A pretty binary star system climbs across the southern sky on summer nights. Gamma Delphini represents the snout of Delphinus, the dolphin. The little constellation is in the east as night falls, with the snout on the left and the tail on the right.

Under dark skies, Gamma Delphini is visible to the unaided eye as a single point of light. But a small telescope shows that it’s not single at all. Instead, the system consists of two stars. On average, they’re separated by about 30 billion miles — a gap that’s wide enough for us to see the system’s individual members.

One of the stars is white or pale yellow, while the other is yellow-orange. Seen side by side, though, the contrast between the two looks much more pronounced. So to most observers, the pale star looks blue, while the other looks orange.

The colors reveal the surface temperatures of the two stars. The pale star is hotter than the Sun, while the orange star is cooler than the Sun.

Not long ago, the orange star would have looked much like its companion does now. But it used up the hydrogen fuel in its core. That caused the core to shrink and get hotter. The extra radiation pushed on the surrounding layers of gas, causing them to puff outward. As they did so, they got cooler, which changed the star’s color from white to yellow-orange.

Before long, the companion will undergo the same process — adding a bit more color to the celestial dolphin.

Script by Damond Benningfield