Well, a constellation is a region of the sky. And each constellation shares a border with its neighboring constellations.
For example: In the same way that the state of Texas is a region of the United States. Ursa Major is a region of the sky.
Also, each constellation has a specific pattern of bright stars in it that are responsible for their name; And because the ancients thought it looked like something; A part-fish,part-goat, scorpion, snake, bear....
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Hope this helps! :)