Old Stars’ Fossil Fields

Astronomers have confirmed that strong magnetic fields are frozen in place deep inside aging stars.
Stars like the Sun puff up and become red giants towards the end of their lives, so they’re much larger even though the masses don’t really change. The red giants (“old” Suns) of the same mass as the Sun do not show strong magnetic fields in their interior, but for stars slightly more massive, up to 60% host strong magnetic fields. University of Sydney
Stars create magnetic fields through convection, the swirling, Ferris-wheel-like motion of hot, ionized gas (or boiling water, for that matter). Where convection happens in a star depends on how massive the star is: low-mass stars, including the Sun, have convective outer envelopes around a non-convective core, but stars a little bulkier — up to a couple Suns’ worth — do have convective cores.
Recently, Jim Fuller (Caltech) and colleagues found that strong core magnetic fields could explain the oddly weak, on-and-off brightening behavior of a sample of red giant stars. These stars are low- to middle-mass and have stopped fusing hydrogen in their centers, so they don’t have convective hearts. They also often have a mismatched, variable glow, with one hemisphere brightening as the other fades. What was strange about the sample the team looked at was that this group didn’t vary as much in brightness as it should have.
Now, Dennis Stello (University of Sydney and Aarhus University, Denmark), Fuller, and their team has expanded this work to 3,600 red giants, observed with the Kepler spacecraft. The astronomers found that here, too, some red giants had “muffled” variations, but just how much they were suppressed depended on how massive the star was. For stars just above the Sun’s  mass and lighter, the stars looked normal. But for the heftiest of the sample — 1.6 to

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