Spectral Lines

Emission lines or absorption lines are discrete lines in the spectrum that are generated by the absorption or emission of a specific frequency. They are produced by hot thin gases in which the atoms do not experience many collisions (because of the low density) which would give a continuous spectrum. Absorption lines occur when light passes through a cold thin gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the spectrum. Due to the quantisations of electron states only some very specific energy levels (frequencies) are allowed. In other words, the emission lines correspond to photons of discrete energies that are emitted when excited electrons in the gas make transitions back to lower-lying levels or absorbed when the electron becomen excited by another energy source.
Although they can only be seen as very discrete lines from thin gasses, they can be noticed as peaks in a continuous spectrum from more or less anything else with some energy (or blocking a continuous energy).
The spectrum from a star has emission lines on top of their continuous spectrum. It is because when atoms are in low pressure, they can only emit or absorb light of certain wavelengths. If some EM wave with continuum spectrum passes through some low pressure atoms (e.g. the outer atmosphere of a star), those atoms will absorb the light it likes and creates an absorption line. Similarly, if some low pressure atoms emit EM wave, they will create emission lines. (All objects radiate (also absorb) infrared (Q: Is this because infrared has the wavelength of atoms (or others) and atoms shaking, makes IR) for the simpliest atom we know, Hydrogen, the formula could be attained to calculate them: 1/l = Rh ( 1/n1^2 - 1/n2^2) Remarks: Let's not forget that the lines have slightly different frequencies/wavelengths in different matter because of the difference in the speed of light is slower in denser materials (wavelength smaller).