Like a shaggy dog in springtime, some black holes have to shed. New computer simulations reveal how black holes might discard their magnetic fields.

Unlike dogs with their varied fur coats, isolated black holes are mostly identical. They are characterized by only their mass, spin and electric charge. According to a rule known as the no-hair theorem, any other distinguishing characteristics, or “hair,” are quickly cast off. That includes magnetic fields.

The rule applies to black holes in a vacuum, where magnetic fields can simply slip away. But, says astrophysicist Ashley Bransgrove of Columbia University, “what we were thinking about is what happens in a more realistic scenario.” A magnetized black hole would typically be surrounded by electrically charged matter called plasma, and scientists didn’t know how — or even if — such black holes would undergo hair loss.