How do pervasive magnetic fields resist gravitational contraction in conductive interstellar gas?
Compression of field lines generates magnetic pressure opposing the contraction.
Magnetic fields thread through the interstellar medium, and because the gas is typically electrically conductive, the magnetic field lines become effectively 'frozen' into the moving gas. As gravity attempts to contract the cloud, these embedded magnetic field lines are forced closer together and compressed. This compression strengthens the magnetic field, which in turn generates a resistive outward force known as magnetic pressure or tension. This tension acts much like an elastic structure resisting deformation. The effectiveness of this resistance depends heavily on how well the field is aligned relative to the contraction path and the density, as high density can lead to decoupling via ambipolar diffusion.
