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Great Square of Pegasus

January 19, 2013

The night sky is filled with circles, triangles, and all other manner of geometric figures. In fact, like the skywatchers of old, you can draw your own figures by connecting different groups of bright stars. And if you look in the west at nightfall at this time of year, one pattern you’re almost certain to see is a big square or diamond standing high in the sky. It won’t be a new picture, though — it’s the Great Square of Pegasus, which has been around for more than 2,000 years.

Alpheratz stands at the highest point of the square, and it’s the brightest of the square’s four stars. Through a quirk of modern mapmaking, though, it’s no longer officially a member of the constellation Pegasus. In the 1930s, astronomers drew precise boundaries for all 88 constellations. Under this system, Alpheratz is just across the border from Pegasus, in Andromeda. Indeed, it’s Andromeda’s brightest star.

Alpheratz is actually two stars, not one. The one that’s visible to us is bigger, brighter, and heavier than the Sun. It’s also near the end of its “normal” lifetime, so it’s getting ready to puff up like a big balloon.

The star at the opposite corner of the square, Markab, is similar to Alpheratz, and it’s in about the same phase of life. It looks a little fainter than Alpheratz mainly because it’s several dozen light-years farther. Markab stands below Alpheratz as they drop toward the western horizon and set in late evening.


Script by Damond Benningfield, Copyright 2012

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