Star Kepler 11145123: Astronomers find the roundest ever celestial object in the universe

Astronomers claim to have found the roundest natural object in the universe ever measured. The object is a star that rotates slowly and is located 5,000 light years away from the Earth.

Laurent Gizon from the Max Planck Institute for Solar System Research and the University of Gottingen in Germany and a team of researchers successfully measured the oblateness of the star named 11145123 with unprecedented precision.

Asteroseismology – the study of the oscillations of stars – was used by the researchers to determine the stellar oblateness. The difference between the equatorial and polar radii of the star is only three kilometres (km), the study revealed. This number is astonishing small compared to the star's mean radius of 1.5 million km; which means that the gas sphere is astonishingly round.

The centrifugal force flattens the stars as they rotate. The faster the rotation, the more oblate the star becomes.

The Sun, which rotates with a period of 27 days, has a radius at the equator that is 10 km larger than at the poles; for the Earth this difference is 21 km. 

Kepler 11145123, which is a hot and luminous star, is more than twice the size of the Sun and rotates three times more slowly than the Sun. 
The star's oscillations were observed for more than four years by NASA's Kepler mission. Different modes of oscillation are sensitive to different stellar latitudes. 

The researchers compared the frequencies of the modes of oscillation that are more sensitive to the low-latitude regions and the frequencies of the modes that are more sensitive to higher latitudes. 

This comparison shows that the difference in radius between the equator and the poles is only three km with a precision of one km. 

"This makes Kepler 11145123 the roundest natural object ever measured, even more round than the Sun," said Gizon. 

Surprisingly, the star is even less oblate than implied by its rotation rate. The researchers propose that the presence of a magnetic field at low latitudes could make the star look more spherical to the stellar oscillations. 

Just like helioseismology can be used to study the Sun's magnetic field, asteroseismology can be used to study magnetism on distant stars. 

Stellar magnetic fields, especially weak magnetic fields, are notoriously difficult to directly observe on distant stars. 

Kepler 11145123 is not the only star with suitable oscillations and precise brightness measurements, researchers said. 

"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," Gizon added.

(With PTI inputs)