A main selection of Scientific American Book Club Magnetic fields permeate our vast universe, urging electrically charged particles on their courses, powering solar and stellar flares, and focusing the intense activity of pulsars and neutron stars. Magnetic fields are found in every corner of the cosmos. For decades, astrophysicists have identified them by their effects on visible light, radio waves, and x-rays. J. B. Zirker summarizes...