When we think of a star, or when we see photos or diagrams of our sun, we see a spherical ball of superheated gas. But no star is perfectly spherical, they're actually oblate - a kind of oval shape. Why is this? Well, as a star spins, as all stars do, the centrifugal force causes the star to flatten and its equator to spread out, squishing the poles very slightly. In the case of our sun, which rotates once every 27 days, the effect is small, but it has been measured. The sun is 10 kilometers (6.2 miles) wider at its equator than it is at its poles.
However, though all stars spin, astronomers have made a surprise discovery and realized that there's a star out there that's not only almost perfectly spherical, but it could be the most perfectly spherical object ever found in nature.
Kepler 11145123 is located around 5,000 light-years from Earth and, using four years of data from NASA's exoplanet-hunting Kepler space telescope, a team of astronomers led by Laurent Gizon, of the Max Planck Institute for Solar System Research and the University of Göttingen, were able to precisely measure the star's oscillations in brightness. And these oscillations revealed Kepler 11145123's secret.
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All stars oscillate, much like a sphere of Jello would wobble when placed on a speaker playing Metallica. By measuring the Jello wobbles, we would be able to calculate the frequency of the sound waves passing through the substance and then, from these measurements, we'd be able to understand how dense the Jello is and even gauge its precise shape. Although stars don't wobble nearly as dramatically as this Jello example, waves ripple through our sun and these waves can be measured to help us understand what our star is physically made of. This field of study is called helioseismology; when studying other stars, it's called astroseismology.
So, by measuring the modes of oscillation (revealed by slight changes in brightness), Gizon's team was able to "see" that Kepler 11145123 supports purely sinusoidal modes of oscillation. By comparing the frequencies that are sensitive to the star's high latitudes and those that are sensitive to the low latitudes, they were able to precisely measure the "oblateness" of the star. And what they found was a surprise. The star is only 3 kilometers (1.9 miles) wider at its equator than the distance between its poles.
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"This makes Kepler 11145123 the roundest natural object ever measured, even more round than the sun" said Gizon in a statement. Though the star is rotating three times slower than our sun, Kepler 11145123 is a lot more spherical than thought possible.
So what's going on? Gizon thinks the star's magnetic field may have a role to play and hopes to use future space telescopes to find other stars in our galaxy with slow rotation rates to gauge how spherical they are.
"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. "An important theoretical field in astrophysics has now become observational."
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