The celestial swan climbs high across the southern sky on summer nights. It’s marked by its bright tail, the star Deneb, which is the lower left point of the wide-spread Summer Triangle. The swan’s body angles to the upper right of Deneb, as though the swan were taking off from a pond. Its long, graceful wings flank the swan’s body. The brightest star of the top wing is known as Delta Cygni. It’s a system of at least three stars. Two of them are a good bit bigger, brighter, and heavier than the Sun, while the third is less massive than the Sun. The two heavier stars orbit each other at an average distance of almost 15 billion miles — roughly 150 times the distance from Earth to the Sun. But the little guy is much farther out. It’s possible that when the system was born, the third star was closer in. But as the stars danced their complex orbital ballet, they swapped some of their energy. The lightweight star was pushed outward, while the heavier stars spiraled closer together. Those sta
