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Astronomers claim to have discovered the roundest object ever measured in nature.

Kepler 11145123 is a distant, slowly rotating star that's more than twice the size of the Sun.

Researchers were able to show that the difference between its radius as measured to the equator and the radius measured to the poles was just 3km.

"This makes Kepler 11145123 the roundest natural object ever measured," said lead author Prof Laurent Gizon.

He added that it was "even more round than the Sun".

Prof Gizon, from the Max Planck Institute for Solar System Research (MPS), and his colleagues used a technique called asteroseismology - the study of how stars pulsate, or oscillate.

Nasa's Kepler space telescope observed the star's oscillations continuously for more than four years.

The periodic expansions and contractions of Kepler 11145123 can be gleaned from fluctuations in its brightness. And from these data, astronomers were able to extract information about its shape.

Using the method, Prof Gizon and his colleagues discovered that the star rotated faster at the surface than in the core, contributing to an unexpected rounding of its form.

The difference of 3km, between the polar and equatorial radii, is tiny compared to the star's mean radius of 1.5 million km.

The authors say that this distortion is probably caused by factors other than rotation alone. They suggest that a weak magnetic field surrounds the star, making the star appear even more rounded.

The research is published in the journal Science Advances.

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