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The star Kepler 11145123 is the roundest natural object ever measured in the universe. Illustration by Mark A. Garlick / Max Planck Institute for Solar System Research

"Roundest natural object ever measured"

The science of figuring out what stars are actually shaped like has taken a giant leap forward.

Astrophysicists in four countries — and affiliated with Center for Space Science at NYU Abu Dhabi — used a groundbreaking new technique to determine with unprecedented precision the specific shape and features of a slowly rotating star that’s more than double the size of the Sun.

The technique is called asteroseismology, explained Laurent Gizon, co-investigator at the Center for Space Science and lead author of a new study published in Science Advances. The information researchers uncovered about this particular star, he said, is 10,000 times more precise than was previously possible.

What star?

Kepler 11145123 is a hot and luminous star more than twice the size of the Sun and rotates four times more slowly than the Sun. NASA's Kepler mission observed the star's oscillations continuously for more than four years (the periodic expansions and contractions of the star can be detected in the fluctuations in brightness of the star). We selected this star to study because it supports very long-lived oscillations whose frequencies that can be measured with astonishing precision.

What did you find out?

We compared the frequencies of the modes of oscillation that are more sensitive to the low-latitude regions of the star and the frequencies of the modes that are more sensitive to higher latitudes. This comparison showed that the difference in radius between the equator and the poles is only 3 km with a precision of 1 km. This makes Kepler 11145123 the roundest natural object ever measured, even more round than the Sun.

Why is this research significant?

Surprisingly, the star is less oblate than implied by its rotation rate. Our research suggests 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.

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An important theoretical field in astrophysics has now become observational.

Laurent Gizon, NYUAD Center for Space Science

Kepler 11145123 is not the only star with suitable oscillations and precise brightness measurements. 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. An important theoretical field in astrophysics has now become observational.

Sizing up a Distant Star With Astonishing Precision
Laurent Gizon, co-investigator at the Center for Space Science at NYU Abu Dhabi.

What is asteroseismology exactly?

Asteroseismology is the study of stellar oscillations. Stars oscillate in high overtone modes. The frequencies of the modes of oscillation can tell us about the mean stellar density, the age of the star, and the rotation of the star. What's new in this research paper is that we use the frequencies of the modes of oscillations to learn about the shapes of stars. The method is based on the fact different modes of oscillation are sensitive to different stellar latitudes. By comparing mode frequencies, we can get a measurement of the relative difference in radius between the equator and the high latitudes, i.e. stellar oblateness.

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Our new method based on asteroseismology is 10,000 times more precise.

Laurent Gizon

How do scientists normally figure out star shapes?

Stars are not perfectly spherical and are so far away that they appear as points in the sky. The traditional technique used to measure the shape of stars is called optical interferometry. This technique can be used to image the shape of some of the largest and closest stars.

Our new method based on asteroseismology is 10,000 times more precise than optical interferometry and applies to stars that support purely harmonic oscillations.

So, stars aren't really round at all?

No. All stars rotate and are flattened by the centrifugal force. The faster the rotation, the more oblate the star becomes. Our Sun rotates with a period of 27 days and has a radius at the equator that is 10 km larger than at the poles; for the Earth this difference is 21 km. Star shapes can also be distorted by magnetic fields.

Where was research conducted?

Several places: Germany, Japan, UK, and Abu Dhabi, and four of the authors are affiliated with the Center for Space Science at NYU Abu Dhabi, which focuses on the study of the Sun and stars. The science to figure out the internal structure and dynamics of stars and the Sun relies heavily on high-quality measurements taken by Kepler and the Solar Dynamics Observatory, both NASA missions.

By Andy Gregory, NYUAD Public Affairs