Nature http://doi.org/10.1038/nature14291 (2015)

What is the role of ambient magnetic fields in star formation? Large-scale magnetic fields pervade the inter-cloud medium from which a giant molecular cloud gathers. These clouds, usually filamentary in shape, then form cores that eventually become stars. To investigate whether the magnetic field is dynamically important, Hua-bai Li and co-workers create magnetic field maps of NGC 6334 (the Cat's Paw nebula in Scorpius) from optical polarimetry data collected by several detectors, covering 100–0.01 parsec scales. The technique works because the polarization of starlight is parallel to the magnetic field, and for close-enough sources, the orientation is preserved.

According to their field maps, the main cloud filament lies perpendicular to the ambient field. Moreover, where the gas column density peaks, the magnetic field lines are 'pinched' together to form an hourglass-shape perpendicular to the elongation direction. This anisotropic contraction shows that the magnetic field plays an important role in cloud fragmentation in NGC 6334, which is the nearest star-forming region producing massive stars (of more than 8 solar masses).