Canis Major

The Great Dog

Canis Major, the Great Dog, follows Orion across the winter sky. It is a modest-sized constellation, but it contains the brightest star of the night: Sirius, often called the Dog Star.

The figure is easy to imagine: Sirius marks the dog's nose or collar, and a group of bright stars to the south-east forms its body, hind legs and tail. Unlike many constellations, Canis Major really does look like a standing dog on a dark night.

How to find it

Find Orion's Belt, the row of three stars in the middle of the Hunter. From the northern hemisphere, follow the Belt down and to the left about 20 degrees. The dazzling star you reach is Sirius, unmistakable in brightness.

From about 41°N, Sirius never climbs higher than about 32 degrees above the southern horizon, so it often flickers through thick air. South of Sirius, the triangle of Adhara, Wezen and Aludra marks the dog's hindquarters and tail. Sirius is also one corner of the Winter Triangle, with Betelgeuse in Orion and Procyon in Canis Minor.

The main stars and their names

Sirius (Alpha Canis Majoris) shines at magnitude −1.46 and lies about 8.6 light-years away. Its name comes from Greek for 'scorching' or 'glowing'. It is a white star about twice the Sun's mass and about 25 times as luminous.

In 1844 Friedrich Bessel noticed that Sirius wobbles slightly as it moves, and he concluded that an unseen companion was pulling on it. In 1862 the American telescope maker Alvan Graham Clark saw the companion while testing a new lens. Sirius B is a white dwarf, the hot leftover core of a star, with nearly the mass of the Sun packed into a ball about the size of Earth. It is about 10,000 times fainter than Sirius A, so it is a hard target even for good telescopes.

Adhara (Epsilon CMa), magnitude about 1.5 and about 430 light-years away, is a hot blue giant; its name means 'the virgins' in Arabic. Wezen (Delta CMa), magnitude about 1.8, is a yellow-white supergiant probably more than 1,500 light-years away. Mirzam (Beta CMa), magnitude about 2.0, rises shortly before Sirius, and its name is often interpreted as 'the announcer'.

Deep-sky objects for binoculars and small telescopes

M41 is an open cluster about 4 degrees south of Sirius, roughly the width of two fingers at arm's length. It shines at combined magnitude about 4.5 and lies about 2,300 light-years away. From a dark site it is visible to the naked eye as a faint patch; binoculars show dozens of stars, including an orange one near the center.

NGC 2362, around the star Tau Canis Majoris, is a young, compact cluster that looks like a bright star surrounded by fainter jewels in a small telescope. The constellation also holds VY Canis Majoris, a red hypergiant among the largest stars known; it is invisible to the naked eye, but a small telescope shows it as a faint reddish star.

Mythology and history

In Greek myth Canis Major is one of Orion's hunting dogs, chasing Lepus the hare at Orion's feet. Some writers identified it with Laelaps, a dog so fast that no prey could escape it. Zeus set it in the sky when it was locked in an endless chase with a fox that could never be caught.

In ancient Egypt Sirius was the goddess Sopdet. Its heliacal rising, its first appearance in the dawn sky after weeks hidden by the Sun's glare, came in midsummer around the start of the Nile flood and helped mark the new year. The Romans associated the hottest weeks of summer with the Dog Star rising with the Sun, which gave English the phrase 'dog days'. In Turkish the constellation is called Büyükköpek.

Practical observing tips

Canis Major is best on winter evenings, from January to March. Watch Sirius when it is low: the atmosphere bends its light into flashes of blue, red and green. This is twinkling, not anything happening to the star.

For M41, put Sirius at the top of your binocular view and look toward the bottom edge. From southern Europe or the southern US, look low in the south for the tail stars on a clear night. To try for Sirius B, you need a steady atmosphere, a telescope of about 8 inches (200 mm) aperture or more and high magnification; the two stars are relatively far apart in their 50-year orbit during the 2020s, which makes this a good decade to try.