Mana - free energy - is radiated from the stars according to the question.
Let us suppose that magic users may absorb mana in order to cast spells. However, there is a limit as to how much mana will impinge upon the magic user, and a limit as to how much mana the magic user can temporarily store in their body, dependent upon their skills. However, the spell may require more mana than the magic user can muster in the timeframe of casting the spell.
However, since mana comes from the stars, it arrives at a constant rate. In order to concentrate mana, it can be changed into another form, a solid or liquid form that is - or contains - potential mana.
This might be accomplished by a magic user, who casts a spell (that manipulates mana) but the spell does nothing but convert the bulk of the spell's mana into potential mana, by placing it into an object or substance.
This might also be accomplished by natural processes, and magic users might be able to find reservoirs of potential mana in the environment.
Potential mana could then be used by the spell caster to boost the amount of mana that they can muster for a spell, and because the mana is coming from the physical object or substance, need not be passed through the spell caster's body, thus increasing both the available mana and the rate at which mana can be delivered to the spell, possibly making the difference between its success and its failure.
As an object or substance containing potential mana is used, it may disintegrate, evaporate/sublime, or change in some obvious way.