SIMBAD references

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.