A normal galaxy like the Milky Way produces a few new stars each year. Starburst galaxies create new stars much more frequently, especially in the central regions, at rates tens or even hundreds of times higher. This phenomenon is still a mystery; a large supply of dense gas collected around the nucleus of the galaxy can supply the raw material for star formation, but it is not clear why or how the gas piles up in this way. Some likely causes include the formation of a galactic bar, with stars and gas following elliptical orbits that pass near the nucleus, or an interaction with another passing galaxy that perturbs the orbital motions. A third influence may be a magnetic field; since interstellar gas is partially ionized and ionic particles tend to move with magnetic field lines, this can ease the flow of gas toward the nucleus. In the last few years, astronomical techniques have been developed that allow the magnetic field to be imaged in starburst galaxies.