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The Little Dipper is famous for the star at the tip of its handle: Polaris, the North Star. Earth’s axis points in that direction, so all the other stars in the night sky appear to circle around it.

The second-brightest star in the dipper is Kochab, at the lip of the bowl. It isn’t nearly as famous as Polaris, but it’s almost as bright.

Kochab is a giant — more than 40 times the Sun’s diameter, and almost 400 times its brightness. It’s so big because it’s nearing the end of its life. The nuclear reactions deep inside the star push on the surrounding layers of gas, making them puff outward.

Just when a star enters the giant phase of life depends on its mass. Heavier stars age much faster, so they “burn out” more quickly. And Kochab is more massive than the Sun.

But just how massive has been the subject of debate. Studies using different techniques have yielded estimates of about 1.3 to 2.5 times the Sun’s mass. If Kochab had a companion star, it would be easy for scientists to measure the masses of both stars. For solitary stars like Kochab, though, astronomers rely on models of how stars behave. Today, the models seem to indicate a mass of about 2.2 times the Sun’s. But that isn’t completely settled. Until it is, we won’t know the complete story of Kochab.

Kochab is moderately bright, and stands to the right of Polaris at nightfall. It rotates directly above the Pole Star in the wee hours of the morning.

Script by Damond Benningfield

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