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Moon and Spica

June 12, 2019

We tend to think of stars as nice round balls. In fact, though, few stars are perfectly round. Most of them bulge out a little around the middle as they spin. Some spin so fast that they’re flattened — they look more like lozenges than balls. And some are distorted by the gravitational pull of a close companion. These stars look like eggs, with the pointed ends aimed at their companions.

A pair of these “egg” stars appears close to the Moon tonight. Spica is the leading light of the constellation Virgo. It’s to the lower right of the Moon as night falls.

Spica consists of two big, bright, heavy stars. The bigger of the two is more than 11 times the mass of the Sun, and about seven and a half times the Sun’s diameter. The smaller star is about seven times the Sun’s mass and less than four times its diameter.

The stars are quite close together. On average, their surfaces are only about seven million miles apart. Because the stars are so heavy, each one exerts a powerful gravitational pull on the other. At such close range, that distorts the shapes of the stars — each one bulges toward the other. So seen in profile, the stars of Spica would look like eggs, with the pointed ends aimed at each other.

This configuration won’t last for long. The bigger star is at the end of its “normal” lifetime. Within a few million years, it will explode as a supernova — blasting away some of the outer layers of its egg-shaped companion.

Script by Damond Benningfield

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