FlipFact of the Day: Given its navigational significance, even people with only the faintest interest in astronomy have heard of the North Star. The brightest star in the Ursa Minor constellation and the 48th brightest star in the sky, Polaris currently marks the North Celestial Pole—the imaginary point where all the stars go around, from our perspective—and is better than any compass at pointing north. The operative word here, however, is “currently,” because in about 2,000 years, Polaris won’t be the North Star anymore.
Polaris is part of a triple star system, located about 433 light-years away. The primary star (Polaris Aa) is a yellow supergiant with two smaller companions (Polaris Ab and Polaris B). Polaris is a Cepheid variable star, which means it regularly dims and brightens over set periods of time.
Interestingly, Polaris isn’t perfectly aligned with the North Celestial Pole. It’s actually off by about 0.7 degree—less than the apparent width of one and a half full moons—meaning it also goes around the pole with the rest of the stars. Polaris only looks like it’s at a fixed point because it happens to be almost precisely above Earth’s northern axis. If we think of the Northern Hemisphere as a casino roulette wheel, Polaris would be sitting extremely close to the center/turret. Thus, every time the Earth spins, Polaris doesn’t seem like it’s moving at all. (Conversely, there is no clearly visible star right above Earth’s south axis right now; hence, we have no South Star.)
This won’t always be the case, though, for the same reason the Moon is inching away from us each year. Due to gravitational forces acting on Earth’s equatorial bulge, our planet undergoes precession, or a change in the direction of its spin axis. (Picture yourself lightly nudging a spinning top; as it wobbles, its axis would no longer be perfectly vertical.) As Earth’s axis shifts, so will Polaris’ location in our sky, from our view.
That’s also why we had a different North Star around 4,800 years ago: Thuban, located in the constellation Draco. A couple thousand years from now, Gamma Cephei or Errai will be our new North Star. Ten thousand years after that, the North Star will be the blue-white star Vega, which is much brighter than Polaris. Fret not, though: Polaris will reclaim its throne in Earth astronomy in about 25,800 years, which is the time it takes for Earth to complete an axial wobble.
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Cover photo: Wang Jinglei/Jia Hao/NASA
References
- http://ns.umich.edu/Releases/1997/Jul97/chr071697b.html
- https://earthsky.org/astronomy-essentials/north-star-movement
- https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question64.html
- https://usm.maine.edu/planet/it-true-vega-will-be-our-next-north-star
- https://www.physlink.com/education/askexperts/ae492.cfm
Author: Mikael Angelo Francisco
Bitten by the science writing bug, Mikael has years of writing and editorial experience under his belt. As the editor-in-chief of FlipScience, Mikael has sworn to help make science more fun and interesting for geeky readers and casual audiences alike.
