The first thing you need to know is that constellations are not real. Constellations are imaginary things that poets, farmers and astronomers have made up over the past 6,000 years or so. The real purpose for the constellations is to help remind us which stars are which in the sky.
On a really dark night away from city lights, you can see about a thousand to fifteen hundred stars. Trying to identify them is hard. The 88 official constellations help by breaking up the sky into more manageable sections. The constellations are mnemonics, or memory aids.
For example, if you spot three bright stars in a row on a winter evening, you might realize that you're looking at Orion. Then, the rest of the constellation falls into place: Betelgeuse is in Orion's left shoulder and Rigel is in his foot.
Once you recognize Orion, you might remember that his Hunting Dogs are nearby. You could recognize the two bright stars in the upper and lower left of the photograph as Procyon in Canis Minor and Sirius in Canis Major, respectively.
A sky atlas would show you a diagram like the black and white graphic on the left. Obviously, this is very different from the photo above. This type of schematic denotes different star brightnesses with different sized stars. Also, there is a standard way of connecting the stars that allows astronomers and others to quickly tell what they are looking at. In almost every star atlas, you will see Orion drawn with these same lines.
Further, every star on the chart is labeled. This chart is useful because it accurately shows the relative positions of the stars in this small region of the sky.
Objects other than stars are labeled on the chart. For example, Barnard's Loop on the left and M42 in the bottom middle are pointed out. Barnard's Loop is a cloud of faintly glowing gas, which we can't see without a telescope. M42 is the Great Orion Nebula - the red splotch in Orion's Sword in the photo above.
Still, where did the constellations come from? We know the constellations are helpful for remembering the stars, but why would people want to do that? Why would farmers care about that?
Around the world, farmers know that for most crops you plant in the spring and harvest in the fall. Yet in some regions, there is not much difference between the seasons. Since different constellations are visible at different times of the year, you can use them to tell what month it is. For example, Scorpius is only visible in the northern hemisphere's evening sky in the summer. Some historians suspect that many myths associated with the constellations were devised to help the farmers remember them. From the constellations, they would know when to begin planting and when to harvest.
This dependence on the sky became a major influence in many cultures. Perhaps it is the mystery of the night sky that makes people want to tell stories about it. The picture at the left is from an ornate star chart printed in 1835. Like the others, it shows the great hunter Orion. In this one, he is holding a lion's head instead of his traditional bow or shield. He is approaching Taurus, the Bull. Behind him, his dog Canis Major is chasing Lepus, the Hare. You can compare this picture to the photo near the top of the page. They are about the same scale and they show the same stars.
The constellations have changed over time. Many constellations have been redefined so that every star in the sky is in only one constellation. In 1929, the International Astronomical Union (IAU) adopted official constellation boundaries for the 88 constellations that exist today.