It is quite remarkable that within only 1.5\(^{\circ}\) of the ONC there are several contiguous, and likely overlapping, groups of young stars (see Figure \ref{fig:1}), although few studies have tackled the entire region as a whole. Unfortunately, the three-dimensional arrangement of star formation regions, in particular massive ones, is far from simple and is essentially unknown given the current distance accuracy to even the nearest star formation regions. It is clear, however, that the ONC is partially embedded in its parental Orion A molecular cloud, which in turn is inside the large \(\sim 200\) pc Orion star formation complex, where groups of young stars with ages from a few to about 10 Myr are seen \citep{Brown1994,Briceno2007}. It has long been suspected that a more evolved group \citep[subgroup Ori OB 1c, including NGC 1981 and NGC 1980,][]{1964ARA&A...2..213B,1978ApJS...36..497W} is in the foreground of the molecular cloud in which the younger ONC population (subgroup Ori OB 1d) is still partially embedded \citep[see][for a large-scale analysis of the possible interplay between these two subgroups]{Gomez1998}.

There are two different views on the stellar population inside the Orion nebula. The first suggests that the core of the ONC, the Trapezium cluster, is an entity distinct from the rest of the stellar population in the nebula, while the second, more prevalent view, suggests that the Trapezium is instead the core region of a larger cluster emerging from the Orion nebula. \citet{1986ApJ...307..609H} performed one of the first charge-coupled devices (CCD) observations of an area centered on the Trapezium cluster (covering \(\sim9.2^{\prime \,2}\)), and from the exceptional high stellar density found they argued that the Trapezium cluster was an entity distinct from the surrounding stellar population, including the stellar population inside the Orion nebula. An opposite view was proposed by \citet{Hillenbrand1998}, who compared optical and near-infrared surveys of the ONC with virial equilibrium cluster models to argue that the entire ONC is likely a single young stellar population.

Confirming which view is correct is critical because they imply different formation scenarios for the ONC, and assuming the ONC is typical, different scenarios for the formation of stellar clusters in general. While the first view implies a bursty formation of the bulk of the stars in a relatively small volume of the cloud, the second, by assuming a more extended cluster, calls necessarily for a longer and more continuos process, allowing for measurable age spreads in the young population, and for substantial fractions of young stellar objects (YSOs) at all evolutionary phases, from Class 0 to Class III. Observationally, the first view argues that the Orion OB 1c subgroup is a star formation event distinct from the 1d subgroup while the second and more prevalent view argues that the two subgroups are the same population, i.e., the Ori OB 1c subgroup is simply the more evolved stellar population emerging from the cloud where group 1d still resides.

If the first view prevails, i.e., if the Trapezium cluster and ongoing star formation in the dense gas in its surroundings represent a population distinct from the rest of the stars in the larger ONC region, then what is normally taken in the literature as the ONC is likely to be a superposition of different stellar populations. If this is the case, then the basic star formation observables currently accepted for this benchmark region (e.g., ages, age spread, cluster size, mass function, disk frequency, etc.) could be compromised.

In this paper we address this important question by attempting to characterize the stellar populations between Earth and the Orion nebula. Our approach consists of using the Orion A cloud to block optical background light, effectively isolating the stellar population in front of it. We then use a multi-wavelength observational approach to characterize the cloud’s and nebula’s foreground population. We find that there are two clearly defined, distinct, and unfortunately overlapping stellar populations: 1) a foreground, “zero” extinction population dominated by the poorly studied but massive NGC 1980 cluster, and 2) the reddened population associated with the Trapezium cluster and L1641N star-forming regions, supporting the first view on the structure of the ONC as described above. This result calls for a revision of most of the star formation observables for this fiducial object.

This paper is structured as follows. In Sect, 2 we describe the observational data acquired for this project as well as the archival data used. In Sect. 3 we present the results of our approach, namely the identification of the two foreground populations and their characterization. We present a general discussion on the importance of the result found in Sect. 4 and summarize the main results of the paper in Sect. 5.