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The two brightest stars of Gemini — Pollux and Castor — are in pretty good view in the east-northeast not long after the sky gets good and dark. Pollux is the brighter one, with Castor close above it. But Gemini is a big constellation, so it’s a long way over to the third-brightest star, Alhena, which marks a foot of one of the twins. It’s off to the right of the other two, by about twice the width of your fist held at arm’s length.

Alhena consists of two stars. The star we see is about three times the Sun’s mass and diameter. It’s about 125 times brighter than the Sun. And it’s thousands of degrees hotter. That makes it a “class A” star.vb

Astronomers classify stars with the letters O-B-A-F-G-K-M. The classes are based on the surface temperatures of the stars. O stars are the hottest and bluest, while M stars are the coolest and reddest.

Class A stars are toward the top of the temperature scale. And they’re not very common. They account for perhaps one percent of all stars in the galaxy. And Alhena is at the very top of the A stars – one of the hottest – so it shines pure white.

Its companion star has never been seen directly – it’s too faint and too close to see through the glare of the brighter star. But instruments reveal some details. It’s about the same mass as the Sun. It’s probably in the same class as well – class G — the spot for yellow stars like the Sun and the unseen star of Alhena.

Script by Damond Benningfield

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