Star Flattened by Spin

July 27, 2001 -- Squeeze a beach ball and it will flatten. Now scientists have shown that a star will bulge in the middle in a similar way if spinning at very high velocity.

For a long time scientists had suspected that centrifugal force would cause the rapidly spinning star, Altair, about 15 light-years from Earth, to flatten slightly in shape. Altair spins at a brisk 470,000 miles per hour. New measurements now prove that outward spinning force around the star causes it to alter shape so it is 14 percent wider than it is tall.

Measurements have shown that Earth, and particularly the planets Jupiter, Uranus and Neptune are slightly flattened by their spin. But this is the first time scientists have proven stars will also flatten with speed.

"Altair is the 12th-brightest star in the sky — you'd think that everything there is to know about this star would have been discovered already," said co-investigator David Ciardi of the University of Florida, Gainesville, in a statement. "It's a good example of the surprises you're going to encounter when you are able to look at even familiar stars with unprecedented resolution."

Two Telescopes Better Than One

To confirm the finding, Ciardi and others measured the star's radius at different angles on the sky using NASA's Palomar Testbed Interferometer, located just outside San Diego, Calif. This system links multiple 20-inch telescopes to patch together a larger, more detailed image of distant celestial objects.

The combined light from the two telescopes used to observe Altair provided an image equivalent in sharpness to one viewed by a telescope as large as a football field.

"Measuring the shape of this star, Altair, was as difficult as standing in Los Angeles, looking at a hen's egg in New York, and trying to prove that it's oval-shaped and not circular," said Charles Beichman, chief scientist for astronomy and physics at NASA's Jet Propulsion Laboratory, in a statement.

After noticing the size of the star changed when viewed at different angles, the scientists were able to conclude it was not a perfect sphere. They then carefully compared measurements of the star at different angles to determine its shape.

Earth Has Slower Spin, Slighter Bulge

Our own planet, Earth, displays a similar distortion, although it is much less pronounced. Earth spins at a slower rate of about 1,000 miles per hour at its equator and this causes the equator to bulge about 12 miles compared to the poles.

The sun also has a broader middle. It rotates much more slowly than Altair at a rate of about 4,000 miles per hour and its equator measures about 0.001 percent greater in diameter than at its poles.

Beichman says understanding how forces control the shape of stars like Altair can help them better understand the Sun and its future.

Ciardi and Gerard van Belle of NASA's Jet Propulsion Laboratory co-authored a paper about Altair's unusual shape, which will appear in the October 1 issue of the Astrophysical Journal.