A survey of older stars with a little more mass than the Sun has revealed that the majority have enormously powerful magnetic fields at the core. The study's authors say the discovery will force a rethink of how large stars evolve.
As stars age they swell to become red giants, hugely bloated objects that have run out of hydrogen to burn in their core. Like many stars, red giants vibrate in a way Dr. Dennis Stello of the University of Sydney compares to a musical instrument, and these vibrations are easier to observe than in younger stars. “For a lot of stars some notes are missing,” Stello told IFLScience. He found the missing notes matched theoretical expectations of what should occur if intense magnetic fields at the stars' cores are suppressing certain frequencies of vibrations.
The fact that this occurred was known before Stello's research. However, it was previously thought that the presence of these fields was restricted to 5 to 10 percent of large stars. In Nature, Stello reveals that 20 percent of red giants with masses above 1.1 times that of the Sun have these suppressed notes. Among stars with between 1.6 and 2.0 solar masses, the proportion rises to 60 percent.
"This is tremendously exciting, and totally unexpected,” Stello said in a statement.
The fields are around a thousand times as strong as the Sun's surface magnetic field, Stello told IFLScience, and 10 million times stronger than Earth's.
Given the challenges to even detect these fields, Stello said that there would be “no direct effect” on any planets orbiting these stars. Nevertheless, if powerful fields change the ways in which these stars evolve, there may be indirect effects, although we don't yet know what these might be.
Although the fields are only detectable during the red giant phase of a star's life, Stello says they are the remnants of fields that form when the star was in its Sun-like phase and have become, in his words, “frozen in.” Earlier on in a star's life, astroseismology, or the study of stars' virbrations, is insufficient with current equipment to reveal their presence.
"Because our sample is so big we have been able to dig deeper into the analysis and can conclude that strong magnetic fields are very common among stars that have masses of about 1.5 to 2.0 times that of the Sun," Stello added.
Stars the mass of the Sun do not show signs of strong magnetic fields, even in the red giant phase, but at larger sizes these become common. Credit: Stello/Sydney University.
The discovery will force a rewrite of how stars evolve, Stello argued. "Because only 5 to 10 percent of stars were previously thought to host strong magnetic fields, current models of how stars evolve lack magnetic fields as a fundamental ingredient,” he said. Now this thinking will have to change.
“It is an open question how these fields change the way stars evolve,” Stello added to IFLScience. “Theoreticians will have to start to implement them in their models.”