What spectral line feature indicates that a molecular core is actively collapsing onto itself?

A blue-shifted component in the line profile

The confirmation of gravitational collapse within a dense core relies on analyzing the Doppler shifts observed in the spectral lines emitted by gas molecules within that core. When material is actively falling inward toward the gravitational center of the forming star, that moving gas exhibits an asymmetry in its observed spectral signature. Specifically, the portion of the gas moving toward the observer (which corresponds to material falling toward the core's center) is characterized by a spectral shift towards shorter wavelengths, known as a blue-shift. The detection of this blue-shifted component alongside the main, stationary line provides direct, observable evidence that matter is actively infalling and feeding the nascent stellar object. The observation of this asymmetry, often facilitated by advanced instruments like ALMA, confirms that the core is not static but undergoing dynamic, inward contraction.