Astronomy

Identifying and Understanding the North Star



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"Identifying and Understanding the North Star"
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Our familiar North Star is Polaris, a Cepheid variable trinary star located 431.42 lightyears from earth. It is not the brightest star in the sky, having an average apparent magnitude of only +1.97; but of all the stars of the sky it has the most historical importance, for it has marked almost exactly due north from Socratic Greece to the present day, including the great period of global exploration and colonisation. Alone among the stars of our sky, it appears to stand almost completely still amid the great rotation of the heavens.

Polaris is the tip of the tail of Ursa Minor, often called the Little Dipper. It can be readily found by first finding the much brighter stars of Ursa Major, also called the Big Dipper or the Plough. Fortunately for us, this constellation is bright enough to be seen even through the less intense urban light pollution. The two stars at the end of the 'bowl' of the Big Dipper, which also happen to be the two brightest stars of the constellation, point directly toward Polaris.

The three stars which are known to make up Polaris are an F7 giant star, an Ab white dwarf, and an F3V main sequence star not too dissimilar from our own sun. The latter two stars are in a very tight orbit of 18.5 astronomical units, closer together than Uranus from our sun. The more powerful telescopes of recent years suggest that these three stars may in turn be part of a loose open cluster of stars. Highly unusually for a Population I Cepheid variable, the period and degree of variability has changed significantly within historical times. It is even possible that Polaris may now be over twice as bright as it was during Julius Caesar's time. If these observations turn out to be accurate, they may well overturn current stellar theory.

Contrary to popular belief, Polaris does not lie directly over the north pole. However, at only 0.7 degrees difference, it is close enough for most practical purposes. In fact, it is close enough that its height above the horizon in degrees can be used to determine an observer's latitude. As a quick rule of thumb, ten degrees translates as approximately three fingers' width at arm's length. At the equator, Polaris lies almost exactly on the horizon. It is not visible at all in the southern hemisphere.

Virtually every northern hemisphere culture acknowledges Polaris in their mythology and their navigational guides, as well as the circumpolar stars which whirl around it, never dipping below the horizon. Some saw its seemingly fixed nature as something steadfast, the Nail of the World which could always be relied upon. Others saw its inability to sink into the ocean to rest, in contrast to most of the other stars, as a curse of the gods. Even after the invention of the compass, local magnetic variance and the distance between axial and magnetic poles reduced the usefulness of this device, so that it was relied upon only when neither stars nor sun were available. Even Christopher Columbus checked his route against the North Star ... and thereby discovered significant magnetic variance near the Canary Islands.

Yet the North Star has not always been Polaris. The axis of the earth wobbles like a top, taking approximately 26,000 years to complete a single precessional cycle: and thus the star closest to the earth's north pole also changes in the same slow cycle. Our north star, Polaris, continues to inch closer to the north celestial pole, until it will reach its closest approach, still nearly half a degree away, on 24 March, 2100. Homeric Greece knew Thuban, in the constellation Draco, as their north star. In another thousand years we will be calling Alrai (Gamma Cephei) our north star, followed by Iota Cephei around the year 5200 CE. At the far end of the cycle, the brilliant Vega will become the north star, twelve thousand years from now.

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