This 1970 cover by Dean Ellis (used on the 3rd through 5th Ace printings) - is my favorite of any edition of this book.

Star Born - The sequel to The Stars Are Ours!. Far from the Terran colony's Homeport on the planet Astra, Dalgard Nordis and his merman companion Sssuri are suddenly confronted by their old enemies, the alien Astrans. Within the Astran's former citadel the two discover that remnants of this nonhuman race (which had once ruled the entire planet) are struggling to recover their lost knowledge and regain their power. Dalgard realizes that the safety of the Terran colony is seriously threatened by this, but has no hope of warning his people in time. A spacehip from Terra has just arrived, and ignorant of the existance of the Terran colony, the crew has been enlisted by the Astrans to aid in their salvage efforts. Of the crew, only the young pilot Raf Kurbi instinctively mistrusts the Astrans. He watches the Astrans make war on primitives, noting that these "animals" wore clothing and went armed. Raf does some snooping, and sees the capture of what looked like a human! Eventually he meets up with Dalgard, joining in the fight against the aliens, whose intentions were deadly for all humans - whether Earth born or Star Born.