellation because of its many bright stars. These include Sirius (Alpha Canis Majoris), the brightest star in the night sky, as well as three other stars above magnitude 2.0. At apparent magnitude −1.46, it is one of the closest stars to Earth at a distance of 8.6 light-years. Its name comes from the Greek word for “scorching” or “searing.” Sirius is also 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 two orbit each other every 50 years. The star marked the Great Dog’s mouth on Bayer’s star atlas. Flanking Sirius are Beta and Gamma Canis Majoris. Also called Mirzam or Murzim, Beta is a blue-white Beta Cephei variable star of magnitude 2.0 and about 500 light-years from Earth, and its traditional name means “the announce,” referring to its position as the “announcer” of Sirius, as it rises a few minutes before Sirius does. Gamma, also known as Muliphein, is a fainter star of magnitude 4.12, in reality, a bright blue-white giant of spectral type B8IIe located 441 light-years from earth. Epsilon, Omicron2, Delta, and Eta Canis Majoris were called Al Adzari “the virgins” in the medieval Arabic tradition. Marking the dog’s right thigh on Bayer’s atlas is Epsilon Canis Majoris, also known as Adhara. At magnitude 1.5, it is the second-brightest star in Canis Major and the 23rd-brightest star in the sky. It is a blue-white supergiant of spectral type B2Iab, around 404 light-years from Earth. It is a binary star; the secondary is of magnitude 7.4.

Canis Major is also home to many variable stars. EZ Canis Majoris is a Wolf–Rayet star of spectral type WN4 that varies between magnitudes 6.71 and 6.95 over a period of 3.766 days; the cause of its variability is unknown but thought to be related to its stellar wind and rotation. VY Canis Majoris is one of the largest stars known, a remote red supergiant located around 3800 light-years away from Earth. Estimates of its size, mass and luminosity have varied, with figures of 600 to 3000 times the radius, and 60,000 to 500,000 times the luminosity of the Sun. It was observed in 2011 using interferometry with the Very Large Telescope, yielding a radius of 1420 ± 120 solar radii, a surface temperature of around 3490 K (and hence spectral type M4Ia) and a luminosity 270,000 times that of the Sun. Its current mass has been revised at 9–25 solar masses, having shed material from an initial 15–35 solar masses.

Seven star systems have been found to have planets. One of them, Nu2 Canis Majoris is an ageing orange giant of spectral type K1III of apparent magnitude 3.91 located around 64 light-years distant. Around 1.5 times as massive and 11 times as luminous as the Sun, it is orbited over a period of 763 days by a planet 2.6 times as massive as Jupiter. HD 47536 is likewise an ageing orange giant found to have a planetary system—echoing the fate of the Solar System in a few billion years as the Sun ages and becomes a giant. Conversely, HD 45364 is a star 107 light-years distant that are a little smaller and cooler than the Sun, of spectral type G8V, which has two planets discovered in 2008. With orbital periods of 228 and 342 days.

The only Messier object is M41 (NGC 2287), an open cluster with a combined visual magnitude of 4.5, around 2300 light-years from Earth. Located 4 degrees south of Sirius, it contains contrasting blue, yellow and orange stars and covers an area the apparent size of the full moon, around 25 light-years in diameter. Its most luminous stars have already evolved into giants. The brightest is a 6.3-magnitude star of spectral type K3. NGC 2360, known as Caroline’s Cluster after its discoverer Caroline Herschel, is an open cluster located 3.5 degrees west of Muliphein and has a combined apparent magnitude of 7.2. Around 15 light-years in diameter, it is located 3700 light-years away from Earth and has been dated to around 2.2 billion years old. NGC 2362 is a small, compact open cluster, 5200 light-years from Earth. It contains about 60 stars, of which Tau Canis Majoris is the brightest member. Located around 3 degrees northeast of Wezen, it covers an area around 12 light-years in diameter. It is a very young open cluster as its member stars are only a few million years old. Lying 2 degrees southwest of NGC 2362 is NGC 2354 a fainter open cluster of magnitude 6.5, with around 15 member stars visible through a binocular. Located around 30′ northeast of NGC 2360, NGC 2359 (Thor’s Helmet or the Duck Nebula) is a relatively bright emission nebula, with an approximate magnitude of 10, which is 10,000 light-years from Earth. The nebula is shaped by HD 56925, an unstable Wolf-Rayet star embedded within it. Credit: Wikipedia.