Why do stars show absorption lines?

[zincshow]

On an intuitive level, Why do stars show absorption lines? For example, the 121.6nm photon is produced when an electron falls into the lowest level of a hydrogen atom. A 121.6nm photon is absorbed by a hydrogen atom being kicked into an excited state, where it will eventually emit a 121.6nm photon.

As you go farther from the sun (or any star) surface, you would expect far more 121.6nm photons to be emitted then absorbed, but yet you see in the spectrum of a star, a shortage of 121.6nm photons, when logic (mine anyway, which is clearly wrong for some reason) would suggest you should see an excess of 121.6nm photons.

 

[mathman]

My guess. The outer layer of a star is cooler than the interior so it absorbs radiation passing through.

 

[Bandersnatch]

Imagine a beam of light of continuous spectrum coming at you.
If the photons in the beam encounter an atom of hydrogen on their way to your spectrometre, some of the 121.6nm-frequency photons will interact with the atom by boosting the electron to a higher energy state, and then getting reemitted as the electron falls back to its ground state.

The thing is, the reemitted photons can be moving in any direction. It is extremely unlikely for the reemitted photon to be moving in the exactly same direction that leads to your spectrometre.

So, each of such interactions will deduct photons of certain wavelength from the beam you're observing, giving rise to absorption lines.

 

[snorkack]

Many stars do show emission lines instead of absorption lines.
What is the cause of stars showing absorption not emission lines? How are the stars that have absorption lines different from stars that have emission lines?

 

[Bandersnatch]

Many stars do show emission lines instead of absorption lines.
What is the cause of stars showing absorption not emission lines? How are the stars that have absorption lines different from stars that have emission lines?

It's more 'in addition to' than 'instead of'.

These stars have a layer of hot, low pressure gas that emits them. Since the gas is hot, the energy for emission comes from collisions between atoms, rather than (just)from the incident radiation, so it adds to the background continuous spectrum rather than deducting from it like cool gas does.

 

[Ken G]

There are actually (at least) two reasons for stars to show absorption lines, and two reasons to show emission lines. The subtraction of an absorption line can be due to an atmosphere with a dropping temperature as was mentioned, or it can also just be due to scattering back down of light that is trying to escape, even in an isothermal atmosphere, as was also mentioned. Emission lines can be due to a rising atmospheric temperature, as mentioned, but it can also be due to a geometically extended "halo" around the star (often either a disk or a wind). If atoms in the halo create photons in some line, say due to conversion of continuum starlight that is absorbed in the halo, then they can create a geometrically extended circumstellar glow when the star is looked at in the frequency of that line. The star literally looks larger at that frequency! This can create an emission line, even if the circumstellar gas is cooler than the star, and even if some scattering of starlight is occurring in the line.

Scattering can even join with the geometric effect to give both absorption and emission components in the same line-- as happens when a wind creates a "P Cygni" profile. So your question opens up a lot of interesting physics, it is not at all a simple matter to decide why lines have the shapes they do, but that's why the line shapes convey so much potentially important information.