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February 4, 2017

Our sedate, middle-aged Sun spins on its axis about once a month. This leisurely rate keeps the Sun nice and round, like a beachball. But some stars rotate much faster. In fact, some spin so fast that they flatten themselves out, like a squashed beachball.

An example is Gomeisa or Beta Canis Minoris, the second-brightest star of Canis Minor, the little dog. It’s in the east at nightfall, above much brighter Procyon, the constellation’s leading light.

Gomeisa is several times more massive than the Sun, and several times wider as well. But it spins much faster than the Sun does, so it probably makes one turn in about a day.

Since a star is made of gas, that high-speed rotation forces material outward at Gomeisa’s equator. That gives the star its compressed appearance.

The rotation has another important effect. Gas at the equator is moving at more than half a million miles per hour. And since the equator is bulging outward, it “feels” less of a gravitational grip than material at the poles. So some of the hydrogen gas at the equator gets flung out into space, like water whirling away from a spinning lawn sprinkler. That surrounds Gomeisa with a disk of gas that’s several times wider than the star itself.

If you could get close to Gomeisa, you’d see one other interesting effect. Because the gas at the poles is closer to the center of the star, it’s much hotter. So the star’s poles are brighter than its equator — a two-toned appearance for a flattened star.


Script by Damond Benningfield


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