  To distinguish between the different theories proposed to explain massive
star formation, it is crucial to establish the distribution, the extinction,
and the density of low-mass stars in massive star-forming regions. We analyzed
deep X-ray observations of the Orion massive star-forming region using the
Chandra Orion Ultradeep Project (COUP) catalog. We found that pre-main sequence
(PMS) low-mass stars cluster toward the three massive star-forming regions: the
Trapezium Cluster (TC), the Orion Hot Core (OHC), and OMC1-S. We derived
low-mass stellar densities of 10^{5} stars pc^{-3} in the TC and OMC1-S, and of
10^{6} stars pc^{-3} in the OHC. The close association between the low-mass
star clusters with massive star cradles supports the role of these clusters in
the formation of massive stars. The X-ray observations show for the first time
in the TC that low-mass stars with intermediate extinction are clustered toward
the position of the most massive star, which is surrounded by a ring of
non-extincted low-mass stars. Our analysis suggests that at least two basic
ingredients are needed in massive star formation: the presence of dense gas and
a cluster of low-mass stars. The scenario that better explains our findings
assumes high fragmentation in the parental core, accretion at subcore scales
that forms a low-mass stellar cluster, and subsequent competitive accretion.
