Distant star is the roundest object in the Universe

The star Kepler 11145123 is the roundest natural object ever measured in the universe(Credit: Mark A. Garlick)

When geographers say the Earth is round, they mean it's "sort of" round and a bit squashed at the poles. There are, as it turns out, much rounder things out there. A team of astronomers led by Laurent Gizon from the Max Planck Institute for Solar System Research and the University of Göttingen have found a star 5,000 light years away from us that's not only much rounder than the Earth, but is rounder than any natural object in the Universe.

Looking at pictures of stars and planets, the obvious thing to say about them is they're spherical, but, in fact, they're technically "oblate spheroids." This is because the centrifugal force caused by their rotation causes them to bulge at the equator and flatten at the poles. It's like taking a ball of soft clay, sticking it on a potter's wheel, and giving it a good spin. Without even touching it, the clay in the middle is forced out and the tops and bottom squeeze until the ball looks more like a loaf of Boule bread. The same goes for celestial objects.

Earth, for example, spins once a day and has a diameter at the equator of 12,756.27 km (7,926.38 mi), but it's 12,713.56 km (7,899.84 mi) at the poles. That means the equator bulges out 42.77 km (26.58 mi). However, the Sun is much rounder because it rotates only once every 27 days, so even though it is 1.4 million km (870,000 mi) in diameter, its equatorial bulge is a mere 10 km (6.21 mi). To put this in perspective, that's less than the width of a human hair on a beach ball.

But according to Gizon's team, that's way out of shape compared to the star Kepler 11145123. It's a little bigger than the Sun at 1.5 million km (930,000 mi) and rotates once every nine days, but it has an equatorial bulge of only 3 km (1.86 mi). This makes it not only rounder than the Sun, but rounder than any other natural object in the Universe measured so far. Only artificial silicon spheres used to standardize weights and measures are rounder.

Gizon's team could measure the roundness of Kepler 11145123 because it has purely sinusoidal oscillations. That is, the star periodically expands and contracts in a way that changes its brightness, which was measured by the Kepler Space Telescope over four years. According to the team, these oscillations change depending on the latitudes on the star's surface. The scientists were able to compare the frequencies of these modes of oscillation, so they were able to precisely measure the polar and equatorial diameters of the star.

The team says that what is surprising about Kepler 11145123 is that it's rounder than they expected, even if the rotation rate is taken into account. One theory is that a magnetic field at lower latitudes acts like a sort of cosmic corset to counteract the centrifugal forces.

"We intend to apply this method to other stars observed by Kepler and the upcoming space missions TESS and PLATO," says Gizon. "It will be particularly interesting to see how faster rotation and a stronger magnetic field can change a star's shape. An important theoretical field in astrophysics has now become observational."

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