Eta Aquilae

StarDate: September 10, 2009

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Two hundred twenty-five years ago tonight, a wealthy English astronomer discovered that a bright star in the constellation Aquila flickers a little. His discovery has helped modern astronomers measure the size of the universe.

In 1784, astronomers knew only a handful of stars whose light varied. In fact, until a couple of centuries earlier, the heavens were considered perfect and changeless.

They aren't, of course, and Edward Pigott correctly suspected that many stars were variable. On September 10th, 1784, he discovered one of those variable stars, named Eta Aquilae. Every seven days, it quickly brightens, then slowly fades.

Pigott didn't work alone. He mentored a young deaf-mute named John Goodricke. In October, Goodricke discovered a variable star that resembled Eta Aquilae: Delta Cephei.

Stars like these are crucial to modern cosmology. They're called Cepheids, after Delta Cephei. They brighten and fade because they expand and contract like a beating heart. The bigger and brighter a Cepheid, the longer it takes to pulsate. So simply measuring the length of a Cepheid's pulse reveals its true brightness and hence its distance. Modern astronomers exploit that trait to measure distances to galaxies that are millions of light-years away.

Aquila sails high across the southern sky at nightfall. Its brightest star, Altair, forms the southern point of the Summer Triangle. Fainter Eta Aquilae is below it -- continuing its steady beat.

Script by Ken Croswell, Copyright 2009

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