  Protostars and young stars are strongly spatially "clustered" or "correlated"
within their natal giant molecular clouds (GMCs). We demonstrate that such
clustering leads to the conclusion that the incident bolometric radiative flux
upon a random young star/disc is enhanced (relative to volume-averaged fluxes)
by a factor which increases with the total stellar mass of the complex. Because
the Galactic cloud mass function is top-heavy, the typical star in our Galaxy
experienced a much stronger radiative environment than those forming in
well-observed nearby (but relatively small) clouds, exceeding fluxes in the
Orion Nebular Cluster by factors of $\gtrsim$30. Heating of the circumstellar
disc around a median young star is dominated by this external radiation beyond
$\sim 50\,$AU. And if discs are not well-shielded by ambient dust, external UV
irradiation can dominate over the host star down to sub-AU scales. Another
consequence of stellar clustering is an extremely broad Galaxy-wide
distribution of incident flux (spanning $>10$ decades), with half the Galactic
star formation in a substantial "tail" towards even more intense background
radiation. We also show that the strength of external irradiation is amplified
super-linearly in high-density environments such as the Galactic centre,
starbursts, or high-redshift galaxies.
