A nebula is categorized as "diffuse" when it is characterized by a lack of distinct, well-defined edges and a relatively low density of gas and dust compared to other types of nebulae. Diffuse nebulae are often spread out over large areas of the sky and can appear faint due to their low surface brightness.
There are two main types of diffuse nebulae:
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Emission Nebulae: These nebulae are composed of ionized gas that emits light at various wavelengths, primarily hydrogen. They often glow brightly due to the influence of nearby hot stars that ionize the gas. A well-known example is the Orion Nebula (M42).
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Reflection Nebulae: These nebulae do not emit their own light but rather reflect the light of nearby stars. The light from these stars scatters off the dust particles in the nebula, giving it a bluish hue, as blue light is scattered more effectively than red light. An example of a reflection nebula is the Pleiades cluster.
In general, diffuse nebulae play important roles in the process of star formation, as they are often regions where new stars are born from the collapsing clouds of gas and dust. Their categorization as diffuse highlights their extensive and often turbulent nature, distinguishing them from more compact nebulae, such as planetary nebulae.
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