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Border Star

September 9, 2011

With autumn drawing close, some of the constellations associated with the season are climbing into prominence. Pegasus, the flying horse, spreads his wings across the eastern sky at nightfall, with Andromeda, the princess, to his lower left.

The two are linked both mythologically and in the sky.

In mythology, both owe their lives to Perseus, the hero, whose own constellation rises in mid-evening, to the lower left of Andromeda. Perseus killed Medusa, a magical creature with a head of writhing snakes. When some of her blood hit the foamy sea, it gave birth to Pegasus. And later, Perseus rescued Andromeda from a sea monster.

And in the sky, they’re linked by the star Alpheratz. It forms the northern corner of the Great Square of Pegasus, which outlines the horse’s body. But it’s also within the borders of Andromeda. In fact, it’s tied for honors as the princess’s brightest star.

The difference highlights the different ways to look at the constellations. The Great Square is an ancient “connect-the-dots” outline of the mythological creature. But in the 1930s, astronomers drew precise boundaries for all of the constellations, like the borders between states. In this system, Alpheratz was just across the border in Andromeda.

So in addition to its proper name, Alpheratz has two other names: Alpha Andromedae, and the rarely used Delta Pegasi — names that unite this crossover star to its two home constellations.


Script by Damond Benningfield, Copyright 2011


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