  We utilize high-resolution cosmological simulations to reveal that
high-redshift galaxies tend to undergo a robust `wet compaction' event when
near a `golden' stellar mass of $\sim 10^{10} M_{\odot}$. This is a gaseous
shrinkage to a compact star-forming phase, a `blue nugget' (BN), followed by
central quenching of star formation to a compact passive stellar bulge, a `red
nugget' (RN), and a buildup of an extended gaseous disc and ring. Such nuggets
are observed at cosmic noon and seed today's early-type galaxies. The
compaction is triggered by a drastic loss of angular momentum due to, e.g., wet
mergers, counter-rotating cold streams, or violent disc instability. The BN
phase marks drastic transitions in the galaxy structural, compositional and
kinematic properties. The transitions are from star-forming to quenched
inside-out, from diffuse to compact with an extended disc-ring and a stellar
envelope, from dark matter to baryon central dominance, from prolate to oblate
stellar shape, from pressure to rotation support, from low to high metallicity,
and from supernova to AGN feedback. The central black hole growth, first
suppressed by supernova feedback when below the golden mass, is boosted by the
compaction, and the black hole keeps growing once the halo is massive enough to
lock in the supernova ejecta.
