Usually an electron in a higher energy level decays to a lower energy level very quickly, observing some selection rule. Those decays falling outside any selection rules, while still happening, are much slower to happen, and the correspongemissionlikes are called forbidden lines. Because the higher energy level is is stable, an electron can remain in the state for a sustained period without some external disturbance. On Earth where gases are relatively dense and gas molecules are regularly interacting with each other or some solid object,such a stimulus is readily provided.In deep space, gases are much less dense, and such stimulus is not so available. These emission lines, when they are produced,readily travel through the nearly transparent outer space/

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