The Creation of Mortals and Magic

The stars of Nimbaar are capable of incredible things, far superior to what mortals' magic has achieved.
The gods created every living thing by shaping the same material that forms them.
The stellar matter inside of most creatures keeps them alive and nothing more; it is "asleep", as the people of Nimbaar would say.
Those capable of magic have somehow "awakened" part of the power of the stars from which they were made.

Historical Basis

Magic is the most evident manifestation of the presence of portions of stars inside living beings.
Bards are naturally sensitive to the beauty of creation, they need just a bit of study to be able to reach for their inner power.
Clerics are chosen by at least one of the members of the Court of Stars, who awakens the stellar matter inside them.
Monks call the power derived from the stars "ki", and harness it to overcome their physical limits.
Druids and Rangers feel a strong link to nature, and are able to tap into the divine essence of the world around them.
Paladins are capable of awakening their magic by the mere strenght of their convinctions.
Sorcerer are people whose inner star matter is awakened right from their birth or after some particular event.
Warlocks, through their pacts, channel the magic of their patrons.
Wizards and Artificiers study how to awaken the stellar matter inside them and manifest it in the world.
Many other creatures are naturally capable of using magic, beacause it is simply part of their essence.

Variations & Mutation

Some wizards tried to study how much star matter resides inside the bodies of mortals and if there are differences between spellcasters and those incapable of magic.
Apparently, the races naturally gifted with magic have a bit more stellar essence than the other humanoids, but otherwise there is no significant difference.
According to these wizards, the determining factors are overall individual power, force of will, and application.
They also distinguished this star matter into ten different tiers of purity. The lowest tier is the closest to the mundane stuff that makes up the material plane, and it is an almost infinite source of weak magical power. The higher tiers are closer to an individual's soul and are more and more similar to actual stars, hence capable of greater feats of magic, but harder to reach and to use repeatedly. Some people believe that an eleventh tier might exist, the fundamental component of one's spirit, which, if controlled, may imitate the power of gods.
There are many who disagree with this kind of classification, but up until now noone came up with a better one.

Cultural Reception

Beside magic, many people see a reflection of some aspects of the gods in their personality and behaviour.
For example if you'd encounter someone brave and selfless, you might say that "they're made by the stars of The Hero", someone artistic and jovial may be "created from the stars of The Jester", while someone gifted for manual labours might be "forged with the stars of The Maker".