Of course, as I've written elsewhere, the system hasn't been maintained very well. Thanks to precession of the Earth's axis, those star signs are so badly out of alignment that a thirteenth constellation (Ophiuchus) has moved in, and all the rest are a month or two out of place!
Astrology aside, a great many constellations were identified and named over the millenia. When the first European explorers traveled south of the equator, they discovered thousands of extra stars which had never been seen by European eyes. These men, explorers that they were, immediately attempted to plant a flag in the sky by coining and naming new constellations. It says a lot about the time of these men that many of the new constellations were named for the wonders of technology which had made their expeditions possible (The sextant, the octant, the chronograph, the compass), or the various animals they had encountered (Both amazing and mundane - the Toucan and the Fly are good examples).
Unfortunately the system was never very well organized. Different countries had their own traditions and variations (Even today, Ursa Major is known variously as the Great Bear, the Plough and the Big Dipper), and there was a lot of argument between various explorers as to which new southern constellations were which. This was finally resolved when the International Astronomical Union had a conference, and then a vote (similar to the process which demoted Pluto), to determine 88 standard constellations, mapped out so as to cover every square degree of the sky.
As a bonus of the new charts, we got a new way of naming stars: The Bayer System. Since all stars were now assigned to a constellation (Previously only the bright stars making the pattern were counter. Anything else was merely a star), all stars could be labelled easily without needing their own unique name. The brightest star in each constellation was named "Alpha", the next brightest "Beta" and so on. Thus, the brightest star in Scorpius is called "Alpha Scorpii". Simple, and easy to understand!
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