To destroy some enemies, the player shoots them with bullets. Bullets are another kind of sprite, so you can make a class, Bullet, to describe them. When the player presses the spacebar, a bullet will be created. It travels in the direction the ship was pointing when it was fired until it hits an enemy or flies out of the window. When a bullet hits an enemy ship, both are destroyed in a ball of flame.
You’ve already made two sprite classes, so you’re probably getting the hang of it. When you make a bullet, you tell it where to start and what direction to go. Once you’ve made a bullet, it should keep moving in that direction until it hits an enemy or flies out the window. Here is a diagram for the Bullet class: