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June 22, 2010

In addition to giving proper names to the brightest stars, astronomers also designate most of them with letters of the Greek alphabet. Normally, the first letter of the alphabet -- alpha -- goes to a constellation's brightest star. The bright star Vega, for example, is also known as Alpha Lyrae because it's the brightest star in the constellation Lyra.

But not all stars follow this rule. One dramatic example is Thuban. It's in the long, winding, and difficult-to-find constellation Draco, the dragon, which is north of the Big Dipper. Although Thuban is visible to the unaided eye, it's faint, and at first sight it doesn't seem like anything special. In fact, it's only the dragon's fourth-brightest star.

Yet Thuban also bears the name alpha Draconis. The "alpha" designation hints that there is something special about this star: Long ago, Thuban was the North Star.

Today, the North Star is Polaris, in the Little Dipper. But over thousands of years, Earth wobbles on its axis, so the axis points in different directions. Today it points at Polaris. But close to 5,000 years ago, it pointed at Thuban. The ancient Egyptians used this ancient "North Star" as a guidepost to help them align the Great Pyramid of Khufu -- the first and largest of the pyramids of Giza.

Thuban is actually less modest than it seems. It's a good bit hotter and brighter than the Sun. It looks faint only because it's 300 light-years away -- circling around the North Star.

Script by Ken Croswell, Copyright 2010

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