What does the use of brackets, such as $[ ext{OIII}]$, signify in astronomical notation for emission lines in nebulae?
Answer
The emission line arises from a collisionally excited state only observable in the low densities of nebulae
Emission lines denoted with brackets indicate processes, specifically collisionally excited lines, occurring under the extremely low densities characteristic of nebulae, which cannot be replicated under standard laboratory conditions on Earth.
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