Artistic representation (not to scale) of a red giant star with strong internal magnetic fields. Waves propagating through the star become trapped within the stellar core when a strong magnetic field is present, producing a “magnetic greenhouse effect” that reduces the observed amplitude of stellar pulsations. Credit: Rafael A. GarcÃa (SAp CEA), Kyle Augustson (HAO), Jim Fuller (Caltech) & Gabriel Pérez (SMM, IAC), Photograph from AIA/SDO
Using asteroseismology, which uses sound waves generated by turbulence on the surface of stars to determine their inner properties scientists found the fusion-powered cores of red giants, stars that are evolved versions of our sun, are strongly magnetized. The findings will help astronomers better understand the evolution of stars.
“In the same way medical ultrasound uses sound waves to image the interior of the human body, asteroseismology uses sound waves generated by turbulence on the surface of stars to probe their inner properties,” says Jim Fuller, Caltech. Magnetic fields likely determine the interior rotation rates of stars; such rates have dramatic effects on how the stars evolve.
Until now, astronomers have been able to study the magnetic fields of stars only on their surfaces, and use supercomputer models to simulate the fields near the cores, where nuclear-fusion takes place. “We still don’t know what the center of our own sun looks like,” Fuller says.
Red giants have a different physical makeup from main-sequence stars such as our sun – one that makes them ideal for asteroseismology. The cores of red-giant stars are much denser than those of younger stars. So sound waves do not reflect off the cores, as they do in stars like our sun. Instead, sound waves are transformed into gravity waves. “It turns out the gravity waves that we see in the red giants do propagate all the way to the center of these stars,” says co-lead author Matteo Cantiello, KITP.
“Depending on their size and internal structure, stars oscillate in different patterns,” Fuller says. In one form of oscillation pattern, known as the dipole mode, one hemisphere of the star becomes brighter while the other becomes dimmer. Astronomers observe these oscillations in a star by measuring how its light varies over time.
When strong magnetic fields are present in a star’s core, the fields can disrupt the propagation of gravity waves, causing some of the waves to lose energy and become trapped within the core. Fuller et al coined the term “magnetic greenhouse effect”. The trapping of gravity waves inside a red giant causes some of the energy of the star’s oscillation to be lost, and the result is a smaller than expected dipole mode.
In 2013, NASA’s Kepler detected dipole-mode damping in several red giants. The internal magnetic fields of the red giants were as much as 10 million times stronger than Earth’s magnetic field. “This is exciting, as internal magnetic fields play an important role for the evolution and ultimate fate of stars,” says Prof Sterl Phinney. “The magnetic fields that they find in the red-giant cores are comparable to those of the strongly magnetized white dwarfs…The fact that only some of the red giants show the dipole suppression, which indicates strong core fields, may well be related to why only some stars leave behind remnants with strong magnetic fields after they die.”
The asteroseismology technique the team used to probe red giants probably will not work with our sun. “However,” Fuller says, “stellar oscillations are our best probe of the interiors of stars, so more surprises are likely.” http://www.caltech.edu/news/astronomers-peer-inside-stars-finding-giant-magnets-48498
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