There's a Starry Pooch in the Sky Named Canis Major

The constellation Canis Major with its companion Canis Major.
The constellations Canis Major and Canis Minor lend their bright stars to the Winter Hexagon, which is visible late in the year through the first part of April. Carolyn Collins Petersen

In ancient times, people saw all kinds of gods, goddesses, heroes, and fantastical animals in the patterns of stars in the night sky. They told legends about those figures, tales that not only taught the sky, but contained teachable moments for listeners. So it was with a little pattern of stars called "Canis Major." The name literally means "Greater Dog" in Latin, although the Romans weren't the first to see and name this constellation.

In the Fertile Crescent between the Tigris and Euphrates rivers in what is now Iran and Iraq, people saw the mighty hunter in the sky, with a small arrow aimed at his heart — that arrow was Canis Major.

The brightest star in our night sky, Sirius, was thought to be part of that arrow. Later on, the Greeks called this same pattern by the name Laelaps, who was a special dog who was said to be an incredibly swift runner. He was given as a gift by the god Zeus to his lover, Europa. Later on, this same dog became the faithful companion of Orion, one of his treasured hunting dogs.

Scoping out Canis Major

Today, we simply see a nice dog up there, and Sirius is the gem at his throat. Sirius is also called Alpha Canis Majoris, meaning it's the alpha star (the brightest) in the constellation. Although the ancients had no way of knowing this, Sirius is also one of the closest stars to us, at 8.3 light-years.

It's a double star, with a smaller, dimmer companion. Some claim to be able to see Sirius B (also known as "the Pup") with the naked eye, and it can definitely be seen through a telescope.

Canis Major is relatively easy to spot in the sky during the months that it's up. It trails south-eastward of Orion, the Hunter, frolicking at his feet.

It has several bright stars that delineate the legs, tail, and head of the dog. The constellation itself is set against the backdrop of the Milky Way, which looks like a band of light stretching across the sky.

Searching the Deeps of Canis Major

If you like to scan the sky using binoculars or a small telescope, check out the bright star Adhara, which is actually a double star. It's at the end of the dog's back legs. One of its stars is a bright blue-white color, and it has a dim companion. Also, check out the Milky Way itself. You'll notice many, many stars in the background.

Next, look around for some open star clusters, such as M41. It has about a hundred stars, including some red giants and some white dwarfs. Open clusters contain stars that were all born together and continue to travel through the galaxy as a cluster. In a few hundred thousand to a million years, they'll wander off on their own separate paths through the galaxy. M41's stars will probably stick together as a group for a few hundred million years before the cluster dissipates.

There is also at least one nebula in Canis Major, called "Thor's Helmet". It is what astronomers call an "emission nebula". Its gases are being heated by radiation from nearby hot stars, and that causes the gases to "emit" or glow.

Sirius Rising

Back in the days when people weren't so dependent on calendars and watches and smartphones and other gadgets to help us tell time or date, the sky was a handy calendrical stand-in. People noticed that certain sets of stars were high in the sky during each season. For ancient people who depended on farming or hunting to feed themselves, knowing when the season for planting or hunting was about to occur was important. In fact, it was literally a case of life and death. The ancient Egyptians always watched for the rising of Sirius just about the same time as the Sun, and that indicated the beginning of their year. It also coincided with the yearly flooding of the Nile. Sediments from the river would get spread out along the banks and fields near the river, and that made them fertile for planting.

Since it happened during the hottest time of summer, and Sirius was often called the "Dog Star", that's where the term "dog days of summer" originates.