We know stars have powerful magnetic fields. We also know that some stars can be ripped apart by black holes. So, what happens when you combine the two?
That’s what a new study published in The Astrophysical Journal Letters has looked at. Picked up by Elizabeth Howell for Seeker, it looks at how a star’s magnetic field might be affected by a supermassive black hole like the one at the center of our galaxy, Sagittarius A*.
Stars can be tidally disrupted by black holes when they pass close to them. These events result in gas and other materials being ripped from the star, which then rain down on the black hole. Some of the material can form a flattened accretion disk around the black hole.
But no one has really looked at what happens to the magnetic field of such a star before. So in this latest study, led by James Guillochon from the Harvard-Smithsonian Center for Astrophysics, the researchers simulated what would happen.
Such an event in our galaxy is rare, thought to occur only once every 10,000 years. However, we can see the streams of disrupted stars still feeding the black hole. And, last year, a tidal disruption event between a star and a black hole was seen in a galaxy 5 billion light-years from Earth.
“Half the matter of the star falls onto the black hole and feeds it, and that generates a luminous flare of a billion or 10 billion solar luminosities,” Guillochon told Seeker.
An animation of a star's magnetic field lines being disrupted by a black hole. James Guillochon
The researchers found that if the star just glanced off the supermassive black hole and survived, its magnetic field would amplify by a factor of 20. The star would continue to orbit the black hole though, and repeated encounters would result in the star becoming highly magnetized. If the star gets too close and is destroyed, the strength of the magnetic field doesn't change.
Indeed, they also suggest a “dynamo” process could take place, where only one glancing encounter with the black hole is needed to massively increase the star’s magnetism, although they found no evidence for this in their simulation.
“If a dynamo process acts within a partially disrupted star, repeated encounters may not be required, which would suggest that many thousands of tidally magnetized stars could lurk near the centers of galaxies,” the researchers wrote.
At any rate, it’s a very interesting area of study. There’s clearly still much to learn about black holes and how they interact with other objects.