Magnetic fields in space are sometimes called the last piece in the puzzle of star formation. They are much harder to measure than the masses or motions of star-forming clouds, and their strength is still uncertain. If they are strong, they can deflect or even oppose gas flowing into a young stellar core as it collapses under the influence of gravity. If they are moderate in strength, however, they act more flexibly and guide the flow, but don’t prevent it.
Early measurements of field strengths in molecular clouds were based on radiation from molecules whose energy levels are sensitive to magnetic field strengths. Those data suggested the fields were of moderate strength, but those conclusions were tentative. More recent observations with stronger signals measured the polarized radiation from dust grains aligned with the magnetic field. These observations obtain the field strength from the changes in field direction across the cloud map.
CfA.
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