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Despite Shakespeare’s assertion to the contrary, the North Star, Polaris, is anything but steady. The star pulses in and out like a beating heart, changing in brightness as it does so. The length of the pulses is changing, too, and so is the variation in brightness.
Polaris is a type of star known as a Cepheid variable. It’s nearing the end of its life, so it’s undergoing a series of changes. Nuclear fusion has converted the original hydrogen fuel in its core to helium. The core is shrinking and getting hotter, but it’s not yet hot enough to trigger the next round of nuclear reactions.
As Polaris tries to settle in to that next phase of life, it’s unstable. Its outer layers pulse in and out every four days, changing the star’s diameter by a few million miles. And every year, the pulses get about four-and-a-half seconds longer.
Polaris gets a little brighter and fainter with each pulse, but only by a few percent. This change in brightness isn’t steady, either. A century ago, the change caused by each pulse was much greater than it is now. But by a couple of decades ago, the variation had almost disappeared — Polaris really was steady. Today, though, Polaris’s brightness once again changes as the North Star beats out its own stellar rhythm.
The distance to Polaris keeps changing, too — not because the star itself is moving, but because different techniques for measuring distance give different results. More about that tomorrow.
Script by Damond Benningfield, Copyright 2012