How do dust grains affect starlight passing through a nebula?

Dust grains significantly alter starlight passing through a nebula primarily through absorption and scattering.1

• Absorption: Dust grains are highly opaque to stellar radiation, particularly at shorter wavelengths like ultraviolet. This absorption effectively blocks or dims light, which is why dense, cold molecular clouds appear dark. Conversely, longer wavelengths such as radio and far-infrared radiation can penetrate these dust regions more freely.1
• Scattering and Reflection: Grains act as excellent reflectors of light, ranging from ultraviolet to visible wavelengths. In reflection nebulae, dust grains scatter light from nearby stars, making the cloud visible. This scattering is highly efficient due to the favorable size of the grains, which promotes the redirection of light rather than its total absorption.2

Beyond these optical effects, the interaction between dust and starlight also influences the physical state of the nebula by heating the gas through the photoelectric effect and providing surfaces for chemical reactions.1