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We present a discussion of the similarities and key differences between the transition on to (at the turn‐on) and away from (at the turn‐off) the main sequence, the latter termed the Hertzsprung gap. Using a set of model isochrones and adopting an initial mass function leads us to predict a dearth of G‐type stars for any star‐forming region. This is caused by the (relatively) constant spectral type at which the transition from a fully convective star to a star with a radiative core begins. We also present analysis of the details of this transition in the ONC. In particular we show that a gap in the photometric and spectral type distributions is centred on, and a change in the fractional X‐ray luminosity and rotation rate distribution occurs approximately at, the position of a peak in radiative core size as a function of mass. Whilst photometric signatures of this transition are lost at ages over ∼20 Myr, we show that changes in fractional X‐ray luminosity and magnetic field configuration persist to older ages. Analysis of literature data shows that the mass at which the change in fractional X‐ray luminosity is observed decreases with age.

Observational indicators of the transition from fully convective stars to stars with radiative cores