A neutron star the size of a city spins faster than a kitchen blender, flinging particles around itself. A glowing cloud forms around it — called a pulsar halo. The shape of this cloud depends on invisible magnetic fields that, like ocean currents, steer the particles. Two such halos, around the pulsars Geminga and Monogem, turned out to be asymmetric, and that became the key to the puzzle.
Magnetic fields in interstellar space are not uniform: they resemble tangled threads stretching in different directions. Particles in the halo move along these threads, so the cloud stretches in one direction. Scientists compared the halo shapes with a computer model and found that around Geminga and Monogem the fields point differently. It's as if two boats in different parts of the ocean were bobbing on waves coming from different directions.
It turned out that magnetic fields maintain their direction over distances of about a hundred light-years. That's like having huge, slowly rotating whirlpools the size of a hundred solar systems in the ocean. Knowing this, astronomers can use pulsar halos as natural compasses, showing where the invisible magnetic lines point in our Galaxy.
Pulsar Geminga is so faint in visible light that it went undetected for nearly twenty years after its discovery in gamma rays: it was hiding behind a cloud of dust.
In Stanisław Lem's novel "Solaris," the planet's ocean behaves as if alive — magnetic fields in the galaxy also create whimsical, almost "living" patterns that we are only beginning to read.