All stars rotate and are therefore 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 kilometres larger than at the poles; for the Earth this difference is 21 kilometres. Gizon and his colleagues selected a slowly rotating star named Kepler 11145123. This hot and luminous star is more than twice the size of the Sun and rotates three times more slowly than the Sun.
Surprisingly, the star is even less oblate than implied by its rotation rate. The authors propose 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. Stellar magnetic fields, especially weak magnetic fields, are notoriously difficult to directly observe on distant stars.
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,” Gizon adds, “An important theoretical field in astrophysics has now become observational.”
Some 4 billion years ago, the Sun shone with only about three-quarters the brightness we see today, but its surface roiled with giant eruptions spewing enormous amounts of radiation into space. These powerful solar explosions may have provided the crucial energy needed to create greenhouse gas in Earth’s atmosphere, warming the planet and incubating life.
Astrophysicists at the University of Birmingham have used data from NASA’s Kepler space telescope to look at super-Earths, which are planets outside our solar system with a mass 1-10 times the mass of Earth. In particular, the researchers focused on hot super-Earths whose atmospheres have been stripped away by intense radiation from nearby host stars.