... Credit: W. Pounds Explanation: It's clear who is the biggest star in this binary system. Based on recent results , this artist's vision of the double star Phi Persei , 720 light years away, shows a bright, rapidly rotating massive star surrounded by a disk of gas. A small companion star orbits 100 million miles away. ... Astronomers consider the ultimate fate of such mass-exchanging, interacting binary systems an open question and a challenge for present theories of stellar evolution . ...