What does "Self-absorption" mean?

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Self-absorption is a phenomenon that occurs when light, or more specifically, certain wavelengths of light, pass through a gas and are partially absorbed by the same gas. This can happen when the gas is not hot enough to emit strong light, so it mostly absorbs light from other sources.

In space, this can be observed in areas where gases are not very dense, such as in clouds of interstellar material. When light from stars or other sources shines through these clouds, some of the light is absorbed, leading to darker areas in the spectrum. As a result, scientists can see signs of self-absorption in the light emitted by elements like oxygen and carbon.

Understanding self-absorption is important for studying star formation and the conditions in space. It provides clues about the composition and behavior of interstellar gases and helps scientists learn more about how stars and other celestial bodies form and evolve.