Andre Norton 
Star Born 

The saga begun in The Stars Are Ours! continues as the growing human colony settles onto their new home world only to discover an unforeseen enemy. . . Generations after landing on the planet Astra, humankind is still working alongside the friendly aquatic native merpeople to survive and thrive. But young Dalgard Nordis is yearning to see more.   Taking his man-journey, he sets out to find and explore the ruins of a city rumored to have belonged to the Others—the mysterious beings whom the merpeople despise and fear. With this feat of bravery he hopes to prove his worth to the Council of Free Men. But instead his courage will prove costly, as the long-unknown Others are on their way back to the city.   Even more daunting is the news that a starship has just arrived on Astra, carrying those who forced the colonists to flee centuries ago: the people of Earth.