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November 14, 2010

The stars might seem like steady beacons of light, but they’re not -- they’re more like flickering candles. The light of the North Star, for example, varies by several percent every four days. Even our own star, the Sun, varies by a small fraction of a percent -- although that’s over the course of its 11-year magnetic cycle.

One of the champions of the “flickering-candle” business is in Cetus, the whale, which stretches across the eastern sky in early evening. This star’s brightness can vary by a factor of several hundred.

That dramatic change inspired the star’s name: Mira, a Latin word that means “wonderful.” At its brightest, Mira is one of the whale’s leading lights, easily visible to the unaided eye. At its faintest, you need a telescope to see it.

The star is much farther along its evolutionary path than the Sun is. In fact, it’s quite near the end of its life, so it’s puffed up like a giant balloon.

This process has made the star unstable, so it pulses in and out like a beating heart. Each beat lasts 11 months, and it’s what causes the change in brightness.

The process is also creating a strong “wind” of hot gas from Mira’s surface. And over time, more and more gas will escape into space with each pulse. Eventually, all of the gas in the star’s outer layers will blow off into space, forming a colorful shell around the star’s dead core -- a core that’s likely to produce its own flickering as it slowly cools and fades from sight.


Script by Damond Benningfield, Copyright 2010


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