Located Near North Celestial Pole
Polaris is a white supergiant star located around 430 light-years away in the constellation Ursa Minor. It is has an apparent magnitude of 1.97, making it the 48th brightest star in the sky, and can easily be found by locating the Big Dipper, and using the stars in its “bowl”, Dubhe and Merak, to point to Polaris. Also known as the North Star, or Polestar, it lies within one degree of the North Pole’s axis as seen from Earth. If you were standing at the North Pole you would have to look directly up to see it.
A Useful Gauge of Latitude
While Polaris is seen directly overhead at the North Pole, the further south one travels the closer the star drops towards the northern horizon. At the Earth’s equator, Polaris appears on the horizon, and any further south subsequently disappears from view all together. In fact, for every degree of separation from the North Pole that you traveled, Polaris would move a degree off center, and sailors could always tell what latitude they were at by observing the position of Polaris. For instance, in London Polaris would be 51° away from the horizon; when you got to Venice it would be 45° from the horizon; in Cairo it would be a mere 30° from the horizon, while at the equator it would appear to be sitting on the horizon.
Not Always Pole Star
Due to precession, the stars change their relative position over time. In 3000 BC, for instance, Thuban in the constellation Draco was the ‘north star’, and though Polaris was close enough to the north celestial pole to navigate by, it didn’t actually “arrive” in position until the 5th or 6th Century. As it stands, Polaris will be our closest marker until around 3000 AD when Gamma Cephei in the constellation Cepheus will more closely mark the pole.
More Than 2,500 Brighter Than Sun
At a distance of 430 light-years Polaris is still quite bright (1.97 mag) because it is more than 2,500 times more luminous than the Sun. We’re lucky to have Polaris where it is for navigational convenience. The southern sky is barren at the South Pole. Well, technically there is something there, but it’s a small constellation called Octans that is very difficult to see, and mirroring Polaris’ role in the southern sky is the dim 5th-magnitude sigma Octanis, which lies within two-thirds of a degree of the pole.
Part of Trinary System
Polaris is actually part of a Trinary System composed of an additional smaller and larger pair of stars. Polaris A (the one we ordinarily think of) and the dwarf Ab, which orbit each other, and Polaris B which orbits them both at a distance of 223.2 billion miles or 2400 Astronomical Units (AU). There are two additional stars that may be gravitationally associated with these stars, making it a rare quinary star system.
Closest Cepheid Variable
Polaris A is the closest Cepheid variable to Earth, and is located 133 parsecs from Earth, with 1 parsec representing 3.26 light years. Cepheid variables pulse at a fixed rate, and change diameter as they do so by millions of kilometers. For over half a century since Edwin Hubble discovered variable Cepheids in a neighboring galaxy, and was able to determine that Andromeda wasn’t a nebula, but in fact an entire galaxy, we’ve been using Cepheid variables to determine distances. It is usually straightforward calculating relative distances by measuring how the brightness of a Cepheid variable changes over time.