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.
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Polaris is the closest Cepheid to Earth, which means that getting its distance is critical. With that value in hand, we can then use it to calibrate the distances to other, more distant Cepheids. The problem is, getting the distance to Polaris is hard! It’s a decently bright star and rapidly saturates the detectors of most modern telescopes. This, in large part, is why distance estimates for Polaris have varied pretty widely, from roughly 300 to 450 light-years, which is an unacceptably large uncertainty, given how important this star system is to our fundamental cosmic reckoning.
In 2018 a team of astronomers did something clever: the researchers assumed that the third star, Polaris B, is physically associated with Polaris A (a pretty solid bet) and observed it with the Hubble Space Telescope to measure its distance using a technique called parallax. The result, 521 light-years, is actually even more distant than the top end of the previous estimates for Polaris, so it was a surprise. But is it right?
Subsequent follow-up work suggests that it is—and this is where Polaris’s enigmatic age comes in.
Astronomer Richard I. Anderson took a look at observations of Polaris Aa and tried to use physical models to understand what kind of star it is, eventually publishing his results in the journal Astronomy & Astrophysics. What Anderson found is that almost all the physical characteristics of Polaris Aa are consistent with a star that has seven times the mass of the sun at 521 light-years distant, agreeing with the new distance measurement.
Great! All the pieces are in place and the puzzle of Polaris is solved—except, yeah, no.
The very fact that Polaris is a Cepheid variable means it’s old enough to have run out of hydrogen fuel in its core. It’s gradually turning into a red supergiant similar to Betelgeuse; right at the moment, it’s still a yellow supergiant, but in as little as a few thousand years, it will become redder. Stars amid such transitions tend to be variable because their core is a little unstable, getting hotter and cooler over time. It pumps that energy into the outer layers, which expand and then contract, making the star cyclically brighten and dim—in other words, it becomes a Cepheid variable.
Massive stars run out of core fuel rapidly, and models taking this fact into account indicate that Polaris Aa should be only about 54 million years old. That’s fine in isolation, but recall that Polaris Aa isn’t alone. Worse, astronomers’ best estimates peg the age of its companion, Polaris B, as more than two billion years old! This is problematic, to say the least, because all other signs point to these two stars being the same age, having likely formed together from the same cloud of interstellar gas. What gives?
This is where things get fun. There is a way for old stars to look young again: by colliding and merging with another star. When this happens, the gas gets mixed up in the star, fooling our physical models of how stars age and giving the star the equivalent of a stellar facelift.
This is not a common occurrence in stars, but it has been known to happen. Recall that the tight binary of Polaris A consists of two stars, the more massive Polaris Aa and the less massive Polaris Ab closely orbiting it—but it’s possible that a third star used to be there, too. At some point this hypothetical third star collided and merged with one of the other two, creating Polaris Aa as we see it now. Another outcome of such a merger is the violent ejection of a lot of material into space—mergers are pretty energetic events—and there is some indication of that around the Polaris system. If so, this happened roughly 50 million years ago—interestingly, the age of Polaris Aa as indicated by models.
Without that merger, Polaris Aa would be a lower-mass star that looks its true age of roughly two billion years and would still have a long life ahead of it. Instead, though, the merger added a lot of mass to it. That makes it look like it’s younger in years (50 million instead of two billion years old), but that comes at a high price because higher-mass stars age more rapidly, exhausting their core fuel exponentially faster than lower-mass stars. That’s how Polaris Aa can be approaching its “last gasp” stage of stellar life as a red supergiant despite its apparent youth.
At last, we seem to have a consistent picture of this star: it was born two billion years ago, merged with another star 50 million years ago and is now a Cepheid variable, and the whole system is 521 light-years from Earth.
The idea of such a time-warping merger may seem like something drawn out of thin air, but in fact there’s plenty of precedent for it. Globular clusters are ancient collections of hundreds of thousands of old, red stars, yet many have younger-looking members called blue stragglers. These blue stars were a complete mystery until astronomers realized they were the results of rejuvenating stellar mergers. There’s also the bizarre object V838 Monocerotis, a star surrounded by a cloud of expanding cosmic dust. It, too, is the result of a stellar merger, one that cast off a thick shroud of material called a red nova, so named because the dust causes the star to take on a ruddy appearance for the same reason that smoke after a fire reddens the sun.
If the “stellar merger” idea holds together for Polaris, then we can be confident when we use this star to calibrate the distance to other Cepheid variables and therefore to other galaxies. This is one of the lowest rungs on what we call the distance ladder—which we use to go from measuring the distance of nearby stars to the most faraway galaxies—so it’s literally how we take the measure of the entire universe.
Funny—we use the North Star to guide us when we’re trying to find our way. That’s true for many explorers, whether they’re earthbound or trying to map out the cosmos itself.
Author’s Note: My thanks to Bluesky user @desmigeo.bsky.social for suggesting this topic.
