astrophysics - Why are there spectral lines at all?

My somewhat basic understanding of the concept comes from lectures I've attended about the Bohr-model, which explains the phenomenon as arising from the fact that certain configurations of an atom can only absorb certain wavelengths of light and other configurations can emit the same wavelength and change into the first configuration.

Now what I cannot understand is why these effects do not cancel out, and why in some instances absorption wins out and we observe absorption lines, and in other instances it is the other way around. Also, if I haven't misunderstood there are also some instances where we observe both absorption and emission at once.

So my question boils down to this: Why?

Answer

At your level of understanding, the Bohr atom will do.

Atoms are neutral, composed of orbiting electrons ( negatively charged) around a nucleus (positively charged).

The very basic question coming out of this fact, that an atom is composed of orbiting electrons around a positive nucleus is: how can it be possible when we know that accelerating charges makes them radiate electromagnetic waves away and lose momentum. The electrons should fall into the nucleus since a circulating charge has a continuous acceleration, and matter as we know it could not exist.

Enter the Bohr model: It postulated that there were some orbits where the electrons could run around without losing any energy , quantized orbits.

Enter the absorption lines: The electrons could only change orbits if kicked up by an electromagnetic wave of an energy specific to that particular orbit and discrete. There fore if one shone that specific frequency of light ( E=h*nu) on a specific atom there was a probability to kick an electron up to a higher, called excited, orbit.

Enter the emission lines: Once excited there was a probability for the electron to fall back emitting the specific energy it had absorbed before. This could be observed.

Different experiments will show the different behaviors, even though absorption and emission will be happening continuously in the material.

An experiment shining light on the material and looking at the reflected spectrum will see absorption lines at those frequencies, because the relaxation of the excited electrons will emit back radiation all around randomly, whereas the reflected spectrum is at a specific angle.

An experiment out of the line of the exciting photons will see the emission spectrum . Absorption spectra are useful for identifying elements in stars, the absorption happening in the star's atmosphere and appearing as dark lines in the black body spectrum.

It goes without saying that physics has moved to new horizons from the time that the Bohr atom was news. It has been supereceded by Quantum mechanics which gives tools to accurately predict and classify all spectra as the result of a coherent theory of the way the universe behaves ( i.e. quantum mechanically).