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Moon and Gemini

April 12, 2016

The Moon nips at the heel of one of the celestial twins tonight. It passes quite close to Alhena, a moderately bright star at the foot of one of the twins of Gemini. The star is just below the Moon at nightfall, with the Moon moving a bit farther from it before they set, after midnight.

Alhena actually consists of two stars, but only one of them is visible to the eye. That star is bigger and heavier than the Sun, and much brighter. It’s also nearing the end of its life, so it’s beginning to puff up — a process that’ll make it even brighter.

The other star is similar to the Sun — about the same size, mass, and brightness. From Alhena’s distance of more than a hundred light-years, it’s too faint to see with the eye alone. And it’s so close to its brighter companion that, until recently, it wasn’t visible even through a telescope. Only in the last couple of years have astronomers been able to pick it out.

In fact, the companion was discovered only through its gravitational influence on the other star. When astronomers studied the spectrum of Alhena — its individual wavelengths of light — they saw the imprint of two stars. From that, they were able to calculate the details about both stars. And they found that the stars follow an elongated path around each other. At their closest, the stars are about as close as Earth is to the Sun. But at their greatest range, they’re almost 20 times farther — giving one of the twins a wider foot.


Script by Damond Benningfield

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