In section 5. Galactic structure, stellar clusters and populations
Orion revisited. I. The massive cluster in front of the Orion nebula cluster
The authors find that there is a rich stellar population in front of the Orion A cloud. The spatial distribution of this population peaks on iota Ori and is likely to represent the extended stellar content of the poorly studied cluster NGC 1980. Its spatial distribution, luminosity function, and velocity dispersion are different from those of the reddened population inside the Orion A cloud. The authors infer an age of 4 to 5 Myr for NGC 1980 and estimate a cluster population of about 2000 stars, which makes it one of the most massive clusters in the entire Orion complex. What is currently considered in the literature as the Orion Nebula cluster should be seen as a mix of several intrinsically different populations, namely: the youngest population, including the Trapezium cluster and ongoing star formation in the dense gas inside the nebula; the foreground population, dominated by the NGC 1980 cluster; and the poorly constrained population of foreground and background Galactic field stars.