What causes bright line spectrum?

A bright line spectrum is produced when an electron falls from a higher energy state to that of a lower energy state.

when you add energy, electrons move to a higher energy level (orbit). WHen they come back down, they release energy. If that energy (wave length) is in the visible light range, we see color.

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

It is a spectrum produced by elements when they are heated.

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