Stars and planets might look like perfect spheres to the undiscerning eye, but even the roundest of objects are still imperfect. Our sun, for instance, is 6.2 miles wider at its equator than it is at its poles. That's exceptionally round-- the roundest object in our solar system-- but still oblate.
In fact, scientists have yet to find any object in nature that is a perfect sphere, not in our solar system or in any other. The reason that perfection seems so unattainable is because of the centrifugal force. As any object spins on its axis, as all stars do, the centrifugal force flattens it so that the equator juts out slightly and the poles squish in.
Scientists have found one star, though, that might be as close to being a perfect sphere as it gets. Kepler 11145123, a hot and luminous star located around 5,000 light-years from Earth, is more than twice the size of our sun and rotates three times more slowly. Its slower spin means a weaker centrifugal force, but the star should still be predictably oblate. So you can imagine the surprise when scientists got out their proverbial measuring tape and discovered that Kepler 11145123 is only 1.9 miles wider at its equator than the distance between its poles.
That's still oblate, sure. But it's pretty close to perfect, especially when you consider the size of the star. It might just be as close to perfect as it gets in the universe.
"This makes Kepler 11145123 the roundest natural object ever measured," said Laurent Gizon, team leader on the discovery, in a statement.
The team was able to make such precise measurements by looking at oscillations in the star's brightness, which indicate the dimensions of the star by comparing how it wobbles at various latitudes. At this time it's unclear why Kepler 11145123 is so immaculately round, but researchers suspect it might have to do with the star's magnetic field, which is another factor that can influence an object's shape.
A more thorough study of other stars should help to narrow down the factors that influence roundness, and thus allow researchers to make a more educated guess about Kepler 11145123.
“We intend to apply this method to other stars observed by Kepler and the upcoming space missions TESS and PLATO. It will be particularly interesting to see how faster rotation and a stronger magnetic field can change a star’s shape,” explained Gizon. “An important theoretical field in astrophysics has now become observational.”