Stellar Top: Astronomers find a squashed star

Think of the sun and other giant balls of gas that twinkle in the heavens. Conventional wisdom has it that all these stars are round. But because stars spin, they’re not perfect spheres. The rotation moves material outward more strongly at the equator than at the poles. This nudges the star into a shape reminiscent of that of a toy top.

OUT OF ROUND. Artist’s rendition of the star Achernar. The red arrow indicates the star’s rotation axis. European Southern Observatory

For a slow rotator like our sun, the effect is tiny. But for a rapidly rotating star such as Achernar, which is 145 light-years from Earth and six times the sun’s mass, that distortion can be significant. Although astronomers already knew that Achernar spins at least 225 kilometers per second, they were flat-out astonished by their newest observations. The squashed star is more than 1.5 times as wide as it is tall.

That makes Achernar the flattest star known, report Pierre Kervella of the European Southern Observatory in Santiago, Chile, and his colleagues in an upcoming Astronomy & Astrophysics.

Conventional models can’t explain such a distortion, Kervella notes. In the simplest theory, a star is assumed to be a solid ball with its interior rotating more slowly than its surface. But such a star would fly apart long before it became as distorted as Achernar, says Marc H. Pinsonneault of Ohio State University in Columbus.

The only way Achernar can be so flat is if “the star is rotating much faster in the core than it is at the surface,” notes Pinsonneault. The new data thus give “the first peek” inside a rapidly rotating star, he says.

The finding is “extremely interesting,” Pinsonneault adds, because a star whose interior turns more rapidly than its surface would mix the interior materials, producing less-distinct layers than other stars have. According to models, the mixing would carry metals forged at the core to the surface, enabling the star to more efficiently seed the interstellar medium with heavy elements. At the same time, the churning action would draw into the core a surrounding layer of hydrogen, a star’s main fuel. This could double a star’s lifetime, Pinsonneault says.

Gerard van Belle of the California Institute of Technology in Pasadena cautions that instead of representing an intact, flattened star, the Achernar observations may indicate a star that has exceeded its rotational speed limit and fallen apart, hurling a disk of material from its equator. The disk might be mistaken for a flattened star.

Kervella says his team’s spectroscopic observations make that possibility highly unlikely.

For their study of Achernar, the researchers used a recently installed interferometer at the Paranal Observatory in Chile. Carefully combining the light from two small telescopes tens of meters apart, the astronomers achieved a resolution comparable to that of a single telescope the size of a football field.


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