Guardians of the Pole

The stars Kochab and Pherkad serve several roles. They’re part of the body of Ursa Minor, the little bear. They form the outer edge of the bowl of the Little Dipper. And they’re “guardians of the pole” – they circle around Polaris, the star that marks the north celestial pole.

Both stars are giants – they’ve puffed up at the end of the prime phase of life. Kochab is about 50 times the Sun’s diameter, and 450 times its brightness. Pherkad looks fainter than Kochab, but only because it’s almost four times farther. In reality, it’s more than twice as bright.

The stars are so big and bright because they’ve exhausted their original supply of nuclear fuel. That’s triggered changes in the cores of the stars that have caused them to puff up.

In ages past, both stars were much closer to the pole than they are today. In fact, Kochab was the closest bright star to the pole for a millennium. It was the best pole star about 3100 years ago. But it wasn’t nearly as good a marker as Polaris – it never got closer than about seven degrees, which is almost the width of a fist held at arm’s length. Thanks to an effect called precession, it’s moved away from the pole. So Kochab and Pherkad serve as guardians of the pole.

The stars stand to the right of Polaris at nightfall, and wheel high above the Pole Star later on. Kochab is the second-brightest star of the little bear, shining just a touch fainter than Polaris.

Script by Damond Benningfield