Canis Major is simple to find once you know Orion. Representing Orion’s larger hunting dog, Canis Major follows its master across the sky, with the brilliant Sirius, the brightest star in the night sky, marking the dog’s chest. Sirius can be especially striking when low in the sky, where it can be seen to twinkle in flashes of red, blue and other colors in Earth’s atmosphere. For Northern Hemisphere observers, Canis Major is best seen during the winter months.
How to Find Canis Major in the Night Sky
Start with Orion’s Belt, the distinctive row of three stars at the center of Orion. Follow the line of the Belt downward and away from Orion and it leads to Sirius, which marks the chest of the Greater Dog.
Once you’ve found Sirius, look for the other bright stars surrounding it. Mirzam lies near the dog’s front, while Adhara, Wezen and Aludra trace its body, hindquarters and tail. Together with Sirius, these stars give Canis Major its recognizable outline.
Canis Major Is Orion’s Greater Dog
The name Canis Major is Latin for “Greater Dog,” and the constellation is traditionally pictured as one of Orion’s two hunting dogs. The other is Canis Minor, the Lesser Dog, whose brightest star is Procyon. Sirius in Canis Major and Procyon in Canis Minor represent the two bright “Dog Stars” following Orion across the sky. The scene also includes Lepus the Hare, which lies beneath Orion’s feet as though being pursued by the hunter and his dogs. One Greek tradition identifies the Greater Dog with Laelaps, the mythical hound said always to catch whatever it chased.
The Brightest Stars of Canis Major
Although Canis Major contains many stars, five bright stars form the main recognizable pattern of the Greater Dog: Sirius, Adhara, Wezen, Mirzam and Aludra.
The main stars of Canis Major, with Sirius marking the Greater Dog and Orion nearby. Illustration: AstronomyTrek.- Sirius (Alpha Canis Majoris) — At magnitude −1.46 and only 8.6 light-years away, Sirius marks the dog’s chest and is by far the closest of the constellation’s principal stars.
- Adhara (Epsilon Canis Majoris) — The constellation’s second-brightest star at magnitude 1.50. Although much farther away at about 430 light-years, it is intrinsically thousands of times more luminous than Sirius.
- Wezen (Delta Canis Majoris) — A yellow-white supergiant roughly 1,800 light-years away that marks the dog’s hindquarters.
- Mirzam (Beta Canis Majoris) — A blue-white star about 500 light-years away. Its traditional name means “Announcer” or “Herald,” because it rises shortly before Sirius.
- Aludra (Eta Canis Majoris) — A distant blue supergiant marking the dog’s tail and lying thousands of light-years from Earth.
Although these stars appear to form a single pattern in our sky, they are actually spread across an enormous depth of space. Sirius is practically on our doorstep at 8.6 light-years, while Aludra lies thousands of light-years away.
Sirius Is Actually Two Stars
Sirius appears to the naked eye as a single brilliant star, but it is actually a binary system consisting of Sirius A and Sirius B, which orbit each other roughly once every 50 years. Almost all the light we see from Sirius comes from Sirius A, while Sirius B is a white dwarf about 10,000 times fainter than its brilliant companion.
What makes Sirius B remarkable is how much matter is packed into such a small object: it has roughly the mass of the Sun squeezed into a body about the size of Earth, around 12,000 km (7,500 miles) across. Astronomers even knew it was there before anyone had seen it. In 1844, Friedrich Bessel inferred an unseen companion from a wobble in Sirius’s motion, before Sirius B was finally observed in 1862.
Sirius Hasn’t Always Been the Brightest Star
Sirius may dominate the night sky today, but another star in Canis Major once appeared considerably brighter. Around 4.4 million years ago, Adhara passed much closer to the Sun than it is today, bringing it within only a few dozen light-years of Earth. At its closest, Adhara would have shone at around magnitude −4, making it the brightest star in Earth’s night sky at the time. That’s about as bright as Venus typically appears today, giving some idea of how extraordinary Adhara would have looked as a star.
Sirius Can Lead You to a Star Cluster
The Milky Way runs through Canis Major, giving the constellation a wealth of deep-sky objects to explore, including numerous open star clusters. One of the easiest to find is Messier 41 (M41), a large open cluster lying about 4° south of Sirius. Under a sufficiently dark sky, M41 can be glimpsed with the naked eye as a faint, hazy patch, while binoculars reveal a scattered group of stars covering roughly the same area of sky as the full Moon.
The cluster lies around 2,300 light-years away and contains roughly 100 stars. This means that although M41 appears close to Sirius in our sky, it is actually more than 250 times farther away — another reminder of the enormous differences in distance hidden within the apparently flat pattern of a constellation.
A Massive Star Is Sculpting Thor’s Helmet
One of Canis Major’s most distinctive deep-sky objects is Thor’s Helmet (NGC 2359), a vast cloud of glowing gas around 15,000 light-years away. Stretching roughly 30 light-years across, its unusual arcs and bubble-like structure give the nebula the helmet-like appearance behind its nickname.
The shape is being created by a Wolf-Rayet star near the center of the nebula — a massive, highly evolved star shedding material through extremely powerful stellar winds. These winds plow into surrounding gas and sweep it into the expanding structures we see today.