Planetary nebulae are characterized by an emission spectrum consisting of balmer (hydrogen) emission lines and a pair of strong emission lines at 4959˚a and 5007˚a that were quite mysterious when first seen. describe the process that is responsible for the production of these lines. where does the energy that is radiated in these lines come from? why were these lines so mysterious when first observed?

Answer explained

Explanation:

In gas at extremely low densities, electrons can occupy excited meta-stable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities.

Electronic transitions from these levels in doubly ionised oxygen leads to emission of lines in the visible spectrum primarily at 500.7 nm and secondarily at 495.9 nm.

When these lines were first observed, spectra of oxygen ions were not known. Hence, scientists hypothesized that the line may be due to an unknown element, which was named nebulium

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