The sun may look like a perfect sphere of light and heat, but if you look close, real close (not directly, but using the right sort of instruments), you will find our star is ever-so slightly fatter along its equator than it is along its polar axis. The difference is so slight that the sun was considered the most perfect sphere observed in nature, but that title has now been taken over by another star about 5,000 light-years away.

To find the roundest object in the observable universe, scientists study the oblateness of objects in space, which in simple words, is the difference between an object’s equatorial length and its polar axis. Oblateness is caused by the rapid rotation of these bodies, and in extreme cases, can lead to stars that are visibly squashed at the poles, leading to pumpkin-shaped stars.

In the case of the sun, which has an equatorial diameter of about 1.4 million kilometers, the difference with its polar axis is a mere 10 kilometers. Earth, which is less than 13,000 kilometers across the equator, has a much higher oblateness at about 27 kilometers. And new measurements by scientists from Max Planck Institute for Solar System Research in Germany show the star Kepler 11145123, which is more than twice the size of the sun, has a difference of only 3 kilometers.

Oblateness This illustration shows the star Kepler 11145123, the roundest natural object ever measured in the universe. Stellar oscillations imply a difference in radius between the equator and the poles of only 3 km. This star is significantly more round than the sun. Photo: Max Planck Institute for Solar System Research/Mark A. Garlick

Kepler 11145123 has a diameter of about 3 million kilometers but rotates three times more slowly than the sun, and it is the slow rotation that is partially responsible for such a low degree of oblateness. Magnetic fields of stars is the other factor that influences oblateness.

According to a statement on the institute’s website, the team of researchers led by Laurent Gizon used a technique called asteroseismology — the study of the oscillation of stars — to determine stellar oblateness. The star was observed continuously for four years by NASA’s Kepler space observatory, and the oblateness of 3 kilometers was determined with a precision of 1 kilometer.

“This makes Kepler 11145123 the roundest natural object ever measured, even more round than the sun,” Gizon said in the statement.

“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,” he added.