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Quick Change

August 8, 2015

Stars change as they age, just as people do. The Sun, for example, will change color as it ages, from yellow to orange to red. But it’s a process that’ll take billions of years to play out.

Yet a star in the constellation Sagitta, the arrow, is changing before our eyes. FG Sagittae is at the center of a planetary nebula, a bubble of gas the star blew out into space.

In 1955, FG Sagittae looked blue — a sign that its surface was much hotter than the Sun’s. In the 1960s, though, the star turned white. And in the decades since, it turned yellow and then orange. That change reflected a dramatic decline in surface temperature. Around the same time, strange elements, such as yttrium and zirconium, began appearing on the star’s surface.

Because FG Sagittae is a dying star, the changes may reflect its last hurrah. The star has puffed it up to giant proportions. As gas expands it gets cooler, so the expanding star’s surface temperature fell, explaining the color change. And changes far below the surface lifted elements such as yttrium and zirconium, which the star had forged earlier in its life, up to the surface.

No one knows what’s next for FG Sagittae, but it sure is fun watching this remarkable stellar chameleon.

Unfortunately, though, you need a telescope to do the watching. Sagitta is high in the east at nightfall, with its “arrow” pointing toward the lower left. FG Sagittae is not far from the arrow’s tip.


Script by Ken Croswell, Copyright 2015

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