How is a dark line spectrum formed?

Dark-line spectrum is a "photo-negative" of emission spectrum. It is the gaps that appear in precisely the same location as corresponding bright lines. produced by a cool gas with a hot solid and you

A dark Line Spectrum is caused by atoms in the atmosphere of a star absorbing energy at the "dark line" frequencies.

The dark lines reveal the atoms that are associated with the stars atmosphere. The dark lines are atom energy absorption signatures.

The dark line appears because the energy of the light is absorbed by the matter at the frequency of the dark line. According to where that line is, an astronomer can tell what elements are in a...

Absorption lines