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Why does the Orion Nebula appear irregular at visible wavelengths?

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H II region Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.

The Orion Nebula appears irregular at visible wavelengths primarily due to the presence of dark foreground neutral material that obscures parts of the nebula. While the nebula's appearance in visible light suggests a chaotic or irregular shape, this is largely a spurious effect caused by dust obscuration rather than the actual distribution of the ionized gas.1

When observed using radio waves, which can penetrate this dust unhindered, the ionized gas is revealed to be quite circular and surprisingly symmetrical in its projection on the sky. In reality, the foreground dark material obscures approximately half of the ionized nebula, creating the irregular visual impression observed by optical instruments.1