  Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal its
spectacular appearance in infrared light, highlighting the nebula's special
evolutionary stage. The images feature recently-formed massive protostars and
numerous young stellar objects, and a single O star that illuminates the
surrounding molecular cloud from which it formed, and unveil large-scale,
filamentary dark clouds. The hot dust grains show contrasting infrared colors
in shells, arcs, bow-shocks and dark cores. Multiple protostars are detected in
the infrared, within the cold dust cores of TC3 and TC4, which were previously
defined as Class 0. The cold dust continuum cores of TC1 and TC2 contain only
one protostar each. The Spitzer color-color diagram allowed us to identify ~160
young stellar objects and classify them into different evolutionary stages. The
diagram also revealed a unique group of YSOs which are bright at 24 micron but
have the spectral energy distribution peaking at 5-8 micron. Despite
expectation that Class 0 sources would be "starless" cores, the Spitzer images,
with unprecedented sensitivity, uncover mid-infrared emission from these Class
0 protostars. The mid-infrared detections of Class 0 protostars show that the
emission escapes the dense, cold envelope of young protostars. The mid-infrared
emission of the protostars can be fit by two temperatures of 150 and 400 K; the
hot core region is probably optically thin in the mid-infrared regime, and the
size of hot core is much smaller than that of the cold envelope. The presence
of multiple protostars within the cold cores of Class 0 objects implies that
clustering occurs at this early stage of star formation. The TC3 cluster shows
that the most massive star is located at the center of the cluster and at the
bottom of the gravitational-potential well.
