It is believed that stars are not spheres – during intense rotation, they become flatter under the influence of centrifugal force. A team of researchers led by Laurent Gieson of the Max Planck Institute for the Study of the Solar System, as well as the University of Göttingen, managed to make a measurement with almost incredible accuracy – they calculated the degree of 'flattening' of a slowly rotating star.
The researchers determined the stellar flattening using astroseismology – the study of the vibrations of stars. The technique was applied to a star 5,000 light-years from Earth and showed that the difference between the equatorial and polar radii of the star is within only 3 km – a number that is strikingly small compared to the star's average radius of 1.5 million kilometers. All of this indicated that the gas sphere was strikingly round.
All stars rotate and are therefore flattened by centrifugal force. The faster the rotation, the more flattened 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 that of the poles. For the Earth, this difference is 21 km. Gizon and his colleagues studied the slowly rotating star Kepler 11145123 – a hot and bright star twice the size of the Sun, the rotation speed of which is three times slower than that of our star.
Kepler 11145123 was not chosen by chance – it turned out that it is capable of supporting only sinusoidal oscillations. Previously, the Kepler Space Observatory has observed the star wobble continuously for over four years. It turned out that periodic expansion and contraction of the star was observed in fluctuations in the brightness of the star, and different modes of fluctuations are sensitive to different stellar latitudes. To study them, the authors compared oscillation frequencies that are more sensitive to low latitude regions and frequencies 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 an accuracy of 1 km.
“This makes Kepler 11145123 the roundest natural object ever measured, even more rounded than the sun,” Laurent said.
Sources: Phys
