An article published in recent days in the journal “Science” describes research that has used the technique of asteroseismology to estimate the intensity of the magnetic fields in the vicinity of the cores of some red giants. This allowed to establish that their intensity can also be 10 million times greater than the Earth’s magnetic field. It’s the first time that scientists have been able to investigate within this type of star.
Red giants are stars with a mass similar to that of the Sun that are reaching the end of their lives. When the hydrogen in their cores begin to run out, these stars begin to pass through a series of dramatic changes that leads to greatly expand and then shrink.
Until now, scientists were able to study the magnetic fields of stars only on their surface and used supercomputers to model that simulated them near their cores. To try to dig into red giants, the authors of this study used the technique of asteroseismology. In essence, the seismic waves produced by the turbulence which reach the surface of these stars were used to probe the characteristics of the core.
In cases where a star has become a red giant and the fusion of helium is about to begin or has already begun, mixed seismic waves were detected with the oscillatory nature of gravity in the inland areas and with oscillatory wave acoustics nature in the convection zone.
By measuring these seismic waves, it was possible to determine with precision the age of the red giants. This allowed to understand in which ones there was still hydrogen fusion and in which ones helium fusiona had already begun and to understand that their cores were rotating at a rate at least 10 times greater than the surface.
Jim Fuller, one of the scientists who participated in this research, explained that according to their size and internal structure, stars oscillate differently. One of the possibilities is the dipole oscillation, a rhythmic movement that makes one hemisphere of the star brighter than the other. Astronomers can observe thise oscillations by measuring the change in the star’s light over time.
The presence of strong magnetic fields can disrupt the propagation of gravitational waves, which sometimes lose energy and remain within the core. Fuller and his colleagues called this phenomenon magnetic greenhouse effect. Its consequence is that part of the energy of the star oscillation is lost and there is a decrease in the intensity of the dipole oscillation mode.
The scientists used observations of NASA’s Kepler space telescope to detect very small changes in brightness in different red giants. Through several observations and simulations, they determined that the most likely cause was in that magnetic greenhouse effect. They also calculated that the magnetic fields inside the red giant reached an intensity up to 10 million times that of the Earth.
This mechanism explains the behavior of some red giants and Jim Fuller announced new investigations of this kind to explore in some way the cores of stars. The asteroseismology technique probably will not work with the Sun but in the distant future it also will become a red giant so it’s important to develop this type of technology to better understand the evolution of stars.
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