Moon and Spica

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Moon and Spica

A pair of bright, heavy stars perches just above the Moon at dawn tomorrow. They’re so close together, though, that they’re almost impossible to see as anything more than a single pinpoint of light.

Spica is the brightest star of Virgo. It’s 250 light-years away. And both of its stars are much bigger and more massive than the Sun. The heavier of the two probably will end its life as a supernova — a titanic explosion that will blast the star to bits.

The two stars are only a few million miles apart — so close that only a special kind of telescope can see them as individual stars. Astronomers identified the system as a binary by studying its spectrum. They separated the system’s light into its individual wavelengths or colors. Each chemical element leaves its own “barcode” in the spectrum. But Spica shows two sets of codes. As the stars orbit each other, once every four days, the two sets shift back and forth a bit — showing that Spica consists of two stars, not one.

Binary systems are most common among heavy stars like Spica’s. About 60 percent of such stars have companions. That’s compared to less than half of the stars like the Sun, and just a quarter of the least-massive stars.

The tight separation creates some interesting effects. The stars bulge toward each other, so both are shaped like eggs. And the side of each star that faces the other star shines brighter — boosted by the glow of its companion.

Script by Damond Benningfield

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