Figure 18-16: A Magnetic Model for Bipolar Outflow (a) Observations suggest that circumstellar accretion disks are threaded by magnetic field lines, as shown here. (b) Magnetic field lines move with the material they thread. (c) The contraction and rotation of the disk make the magnetic field lines distort and twist into helices. These helices steer some of the disk material into jets that stream perpendicular to the plane of the disk, as in Figure 18-15.
(Adapted from Alfred T. Kamajian/Thomas P. Ray, “Fountain of Youth: Early Days in the Life of a Star,” Scientific American, August 2000)