A constellation is a group of stars that make an imaginary shape in the night sky.
They are usually named after mythological characters, people, animals and objects. In different parts of the world, people have made up different shapes out of the same groups of bright stars.
It is like a game of connecting the dots. In the past creating imaginary images out of stars became useful for navigating at night and for keeping track of the seasons.
Because all the stars are at different distances, the constellations would look totally different to inhabitants of another planet orbiting another star.
Where did the constellations come from?
OK, so we know the constellations are helpful for remembering the stars, but why would people want to do that (besides astronomers, that is)? After all, I said at the beginning that farmers invented the constellations. Why did they do that? Was it for some religious purpose?
Yes and no. Around the world, farmers know that for most crops, you plant in the spring and harvest in the fall. But in some regions, there is not much differentiation 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 of the myths associated with the constellations were invented to help the farmers remember them. When they saw certain constellations, they would know it was time to begin the planting or the reaping.
This dependence on the sky became a strong part of many cultures. Perhaps there is something about the mystery of the night sky that makes people want to tell stories about the constellations.
The constellations have changed over time. In our modern world, many of the constellations have been redefined so now every star in the sky is in exactly one constellation. In 1929, the International Astronomical Union (IAU) adopted official constellation boundaries that defined the 88 official constellations that exist today.
http://www.astro.wisc.edu/