Rotation periods and projected equatorial velocities of pre-main-sequence ( PMS ) stars in star forming regions can be combined to give projected stellar radii .
Assuming random axial orientation , a Monte-Carlo model is used to illustrate that distributions of projected stellar radii are very sensitive to ages and age dispersions between 1 and 10 Myr which , unlike age estimates from conventional Hertzsprung-Russell diagrams , are relatively immune to uncertainties due to extinction , variability , distance etc .
Application of the technique to the Orion Nebula cluster reveals radius spreads of a factor of 2–3 ( FWHM ) at a given effective temperature .
Modelling this dispersion as an age spread suggests that PMS stars in the ONC have an age range larger than the mean cluster age , that could be reasonably described by the age distribution deduced from the Hertzsprung-Russell diagram .
These radius/age spreads are certainly large enough to invalidate the assumption of coevality when considering the evolution of PMS properties ( rotation , disks etc . )
from one young cluster to another .