From Galaxy Attack: Alien Shooter Wiki

Campaign Mode is single player. You start at Level 1, and see how far you can get. There are currently 182 Levels (additional levels get added periodically). Each Level has three modes: Medium, Hard and Crazy. You are awarded a Star the first time you complete any level at each mode (so three stars possible for each level). Stars are used to restrict progress in the game, such as playing PvP, upgrading ships or selecting higher levels.

Awards are given for completing each level. This is usually Gold, but can include Drone Tickets and Evolve Stones.