Emission nebulae do just what the name suggests: they emit radiation. We can see these by the spectrum that we observe as we look at these clouds.
Emission nebulae are clouds of gas and dust that surround or are in front of a hot star. The radiation from the star heats up the surrounding cloud, exciting the electrons in the atoms of the gas and dust. As the electrons lose energy, they emit radiation.
These
nebula are typically red because of the strong hydrogen-alpha (Hα) lines which occur when an electron falls from second excited state (n=3) to first excited state (n=2).
Emission nebula always found near stars because they must emit radiation to glow.
Rosette Nebula, an emission nebula in Monoceros (Rhinoceros)
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