In the field partsH, K, L andM of the Orion Nebula, indicated in Figure 2, no obvious differences do appear in the monochromatic photographs obtained in Hα+[Nii], [Oii], the visual continuum and the range of the Balmer continuum. A different situation we meet in the rest of the field, where one observes two types of featuresA andB, distinguished in Figure 1 by solid and dashed bordering lines respectively. Relative to the conditions in the Hα+[Nii] pictures, the typeA areas gain in intensity in the photographs taken in the visual continuum. the emission in the forbidden [Oii] lines at λ 3727 Å is correlated with Hα+[Nii], the emission in the range of the Balmer continuum with the visual continuum. According to these properties theA-areas must have a particularly high percentage of scattered star light.
Most of the areas with identical monochromatic features show a high deficiency of cluster stars correlated with a low surface brightnesss and a reduced gas density. This is explained by an opaqueness of the emission strata in the direction in the line of sight and a position of the same nearer to the observer than the extension of the cluster. There appear surface structures at large distances from the Trapezium which show a correlation between the intensity of scattered star light and the intensity of the emission of the higher ions ([Oiii], [Neiii]). This observation is considered as a proof that ‘canals’ through the nebular cloud complex allow in some directions the exciting radiation to reach large distances from the star without having suffered an appreciable absorption or scattering.
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Wurm, K., Perinotto, M. Optical surface features and morphologic properties of the Orion Nebula. Astrophys Space Sci 9, 383–397 (1970). https://doi.org/10.1007/BF00649578