Stars and planets are not perfect spheres. When they spin, they become flat as a result of centrifugal force.
However, a team of scientists recently announced that it had measured one of the roundest known objects – a star named Kepler 11145123.
According to a study published in the journal Science Advances, the scientists were able to establish the stellar oblateness of the star using asteroseismology, the study of stellar oscillations. The study team found the difference between the equatorial and polar radii of the star is just 1.9 miles, a very small number compared to the star’s average radius of 930,000 miles; meaning the gas sphere is exceptionally round.
All stars spin and are consequently flattened by the centrifugal force. The more rapid the rotation, the flatter the star becomes. Our Sun rotates every 27 days and has a radius at the equator that is 6.2 miles bigger than at the poles; for the Earth this difference is 13 miles. Kepler 11145123 is a slow spinning star that is more than double the size of the Sun and spins three times slower than the Sun.
A Strange Star
The study team said they chose the star because it supports solely sinusoidal oscillations, the regular fluctuations in a star’s size that can be seen in its brightness. NASA’s Kepler mission identified the star’s oscillations regularly for greater than four years. Various modes of oscillation are responsive to various stellar latitudes. For their study, the authors contrasted the frequencies of the modes of oscillation that are more responsive to the low-latitude regions and the frequencies of the modes that are more responsive to greater latitudes. This contrast revealed the difference in radius between the equator and the poles.
“This makes Kepler 11145123 the roundest natural object ever measured, even more round than the Sun,” study author Laurent Gizon from the Max Planck Institute for Solar System Research in Germany said in a news release.
The study team noted that the star is even more round than its rotation rate might indicate. The scientists said the existence of a magnetic field at low latitudes might make the star seem more spherical to the stellar oscillations. The same way helioseismology can be used to analyze the Sun’s magnetic field, asteroseismology can be used to analyze magnetism on faraway stars. Stellar magnetic fields, particularly weak magnetic fields, are infamously challenging to directly view on faraway stars.
Kepler 11145123 is not the only star with suitable oscillations and precise brightness measurements, the team added.
“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,” Gizon said, “An important theoretical field in astrophysics has now become observational.”
Image credit: Mark A. Garlick
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