Yellow supergiant stars are the same color and temperature as the Sun -- but that's about all they have in common. These stars pour out thousands of times more light than the Sun, and they're much younger. What's more, they're quite rare. Yet one constellation features two of them shining side by side.
The constellation is Aquarius, the water bearer. Its two brightest stars -- Alpha and Beta Aquarii -- are yellow supergiants that look as bright as the faintest star of the Big Dipper.
Yellow supergiants are rare for the same reason that a yellow traffic light is rare. A yellow traffic light represents a brief transition from green to red. Likewise, a yellow supergiant is a brief transition from blue to red in the life of a massive star.
Both Alpha and Beta Aquarii were born with more mass than the Sun, so they once shined hot and blue. They generated energy in their cores by converting hydrogen to helium. But their cores ran out of hydrogen, so the stars expanded and cooled, turning from blue to yellow. Eventually, they'll puff up even more, becoming red supergiants.
For now, each of the stars shines about 2,000 times brighter than the Sun, so in a single night each star emits as much light as the Sun will during the next three years.
Alpha and Beta Aquarii are hundreds of light-years from Earth. They move through space together, which means they were born together -- explaining why a single constellation has two stars of such a rare type.
Script by Ken Croswell, Copyright 2009, 2012