Buy Star Map -

(Definite Article): Use "the" if you are referring to a specific map you’ve already discussed, such as one customized for a certain date or a particular brand. Quick Reference

The proper article depends on whether you are referring to a generic item or a specific, already-identified map: buy star map

(Indefinite Article): Use "a" if you mean any star map in general or if you haven't picked a specific one yet. (Definite Article): Use "the" if you are referring

"I want to buy a star map for my anniversary". buy star map

"I decided to buy the star map we saw online".