Constellations take their names from ancient patterns of Greek and Romans, plus some new ones in the Southern part of the sky only charted by Westerners during the age of exploration, and named in the 16th and 17 centuries. Constellations officially are regions that cover the sky. International agreement in 1930 divided the entire sky up into 88 regions, each holding a classic star pattern that gives it its name. Stars and planets don't all appear the same brightness. Objects in the sky are assigned magnitudes that are an indication of brightness.
Magnitudes can be negative for very bright objects.
Naked eye astronomy.