Spectroscopy

A star emits light across the spectrum — a continuum. When white light goes through a prism, it forms a rainbow, its spectrum. In the same way, as light from a star goes through the gas of a nebula — or even just the atmosphere of the star — specific colours (or wavelengths) are absorbed by the elements contained in the gas, producing dark lines over the continuum. This is an absorption spectrum. The energy that is absorbed by the gas is then re-emitted in all directions, also at the specific colours characteristic of the elements present in the gas, producing bright lines at certain wavelengths; this is known as an emission spectrum.