Phil Plait writes:

Polaris, the North Star, is one of the most famous stars in the sky, but it’s also quite an enigma. A recent reappraisal of its basics—such as its mass and distance from Earth—suggests that the star is paradoxically youthful, appearing to be only a small fraction of its true multi-billion-year age, like a middle-aged human who somehow passes for a toddler. This is deeply strange; you’d probably assume astronomers have simply miscalculated this star’s age. But in fact, the truth may be even stranger: it turns out that stars can sometimes turn back the cosmic clock to rejuvenate themselves. And understanding how this may have happened for Polaris could prove crucial for nothing less than our conception of the universe itself.

To explain this enigma, the first thing to know is that Polaris is actually a multistar system in which several stars orbit one another. Even a quick glance through a small backyard telescope will reveal Polaris to be two stars: a bright one called Polaris A and a fainter one quite close to it called Polaris B. More sophisticated observations further reveal that the brighter star is itself actually a very tight binary consisting of two stars (called Aa and Ab), which orbit each other so closely that they appear as one in most images.

Polaris Aa is a giant star and by far the brightest of the three—when astronomers talk about Polaris, they usually mean this star specifically. It’s also a very special kind of star called a Cepheid variable, one that grows brighter and then dimmer periodically. Polaris Aa changes in brightness by about 4 percent over the course of about four days. Cepheid variables are critical in astronomy: the length of time it takes them to go through a complete cycle of dimming and brightening is related to how much energy they emit. That means that if you can measure their variability, you can get their absolute brightness. Comparing that intrinsic brightness with how bright a star appears in Earth’s sky is a way of determining cosmic distances (because more distant objects look fainter). We can spot such stars in nearby galaxies, which means we can measure the distance to that galaxy, which is otherwise difficult to do! That’s a very big deal indeed. [Continue reading…]