Just about every star varies a bit — it gets brighter and fainter over a period of time. And in most cases, astronomers can tell you what’s going on. Some stars pulse in and out like a beating heart. Others produce huge dark “starspots,” or brilliant eruptions known as flares. Even the Sun varies a tiny bit, over an 11-year magnetic cycle.

One of the stars of the Little Dipper varies, too, but astronomers aren’t sure why. They’ve come up with several ideas, but none of them seems to work.

Pherkad forms the lower outer corner of the dipper’s bowl. It’s about 500 light-years away. It’s a good bit bigger and heavier than the Sun, and about a thousand times brighter. Over a period of a couple of hours, though, that brightness varies by a few percent.

Astronomers suspect the difference is caused by the star’s stage in life. It’s nearing the end of its “normal” lifetime, which is triggering a series of changes in the star. In stars of similar mass and life stage, those changes can cause the stars’ outer layers to pulse in and out. In fact, that’s happening to one of Pherkad’s neighbors — Polaris, the North Star, at the end of the dipper’s handle.

Such stars don’t change brightness so quickly, though. Flares and starspots don’t cause such regular changes. And there’s no evidence of another object passing in front of the star, which would change the system’s brightness as well. So the cause of Pherkad’s changing brightness remains a mystery.


Script by Damond Benningfield

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