U Mon's Primary Star (IMAGE) NASA/Goddard Space Flight Center Caption U Mon's primary star, an elderly yellow supergiant, has around twice the Sun's mass but has billowed to 100 times the Sun's size. Scientists know less about the companion, the blue star in the background of this illustration, but they think it's of similar mass and much younger than the primary. Credit Credit: NASA's Goddard Space Flight Center/Chris Smith (USRA/GESTAR) Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.