We review the characteristics and discuss the nature of a dense group of compact radio sources found projected toward the
Trapezium cluster of the Orion nebula. There are twenty-six radio sources, with flux densities greater than 2 mJy, clustered
within a region of 35″ radius around ϑ
1C Orionis, the most luminous star of the Trapezium. The density of radio objects, of 1.4×10
4 pc
−3, is extraordinarily high, about a thousand times greater than the density of stars in typical galactic clusters.
Most sources show flat, or slowly rising, spectra between 5 and 15 GHz, brightness temperatures smaller than 104 K, and flux densities that are constant on time scales of days to a few months. These characteristics suggest that the radio
emission is free-free radiation from an HII region. The ionization must be external however, most likely produced by the UV
radiation from ϑ1C. Possible models for these objects are: (1) neutral condensations surrounded by ionized envelopes; and (2) low mass stars
surrounded by evaporating protostellar neutral accretion disks. The ionized gas flowing out from either the globules or the
stellar disks and expanding into the surrounding medium is likely to produce the turbulence observed in the central region
of the Orion nebula.