coldens lic 100 largerMolecular clouds are the birthplaces of stars in galaxies. Those clouds are extremely cold (about minus 260 degree Celsius). Therefore, the low internal thermal pressure allows the condensation of stars out of these clouds due to their own gravitational weight. Molecular clouds are also permeated by the galactic magnetic field. Similar to the thermal pressure, magnetic fields act as counterforce against gravity, which in principle could hinder the formation of stars. Given the fact that the galactic magnetic field is fairly strong (about 6 micro Gauss), the question remained how star-forming clouds could build up within galaxies.

In a recent study, researchers from the Hamburger Sternwarte and the McMaster University in Hamilton, Canada, were able to show that, via numerical simulations of entire disc galaxies, a buoyant magnetic field (known as the Parker instability) can induce converging gas motions along magnetic field lines. The gas streams are sufficiently dense and long that the stabilizing effect of magnetic pressure is overcome. In contrast to previous studies, the authors showed that magnetic fields buckling out of the galactic plane do not lead to the formation of molecular clouds directly. Rather, the field buoyancy induces the formation of thousands of lightyears long filamentary structures, which carry enough mass to become gravitationally unstable and fragment into (star-forming) molecular clouds. Furthermore, this process is independent of the initial magnetic field strength or the turbulent properties of the gas in the galaxy.
The results thus suggest a solution to the question of how to form stars out of a magnetically dominant, diffuse interstellar medium, posed almost 70 years ago.

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