These are stellar nurseries called HII regions. The young stars found in HII regions are extremely hot and provide a lot of energy for lighting up nebulae. Some types of gas clouds are not visible in the optical regions of the electromagnetic spectrum. We need to explore some other wavelengths to observe these type of gas clouds. For example, neutral hydrogenemits energy in the radio region of the electromagnetic spectrum.
Dust absorbs the optical and ultraviolet light of stars and re-emits it as infrared radiation.
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