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July 17, 2016

The Sun is a fairly big, middle-aged star. And as sometimes happens in those years, it moves a little more slowly than it did when it was younger — it takes almost a month to complete one full turn on its axis.

Many other stars spin much faster than the Sun does. An example is Altair, the brightest star of the constellation Aquila, the eagle.

Altair is almost twice as wide and heavy as the Sun. Yet despite its great bulk, it makes a complete turn in only about 10 hours. At that rate, a point at its equator moves at 130 miles per second — fast enough to cross from New York to Los Angeles in less than half a minute.

One reason Altair spins so fast is that it’s much younger than the Sun. The Sun probably turned much faster when it was young, too. But its magnetic field acts as a brake, slowing it down.

One of the effects of Altair’s high-speed rotation is that its gas is forced outward at the equator, making the star look a bit like a flattened beachball. It’s about 14 percent wider through the equator than through the poles. There’s a limit to how flattened the star can get, though; if Altair twirled about twice as fast as it does now, it would fly apart.

Altair forms the southern point of the bright Summer Triangle, which stands high in the east at nightfall. The highest and brightest point is the star Vega, with Deneb at the triangle’s left point. The triangle is still in view, high in the west, at first light.

Tomorrow: One year after Pluto.

Script by Damond Benningfield

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