Polaris
What is Polaris?
Stars· Distance ~445 ly · size ~46× Sun. The star that lies close to where Earth's axis points. The whole night sky seems to turn around it, so for thousands of years it has been a guide to north.
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Distance and spectral type
Polaris is about 433 light-years away, a yellow-white star (spectral type F7:Ib-II), in the constellation Ursa Major. Its apparent magnitude is 2, visible to the naked eye.
Stellar figures
| Distance | ~445 ly (estimates 323–445, debated) |
|---|---|
| Constellation | Ursa Minor |
| Note | within 0.7° of the celestial pole |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Latin "pole star". It sits 0.7° from the pole; around 3000 BC the role belonged to Thuban in Draco.
The pole star's pulse that faded and came back
The main star of Polaris swells and shrinks about every four days, and its brightness rises and falls with it, as regularly as a heartbeat. But the size of that heartbeat has changed. Before 1963 its brightness varied by more than 0.1 magnitude, slowly shrinking. After 1966 the pulse weakened quickly, dropping below 0.05 magnitude, and some predicted that it might stop altogether.
The pulse did not stop. More recently, astronomers have reported that it is growing stronger again, a reversal not seen in any other Cepheid. The length of each cycle is also growing by about 4.5 seconds a year, which is read as a sign that the star is slowly swelling as it ages. Comparing old records, one study suggests Polaris is 2.5 times brighter now than when Ptolemy observed it in the second century. The nearest Cepheid turns out to be one of the least predictable.
The star in depth
Polaris is a compass made by coincidence. Extend Earth's spin axis northward and it meets a point in the sky called the north celestial pole. Polaris is now within 0.7 degrees of that point. As Earth turns, the whole sky seems to wheel around the pole, so Polaris stays almost still all night. Its height above the horizon is also nearly equal to the observer's latitude. From London it stands about 51.5 degrees high and from New York about 41 degrees; south of the equator it cannot be seen.
Polaris will not always be the pole star. Earth's axis slowly changes direction, tracing a circle like a wobbling top. This is called precession. Around 3000 BC, Thuban in Draco was near the pole, and in about 12,000 years Vega will be close to it. Polaris itself will come nearest to the pole around the year 2100, at about 0.45 degrees, and then drift away again.
Polaris is actually three stars. The brightest is a yellow supergiant about 46 times as wide as the Sun. A small star circles it about every 29 years, and a third star lies farther out. The main star is a Cepheid variable, changing slightly in brightness about every four days. The timing of a Cepheid's changes reveals its true brightness, so these stars are used as rulers for measuring distances in space. Polaris is thought to be about 445 light-years away, but some studies give as little as 323, and the question is still debated.
At magnitude 2, Polaris is not as bright as many people expect. To find it, use the Big Dipper: extend the line through the two stars at the end of the bowl about five times, and you reach Polaris. Turn on 'track Polaris' in the Sky view to watch the stars circle around it.
Frequently asked
- How far is Polaris from Earth?
- The distance of Polaris is ~445 ly (estimates 323–445, debated).
- What is notable about Polaris?
- The note of Polaris is within 0.7° of the celestial pole.
- Which constellation is Polaris in?
- Polaris lies in the constellation Ursa Major.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.