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The @stro object for the week of 04/17/2000

image of Polaris

image (c) 1997 Roger D. Herzler

Polaris. Polaris, commonly referred to as "the North Star", is a star visible in the northern hemisphere who's most commonly known trait is that it is very close to the North Celestial Pole (NCP). The NCP is the imaginary point in the northern sky around which the stars seem to circle as the Earth rotates during the night. The NCP in the sky corresponds to the North Pole on Earth.

Polaris is circumpolar meaning that it never sets so its visible all night from sunset to sunrise. It is this property plus its closeness to the NCP that allowed Polaris to be used as a major navigational reference point for sailors and others who were traveling at night. Even today Polaris is used for navigation and is often used by astronomers aligning their equatorially mounted telescopes to the night sky.

All stars appear to "drift" very slowly through the constellations (not to be confused with the normal nightly rotation of the Earth) and change their absolute positions due to their motion and the Earth's motion through space. This means that Polaris hasn't always been the "North Star" and other stars have been closer to the NCP in Earth's history. Polaris is very close to the NCP right now but sometime around the year 2100 it will slowly begin to drift away from the NCP from our perspective and another star will eventually take its place as the star closest to the NCP.

Polaris is a Class F yellow supergiant star and is a "Cepheid Variable". Cepheid Variables are stars with a regular dimming and brightening pattern that has earned them importance in astronomy because they can be used to determine celestial distances. Polaris is only around magnitude 2.0 and even though its often thought of as a "bright star", its magnitude is much lower than some other stars in the sky. It can be difficult to see with the naked eye in light-polluted urban environments.

Photos taken of Polaris by using the technique called "star trails" can produce images like the example above. The star in the center of the image that displays the least amount of trailing is Polaris.

Current information for Polaris (Northern Hemisphere):
Rise: <never rises>
Set: <never sets>
Magnitude: 2.0
Constellation: Ursa Minor
Distance: Approximately 384 light-years +/- 54 LY
Luminosity: ~2200 Suns

For more information visit:

APOD of Polaris...

more on Polaris...

more on Cepheid Variables...

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