Aquila, the Eagle

Aquila glides on outstretched wings through the glowing band of the Milky Way. Look for it high in the south in late summer.

Aquila, the Eagle

The brightest star in Aquila is Altair, "the eagle," a white star that is about 17 light-years from Earth. Altair is the southern point of a pattern of three bright stars called the Summer Triangle. (Deneb, in the constellation Cygnus, forms the triangle's northeastern point. Vega, in Lyra, the harp, is in the northwest. Altair is nice and bright and easy to find right up to the beginning of winter.)

Altair is about twice as massive as the Sun, so it will live only about two billion years versus the Sun's 10 billion. Despite its size, Altair appears to turn on its axis once every 10 hours, compared to about four weeks for the Sun. One of the effects of Altair's high-speed rotation is that its gas is forced outward at the equator, giving the star a flattened appearance -- it is about 14 percent wider through the equator than through the poles. If Altair spun about twice as fast as it does now it would fly apart.

Aquila's two "tail" stars are Zeta and Epsilon Aquilae. Together, they also have an older Arabic name: Deneb al Okab, "the eagle's tail." The northern star in the tail is actually a system of three or more stars. Only one of them is bright enough to see with the unaided eye. It's a type of star known as a red giant. That means it's in the final stages of life. Its core is getting hotter and denser, while its outer layers are puffing out and getting cooler. That's the same process that awaits the Sun in several billion years. The southern star is a multiple-star system, too. Its brightest star is fairly early in life. It's bigger, heavier, and hotter than the Sun, and it spins close to 200 times faster than the Sun, so like Aquila it bulges out at the equator, so it looks like a squashed beachball.

The Pioneer 11 spacecraft, which was launched in 1973, is heading toward one of the eagle's stars, Lambda Aquilae, which is 125 light-years away. Pioneer will pass the star in about four million years. Although the spacecraft has already expired, it carries a message from home: a small plaque with information about the craft and its makers -- a greeting to the galaxy from the people who made Pioneer 11.

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