greatly, e.g. about 100 R, occupying the supergiant part of the Hertzsprung-Russell diagram before returning to the main sequence at higher mass. This was not because of deuterium burning, but simply because the accretion timescale ($M/{\mbox{$\stackrel{.}{M}$}}$) is shorter than the thermal adjustment or Kelvin-Helmholz timescale. The expanded phase lasts for the accretion timescale if the accretion rate is constant, which is 10$^{4}$ years for a 10 M star accreting at $\sim$10$^{-3}$ . If the accretion rate is increasing with time this phase would be longer still.

Overall, if the accretion rates during the hot core/massive YSO phases are as high as $\sim$10$^{-3}$ , then it is plausible that the reason for the lack of an H II region is simply that ongoing accretion keeps the effective temperature of the star too low. This scenario would also explain several other features of massive YSOs. If the ongoing accretion keeps the stellar radius high then it also lowers the surface gravity. It has been noted several times in the literature how the spectroscopic characteristics of massive YSOs resemble those of evolved OB stars (Simon & Cassar 1984). Most of the massive YSOs amenable to near-IR spectroscopy have luminosities around a few 10$^{4}$ 