Constructed of a hot plasma, the Sun is almost spherical and has a diameter of approximately 1,392,684 km, making it 109 times the size of the Earth. Recent studies have shown, however, that the size of the sun varies slightly depending on its activity. When the sun is most active, its radius decreases by 1 or 2 kilometers, according to scientists in The Astrophysical Journal.
The issue of the exact size of the Sun is not as obvious as in the case of heavenly bodies with a fixed surface. One way to define the size of our star is to determine how the brightness of the Sun decreases from the center. In 2010, however, Jeff Kuhn from the University of Hawaii in Maui and his colleagues found no sign that the sun's radius changed over the course of the solar cycle, based on an analysis of changes in brightness. A new reference point, called the seismic radius, has been used in the new research. It is measured by means of seismic waves passing through the interior of the Sun.
Alexander Kosovichev from the New Jersey Institute of Technology and Jean-Pierre Rozelot from the Université Côte d'Azur in Nice inform that the new method allows them to obtain more precise results. Scientists have used 21 years of wave frequency data collected by two spacecraft probes to produce a seismic radius. It turned out that individual layers of the Sun at different depths expand, while others shrink. The final result, however, was that the seismic radius was reduced, and thus the size of the Sun was reduced when it was most active.
In addition, further research into the seismic radius of the Sun can help scientists understand the varying magnetic field intensity at different depths of the star. Interestingly, scientists say that the new research does not replace the previous ones, which are considered less accurate. As they themselves say, both measurements are based on different techniques and therefore explore different aspects of the behaviour of the Sun.