Galactic Dust and Nebulae

If the interstellar medium is so thin, how can we see nebulae at all?

Even though interstellar gas clouds are incredibly thin by terrestrial standards, they make up in size what they lack in density. Interstellar nebulae can be many light-years across, so the total amount of gas we see from a distance can far exceed even the thickest cloud in Earth’s atmosphere, making them quite visible.


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