The initial velocity dispersion of newborn stars is a major unconstrained aspect of star formation theory .
Using near-infrared spectra obtained with the APOGEE spectrograph , we show that the velocity dispersion of young ( 1-2 Myr ) stars in NGC 1333 is 0.92 \pm 0.12 km s ^ { -1 } after correcting for measurement uncertainties and the effect of binaries .
This velocity dispersion is consistent with the virial velocity of the region and the diffuse gas velocity dispersion , but significantly larger than the velocity dispersion of the dense , star-forming cores , which have a sub-virial velocity dispersion of 0.5Â km ^ { -1 } .
Since the NGC 1333 cluster is dynamically young and deeply embedded , this measurement provides a strong constraint on the initial velocity dispersion of newly-formed stars .
We propose that the difference in velocity dispersion between stars and dense cores may be due to the influence of a 70 \mu G magnetic field acting on the dense cores , or be the signature of a cluster with initial sub-structure undergoing global collapse .