Mystery of Magnetic Stars Solved

In a simulation, magnetic field lines in some stars form a ring of lines twisted around each other (blue). Field lines protruding through the surface (red) are held together and stabilized by the twisted ring inside. This is illustrated by the schematic sketch (the lower right) and the cut through the star (upper right). The field drifts slowly outward over hundreds of millions of years, then distorts into the shape of the seam on a tennis ball (lower left), after which it disappears from the star. CREDIT:

While you may never have pondered the similarity between a common bar magnet and a star, astronomers do, and they believe they have figure out why the two disparate bodies are sometimes strikingly similar.

Magnetic activity on many stars, such as our Sun, varies a lot over days, weeks and years. Magnetic fields pop in and out of existence at different spots and overall intensity changes with time. But other stars have strong, consistent magnetic fields that behave just like the smooth and static field of a bar magnet. Astronomers call them magnetic stars.

In these magnetic stars, as with bar magnets, magnetic field lines emanate from each pole, north and south, and loop outward like the skeletal lines of a perfect pumpkin, connecting one pole to the other.

There are three types of magnetic stars:

• Magnetic A-stars are otherwise normal and about two to 10 times as hefty as the Sun. One example is Alioth, the third star in on the handle of the Big Dipper.
• Some white dwarfs, which are burnt-out stellar corpses, have magnetic fields 100,000 times stronger than the typical magnetic A-star.
• Magnetars are ultra-dense neutron stars that have fields 100 billion times stronger than a commercial bar magnet.

For the past five decades or so, there have been two competing ideas for how magnetic stars pack such strong and consistent power. One says the magnetism is generated by movement deep within the star, similar to how Earth's ever-present magnetic field is created.

The other idea, known as the fossil field hypothesis, holds that the magnetic field of a giant gas cloud is sometimes retained after the cloud collapses to form a star. That fits with the fact that fields of magnetic stars don't change with time. But there's a problem: The magnetic field in a star should decay in a few years, other theories state, so something would have to rejuvenate it. Or, perhaps the theory hasn't been fully fleshed out.

Researchers at the Max Planck Institute for Astrophysics have made new numeric simulations in which magnetic fields of various initial configurations become stable as a star develops, supporting the fossil field idea.

"The clouds from which stars form contain a very large amount of magnetic field lines, more than even the stars with the strongest fields observed, explained Max Planck researcher Hendrik Spruit. "Most of the magnetic flux decays away in all cases, but in some a bit (or even a lot) remains."

The fields always end up the same, with a ring of twisted field lines. The mess looks something like a car tire in which broken steel from the internal wire mesh sticks through the surface at various angles, Spruit and his colleagues reported in the Oct. 14 issue of the journal Nature.

"Which cases lead to a strong final state is a matter of chance," Spruit told SPACE.com. "Some initial conditions are more favorable for a magnetic field to survive. One such condition involves the initial field being somewhat concentrated towards the center of developing star.

The researchers caution that the results involve simulations, not actual observations.

Why some stars have incredibly strong magnetic fields compared to others is no great mystery. As stars age and evolve, the field strength increases because the stars get smaller and retain a lot of their original mass. A white dwarf, for example, can be 100 times smaller than a normal Sun-like star. And a neutron star, still packing more mass than the Sun, is typically no larger than a city.

As Spruit puts it: "If I take a bar magnet and squeeze is to half its linear dimensions, the field strength in it goes up a factor of four."

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