  Using a recently-developed technique to estimate gas temperatures
($T_\textrm{SF}$) in star-forming regions from large photometric surveys, we
propose a diagram, analogous to the Hertzsprung-Russell diagram for individual
stars, to probe the evolution of individual galaxies. On this
$T_\textrm{SF}$-sSFR (specific star formation rate) diagram, a small fraction
of star-forming galaxies appear to be dominated by different feedback
mechanisms than typical star-forming galaxies. These galaxies generically have
younger stellar populations, lower stellar masses and increase in relative
abundance towards higher redshifts, so we argue that these objects are in an
earlier stage of galactic star formation. Further, Hubble observations find
that these "core-forming" galaxies also exhibit distinct morphology, and that
tracks on the $T_\textrm{SF}$-sSFR diagram are also a morphological sequence.
Thus, unlike starburst phases which can be triggered environmentally, these
earliest, core-forming galaxies, appear to be a stage that typical galaxies go
through early in their star formation history. We therefore argue that most
galaxies first go through a core formation stage, then subsequently disk
formation, and finally become quiescent.
