Galaxian

Destroy waves of attacking Galaxians by moving left and right and firing. The
Galaxians will sit in formation above you, and you can pick them off with
your Cannon. Beware – You can only fire one shot at a time. Your next
bullet will not reappear until your last one has disappeared. Also, the
Galaxians become more and more aggressive as the waves proceed. They
will break formation and swoop down on you as they attack.