Editorial on the Research Topic The Role of Magnetic Fields in the Formation of Stars The subject of how stars and planets form is one of the most fundamental outstanding questions in astronomy. Many theories have been proposed to explain the various processes involved. One of the key unanswered aspects of this whole question is exactly what role magnetic fields play in the overall process. It has been known for many years that magnetic fields exist in the interstellar medium, although their

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... e is hotly debated. Some theories have magnetic fields as the key agents of evolution, whilst other theories ignore magnetic fields altogether, as being only a minor perturbation on an otherwise turbulent picture. However, the recent advent of new telescopes capable of measuring inter-stellar magnetic fields, with previously unheard-of sensitivity and resolution, such as ALMA, NOEMA, CARMA, SMA, and new instruments on existing ground-based telescopes such as JCMT, Nobeyama and IRAM, and airborne/space-based telescopes such as SOFIA and Planck, has meant that it is now possible to revisit this question with fresh eyes, based on new data. In addition, the huge increase in the power of High-Performance Computers (HPCs) means that the current generation of simulations can include more details of more aspects of astrophysics than ever before. In this Research Topic we revisit the question of the role of magnetic fields in the star formation process and bring together the latest observations with the latest theories to see what progress can now be made in addressing this question. The ordering of this Editorial follows the broad theme of observations followed by theory, with each scaling roughly from large scales to small-from entire molecular clouds to individual protostars. Crutcher and Kemball begin the observation section with a discussion of the use of the Zeeman Effect to measure the line-of-sight strength of magnetic fields in molecular clouds and the general inter-stellar medium (ISM). This has only been detected in three species in the general ISM, HI, OH and CN, and in three species in masers, OH, CH 3 OH, and H 2 O. The Zeeman Effect calculates the line-of-sight field strength from measurements of the hyper-fine splitting of a degenerate line, where the amount of splitting is directly proportional to the field strength, with the constant of proportionality relating to the Bohr magneton. The magnetic field strength can then be used to derive the mass to magnetic flux ratio to determine whether a cloud is magnetically super-critical (prone to collapse) or sub-critical (supported by the magnetic field) using a version of the magnetic virial theorem. The magnetic field strength shows a behavior with respect to the column density as follows (see their Figures 4, 5) : below a column density of order 10 21−22 cm −2 the field is essentially independent of column density (at around 10-20 µG); above this column density the field strength increases with increasing column density (with a relation of B α n 0.65 ). In terms of volume density this transition occurs at roughly 300 cm −3 . This can be interpreted in a way that says that low-density gas is sub-critical and high-density gas is super-critical.