The Black Star Trilogy — John W. Campbell

The Black Star Passes - A sky pirate with superior weapons of his own invention. First contact with an alien race dangerous enough to threaten the safety of two planets. The arrival of a dark sun whose attendant marauders aimed at the very end of civilization in this Solar System. Islands of Space - As Earth's spaceship hung in the void between galaxies, they could see below the mass of stars that formed their own island universe. They raced through the darkness of space. But suddenly they saw two shining ships paralleling their course Invaders from the Infinite - The scientific trio challenged by the most ruthless aliens in all the universes, blasted off on an intergalactic search for defenses. Worlds were visited, secrets unleashed, and turned to mighty weapons of intense force - and still the enemy seemed to grow in power . The Earth team hurled bolt after bolt of pure ravening energy - until it appeared that the universe itself might end in one final flare of furious torrential power.