Constellations Monoceros, Canis Minor and Canis Major, based on the star atlas of Johannes Hevelius [WallHap.com]

There are several canines in the night sky, including a wolf and a fox. But the best known canine constellations are Canis Major (the Greater Dog) and Canis Minor (the Lesser Dog).

1. Kyon and Prokyon
There's been a dog constellation since the earliest days of Greek astronomy. It was called Kyon (the Dog). Then Kyon was joined by Prokyon (before the Dog), a constellation with only two stars. The star Prokyon — now spelled Procyon — got its name because it rises before Sirius The Dog Star.

2. Canis Major and Canis Minor are Orion's hunting dogs.
Around 150 AD Claudius Ptolemy listed the two dog constellations in his star catalog. Sometime later, Kyon became Canis Major and Prokyon became Canis Minor. There are some myths and stories about the two constellations, but for many centuries we've known them as Orion's hunting dogs.

3. Both constellations have one very bright star.
Sirius is both the brightest star in Canis Major and the brightest one in our night sky. Canis Minor's Procyon is the 8th brightest star we can see.

4. The Winter Triangle
The dogs are easy to spot if you first find Orion. Just follow the three stars of Orion's belt to the southeast. They point towards Sirius. Then Sirius and Procyon, along with Orion's red giant Betelgeuse, make the Winter Triangle.

5. Brightness matters, but distance matters more.
Procyon is eight times brighter than the Sun, and Sirius is over twenty times brighter. However, almost all the other stars of Canis Major and Canis Minor are actually much brighter than Sirius and Procyon. For example, Wezen doesn't seem as bright at Procyon, even though it's a supergiant 50,000 times brighter than the Sun. That's because it's 1600 light years from us. At 8.6 light years away, Sirius is a near neighbor, and so is Procyon at 11 light years. If Wezen were as close as Sirius, it would look as bright as the half Moon.

6. Sirius and the “dog days” of summer.
For the ancient Greeks, the dog days were the hottest part of the summer. They began around the time that Sirius first rose in the morning sky after being behind the Sun for a few months. The star is so bright that they thought it added to the heat of mid-summer.

7. The Twins — gods of Mesopotamia
Although Ptolemy listed Prokyon as a constellation, with just two stars it wasn't much of a constellation. The second star Gomeisa [go-MAY-suh] is near to Procyon, but not as bright. Yet as a pair, they were known as "The Twins" and represented gods three thousand years ago in Mesopotamia (modern day Iraq).

8. Anubis and Isis – gods of Egypt
In ancient Egypt, Canis Minor was associated with the god Anubis. He was represented as a man with the head of a jackal. Anubis was the guardian of the underworld. He was also linked with Sirius, the sacred star of the goddess Isis who had raised him.

9. Both Sirius and Procyon are binary stars.
A binary star is a pair of stars orbiting each other. No one knew that either Sirius or Procyon had a companion until the 19th century. In each case, the first sign was a little wobble in the star's position caused by the pull of an unseen star. Then, in both cases, several decades later someone finally saw the companion.

10. How to name a binary star
The brighter star of the pair is “A” and the other one is “B”. For Sirius, the star everybody had been seeing for thousands of years is Sirius A. The tiny companion is Sirius B. Sirius A is also often called by its old name, The Dog Star, and Sirius B's nickname is The Pup.

11. The companions are both white dwarfs
No one understood what the little companions were at the time. These stars are about the size of the Earth with the mass of the Sun. They're called white dwarfs. A white dwarf is the remains of a sunlike star that has run out of fuel and collapsed, so that even its atoms are squashed.

12. Thor's Helmet in Canis Major
There's a nebula nicknamed Thor's Helmet. It looks a bit like a cartoon Viking helmet. (Vikings didn't really have horns on their helmets.) The nebula has formed around a large unstable star that's 280,000 times brighter than the Sun, and likely to explode as a supernova one day. [Image: Adam Block/Mount Lemmon SkyCenter/University of Arizona]

13. Another interesting object in Canis Major was discovered in 1835 by English astronomer John Herschel (1792-1871).
It was just a fuzzy object in the 19th century. We now know that NGC 2207 and IC 2163, are a pair of beautiful spiral galaxies face-on to us. Gravity is pulling them together, and one day they'll unite into one. Already their gravity is disturbing the gas and dust, and that has started new star formation. [Image: NASA/ESA Hubble Space Telescope]

14. A cannonball star?
PSS 544-7 is a strange red dwarf star in Canis Major, 1500 light years away. It seems to be racing away from the center of the Galaxy at high speed. Astronomers think it was born in one of the Milky Way's star clusters, but has been kicked out. A star like that is called a cannonball star.