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Scorpius X-1

To the human eye, the brightest star in the scorpion is Antares, its orange heart. If our eyes were sensitive to X-rays, though, scorpion’s leading light would be Scorpius X-1. In fact, it would be the brightest star in all the night sky.

Scorpius X-1 was discovered 60 years ago — the first source of X-rays ever seen beyond our own solar system.

It’s actually two stars locked in orbit around each other. Yet the X-rays aren’t directly produced by either star. Instead, it’s the interaction between them that sets the system ablaze.

One of the stars is a neutron star — the ultra-dense corpse of a once mighty star. It’s almost one-and-a-half times the mass of the Sun, yet only as big as a small city. Its companion is a “normal” star, but it’s less than half the mass of the Sun.

The two stars are quite close together. The neutron star pulls gas off the surface of the companion. The gas forms a swirling disk around the neutron star. As the gas spirals inward, it’s heated to millions of degrees — creating X-rays. Eventually, the gas slams into the neutron star at a large fraction of the speed of light, creating even more fireworks.

Antares is almost due south at nightfall, about a third of the way up the sky. Scorpius X-1 is directly above it, by the width of your fist held at arm’s length. It’s too faint to see with the eye alone — unless you just happen to have X-ray vision.

More about the scorpion tomorrow.

Script by Damond Benningfield

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