Something that's minor is considered of low importance — a minor injury is not very serious, and a college student's minor subject is a secondary field of study. Likewise, the star constellation Ursa Major is a larger grouping than Ursa Minor.
The word minor has retained its spelling from the Latin. There, minor means something "lesser," and is related to the Latin word minuere, which means "to diminish." It is usually used as an adjective, but as a noun it gained the meaning "underage" in the 16th Century, and is now used to refer to children and teens under the age of legal responsibility. It is also used in musical terms to refer to a key that has a flatted third in its scale.