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Why are ultracompact H II regions difficult to observe with visible light?

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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.

Ultracompact H II regions are difficult to observe with visible light because they are located within very dense, young molecular clouds. These clouds contain thick dust that obscures the visible light emitted by the ionized gas.1

Because visible light cannot penetrate this dense material, astronomers must rely on radio and far-infrared radiation to observe these regions. These wavelengths are capable of passing through the thick dust, allowing researchers to study the ionized gas and the surrounding dust warmed by the embedded star.1