Moon and Spica

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Moon and Spica

To the eye alone, Spica looks like a bright blue-white star. But the system is far more complicated. It consists of two massive stars locked in a tight orbit. The stars are so close that they distort each other. The bigger of the two pulses in and out like a beating heart. And even though the system is quite young, one of the stars is nearing the end of its life.

Spica’s main star is 11 or 12 times the mass of the Sun, more than seven times the Sun’s diameter, and more than 20 thousand times its brightness. The other star is a little smaller and fainter, but still impressive.

The official distance between the stars is about 12 million miles. But that’s measured from the centers of the stars. When you figure in the sizes of the stars, the distance is only about half of that — just a few percent of the distance from Earth to the Sun.

At that range, gravity distorts the stars. They puff toward each other, so they’re shaped like eggs. As they orbit each other, the angle at which we see them changes. That causes a small change in Spica’s overall brightness.

The system is only about 12 million years old — a tiny fraction of the Sun’s age. But the bigger star is near the end. In a few million years, it’ll explode as a supernova. After that, it’ll fade away, and Spica will lose much of its luster.

Spica is especially easy to spot tonight. It’s quite close to the Moon as they climb into good view, shortly before midnight.

Script by Damond Benningfield

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