Polaris, the North Star, barely seems to move while the rest of the northern sky turns around it. But its famous position in our night sky is only one of the things that makes this unusual star so fascinating. Here are some interesting facts about Polaris.
Polaris lies less than 1° from the north celestial pole
Polaris, formally designated Alpha Ursae Minoris, is a yellow-white supergiant located about 450 light-years away in the constellation Ursa Minor. Its fame as the North Star comes from where it happens to appear in Earth’s sky. Polaris lies less than one degree from the north celestial pole, the point in the sky directly above Earth’s geographic North Pole. If Earth’s rotational axis were extended northward into space, it would point almost directly at Polaris. And if you were standing at the geographic North Pole, the star would appear almost directly overhead.
Polaris isn’t completely stationary. As Earth rotates, it traces a very small circle around the true celestial pole — so small that to the naked eye it can seem almost fixed in place. This is why, while other stars appear to move across the night sky, Polaris seems to remain in almost the same position.
Northern Hemisphere observers are particularly fortunate. The south celestial pole has no similarly bright marker: Sigma Octantis, sometimes called Polaris Australis or the South Star, lies near the pole but is much fainter and harder to spot with the naked eye.
Polaris isn’t especially bright — but it’s easy to find
Despite its fame, Polaris isn’t among the brightest stars in the night sky. With an apparent magnitude of about 2, it is readily visible to the naked eye, although dozens of stars appear brighter. Fortunately, its position makes it surprisingly easy to find. First locate the Big Dipper, the familiar seven-star pattern in Ursa Major. The two stars forming the outer edge of its bowl, Merak and Dubhe, are known as the Pointer Stars because they lead almost directly toward Polaris.
Draw an imaginary line from Merak through Dubhe and continue it about five times the distance between the two stars. That line points toward Polaris, which marks the end of the handle of the Little Dipper in Ursa Minor.
Polaris can show you north, your latitude and even the time
Once you’ve found Polaris, you have one of the most useful reference points in the northern sky. Because it lies so close to the north celestial pole, facing Polaris means you are looking almost due north. Its height above the horizon can also tell you approximately how far north you are. The angle of Polaris above the northern horizon roughly matches your latitude in degrees: from 50° north, for example, Polaris appears about 50° above the horizon. Travel farther north and it climbs higher; travel south and it sinks lower. At the geographic North Pole it is almost directly overhead, at the equator it sits close to the northern horizon, and farther south it disappears from view altogether.
Polaris can even serve as the center of a giant celestial clock. Imagine a line running from Polaris through the Pointer Stars of the Big Dipper as the clock’s hour hand. As the Big Dipper appears to circle Polaris, that imaginary hand completes roughly one turn every 24 hours — about 15° each hour. But because the stars return to the same position about four minutes earlier each night, the Big Dipper’s position at a particular clock time gradually changes throughout the year. Estimating the actual time therefore requires taking both the position of this celestial hour hand and the date into account.
We explain these techniques in more detail in our guide to using the stars for direction, latitude and time.
Polaris hasn’t always been — and won’t always be — the North Star
Polaris is an exceptionally good North Star today, and the north celestial pole is still moving slightly closer to it. The two will pass closest around the beginning of the 22nd century, when they will be separated by less than half a degree. After that, the pole will slowly begin moving away again. This happens because the direction of Earth’s rotational axis is not fixed forever. Over a cycle of roughly 26,000 years, the axis slowly traces a circle against the background stars, rather like the axis of a wobbling spinning top. This motion is known as axial precession, and as the axis changes direction, so does the position of the north celestial pole.
Around 5,000 years ago, when the ancient Egyptians were building the pyramids, Thuban in the constellation Draco lay much closer to the north celestial pole and was the pole star of that era. Nor will Polaris be the last. As the celestial pole continues its slow journey, Gamma Cephei in the constellation Cepheus will become a future North Star around 4000 CE. As precession continues, other stars will take their turn, and roughly 12,000 years from now the pole will lie near bright Vega.
So Polaris isn’t the North Star because of anything inherent to the star itself. It simply occupies an extraordinarily convenient position in Earth’s sky during our particular period of history.
Polaris is far larger and more massive than the Sun
Polaris Aa, the primary star in the system, is an evolved yellow supergiant with a mass of about five times that of the Sun. Recent measurements put its diameter at roughly 46 times that of the Sun, or around 64 million kilometers (40 million miles) across. Its diameter is therefore wide enough to fit roughly 5,000 Earths side by side. Yet despite being far more evolved than the Sun, Polaris is considerably younger. Estimates place its age at around 50–70 million years, compared with about 4.6 billion years for the Sun.
So how can Polaris be much younger than the Sun yet already be so much further along in its evolution? The answer lies in its greater mass. More massive stars burn through their nuclear fuel much faster, so Polaris Aa has already left the main sequence and expanded into a supergiant, while the much older Sun remains a main-sequence star.
Polaris is actually a triple-star system
The North Star may look like a single point of light, but Polaris is actually a triple-star system. The supergiant Polaris Aa has two much smaller companions: a close companion called Polaris Ab and a much more distant star called Polaris B.
Polaris Ab is an F-type main-sequence star with roughly 1.3 times the Sun’s mass. It forms a close pair with Polaris Aa, orbiting their common center of mass roughly once every 30 years at an average separation of about 18.5 astronomical units (AU) — roughly the distance between the Sun and Uranus. Astronomers knew Ab was there before they could see it because its gravity affected the motion of Polaris Aa. But the smaller star is so close to the vastly brighter supergiant that it is easily lost in its glare, and it took the Hubble Space Telescope to resolve the pair directly.
Polaris B is another F-type main-sequence star, with roughly 1.4 times the Sun’s mass, but lies about 2,400 AU from the central pair — around 130 times farther away than Ab. Despite that greater distance, its wide separation in the sky makes Polaris B far easier to distinguish from the brilliant supergiant. William Herschel discovered it around 1780, and it can still be seen through a modest telescope.
By carefully tracking the orbit of Aa and Ab, astronomers can use their motion to calculate the stars’ masses — effectively weighing them through gravity. That is particularly valuable because accurately measuring Polaris Aa’s mass can help astronomers understand another unusual property of the star: its regular pulsations.
Polaris is the nearest Cepheid variable to Earth
Polaris Aa belongs to an important class of pulsating stars known as Cepheid variables. Cepheids repeatedly expand and contract, causing their size, temperature and brightness to change in a regular cycle. Crucially, the length of that cycle is related to the star’s true luminosity: in general, the longer the period, the more luminous the star.
Polaris completes its own pulsation cycle in roughly four days, although the changes are too subtle to make it noticeably flicker to the casual observer. By measuring a Cepheid’s period, astronomers can work out how luminous it really is and compare this with how faint it appears from Earth. The fainter it appears relative to its true luminosity, the farther away it must be. This makes Cepheids powerful cosmic distance markers, or what astronomers call standard candles. In the 1920s, Cepheid observations of Andromeda provided decisive evidence that it was a separate galaxy beyond the Milky Way, revealing that our galaxy was only one among many galaxies in a vastly larger universe.
Its relative proximity makes Polaris especially valuable to astronomers. They can study its mass, pulsations and other physical properties in exceptional detail, providing a nearby test of the same kind of stars used to measure distances across the universe. Recent observations have even revealed large bright and dark regions changing across Polaris’s surface — the first time the surface of a Cepheid has been imaged in this way. Astronomers are now studying whether these features are linked to the star’s rotation, four-day pulsations or convection in its outer layers, giving them a new way to investigate how Cepheid stars behave.