The formation of absorption lines in solar spectrum
It is said that absorption lines originate from regions higher in the photosphere where the gas is cooler.
The gas should be absorbing photons, then re-emitting them... absorb, re-emit, absorb, re-emit...
My question is where are the photons produced by re-emission? Why are there absorption lines?
Although photons could be scattered, photons at other places could be scattered to our line our sight. Surely the total number of photons does not drop the further out you go? So while photons originally headed in the direction of an observer are possibly absorbed and re-emitted in a direction away from the observer, surely this will be made up for by a photon being emitted from another location in the direction of the observer? Otherwise wouldn't there be a location somewhere else where a bunch of extra scattered photons can be observed?
If we integrate all the photons at the lower surface of the photosphere A and integrated all the photons at the upper surface of the chromosphere C, surely there are the same number of photons? Otherwise, where have they gone? Where is the lost energy?
Are there less photons emitted from the surface C because some have been emitted back down into the Sun? But that has to reach an equilibrium, or there would be a cascading increase in the number of photons inside the sun.
These can be two different questions. Since you seem to actually understand the answer to your own second question, the short answer to the first one is: they are scattered away from the line of sight.
I've redrafted the question to add to it comments by @questionhang made in the answers already provided. I have created a fresh diagram so we aren't using a drawing we don't have rights to. I have avoided using the terms ionization and recombination as emission and absorption lines are not limited to ionization and recombination, as only the excitation of an electron to a different energy state is required.
In the upper photosphere, the electrons join with the ions to neutralize the atom. When a photon strikes this atom (the atom absorbs the photon), it promotes a electron from a low energy shell to an higher energy shell. The electron then falls back to the lower energy shell emitting a photon in a different direction. When you break the sunlight into wavelengths (by a prism), you will see dark bands where the light was absorbed by the atoms and emitted in a different direction. These bands correspond to the energy absorbed and released when an electron changes energy shells. Here is the sun's light spread out to show the absorption bands.
Added: Here is a high resolution of the wavelengths from the sun.
Apr 20: You have the wrong interpretation of Figure 16.9. At level A, there is about the same number of photons at all wavelengths coming at the observer on Earth. As the photons pass through the Chromosphere, the photons with the wavelengths of the atom's electron transition are absorbed and redirected away from the observer. The photons with different wavelengths of the atom's electron transition eventually arrive at the observer. This is why there are dark lines in the spectrum. It is because most of the photons are absorbed and redirected in a different direction.
The absorption lines are formed at a higher region since the photons are removed from the spectrum.
Let's take Halpha as an example. The total amount of Halpha photons drops after they leave photosphere? Although photons could be scattered, photons at other places could be scattered to our line our sight.
Jeremy means the absorption is not the main reason，but the recombination？ The photons produced by recombination will have a wavelength dispersion?
@questionhang I'm don't understand what you mean by H-alpha. But the photons from the sun (at the absorption by hydrogen) are nearly all absorbed (maybe 90%) and less than 1% are emitted at Earth, so the total light at this one wavelength is about 1/10 (or less) as bright as the wavelengths that have no absorption.
@Jeremy The absorption and emission are at the same wavelength. The energy levels don't have the time to change wavelengths as what happens in fluorescence.
@LDC3 Like I said, the total amount of Halpha photons drops after they leave photosphere? Although photons could be scattered, photons at other places could be scattered to our line our sight.
@questionhang Are you just talking about the sun's photosphere? If you look at the first link (NOVA), this is what a telescope in space would see when looking at the sun. On the lower spectrum, you will see an absorption line at about 490 nm. This is caused because 90% of the light is absorbed (by the atoms in the upper photosphere) and emitted in a different direction (than our telescope). BTW, I still don't understand what you mean by H-alpha.
There is a H-alpha absorption line(6563A) at your first link (NOVA). Although photons in our line of sight originally could be scattered and emitted in a different direction,photons at other places could be scattered to our direction. `Like I said, the total amount of Halpha photons drops after they leave photosphere? Although photons could be scattered, photons at other places could be scattered to our line our sight.`
@questionhang There is very little substance to scatter or reflect photons in space. Lets look at a stream of photons with energy at a Hydrogen absorption band. It starts at the surface of the photosphere and comes directly at you on Earth. It encounters hydrogen in the upper photosphere and most of the stream is absorbed and redirected. The remaining stream (about 90%) travels to Earth through space (less than 0.001%) is absorbed and redirected. The photons enter the atmosphere and come to you. The photons slightly off the path are scattered to make the sky blue.
@LDC3 Please forgive my ambiguous expression. What I mean is not about the space between the photosphere and the observer. The photons can be absorbed by the colder gas at the top of the photosphere, but there are recombinations. Although photons may be redirected, it should not the the reason of absorption line. Because photons at other regions of photosphere can be redirected to our line of our sight too. So the amount of Halpa photons really drops because of photospere?
@questionhang The nuclear reactions create the photons and they emerge from the surface. [Here] is a cutout of the sun. As you can see, the photosphere is under the chromosphere. The chromosphere (upper photosphere) contains the cooler hydrogen atoms that absorb the light at various wavelengths. I think you are confused that an equal amount of light is given off in our direction. Put a point on a basketball. What is the area of the point to the whole ball? That is how little light comes our way. : http://public.media.smithsonianmag.com/legacy_blog/sunstructurelg.png
@LDC3 My confusion is why there are absorption lines. There is absorption, but there is recombination too.
I add an image to my post. Both ionizations and recombinations take place at layer B. What the observer sees is an integrated effect. He really sees less Halpha photons? A's Halpha photons does not outnumber B.
All the photons being emitted do not 'fill in the blank' of the absorption line, because when they are emitted, most do not come towards us. You suppose that there will be some emitted from a different hydrogen atom elsewhere, that will just happen to aim a photon at us, but where will it come from? Think about how much of the sky the Sun subtends from our perspective. Now think about how much of the sky the Earth subtends from the perspective of an atom of hydrogen in the Sun's photosphere that is thinking about popping a photon out and wondering if it will head in the direction of Earth. Here is a 2D diagram - bear in mind that the atom actually is in 3D space and instead of being limited to only 360 degrees, can pop that photon off in any direction at all.
So it doesn't matter where ANY of the hydrogen atoms are in the photosphere, they have next to no chance of actually hitting Earth with a photon that they emit. Even though there are a lot of them, they are ALL firing off photons in almost every direction EXCEPT towards Earth.
＠LDC3 @Jeremy If we make two integrations at the whole surface of the photosphere. One is at the bottom surface of photosphere. The other is at the bottom surface of the photosphere. The Halpha photon numbers are the same? If they are not the same,where is the lost energy?
Emission and Absorption lines are due to Kirchhoff's Laws that explain what happens when light passes through a gas. This is due to photons being absorbed by an atom or ion causing an electron to jump to a higher energy state, and photons being emitted when an electron returns to a lower energy state.
Photons will only be absorbed by an atom or ion if they are of precisely the energy required to bump an electron up to an excited state. If they are not of the right energy, they pass unimpeded.
Photons of particular wavelengths that are not absorbed by ions in the photosphere come from deep within the photosphere where it is hotter, and travel straight through unimpeded.
Photons of the right energy to be absorbed by a gas present in the photosphere, say hydrogen seeing as you have mentioned h-alpha, have a high probability of being absorbed by an atom on the way through the photosphere, and then being re-emitted in a random direction (including back down!)
Photons that have a high probability of being absorbed and re-emitted are therefore more likely to be coming from the top of the photosphere, or the lower chromosphere, where it happens to be cooler. (During the travel from the lower to upper photosphere, they will have been absorbed and re-emitted).
According to the Stephan-Boltzmann Law for black bodies, the energy radiated per unit of surface area is related to the temperature of the black body. A black body that is cooler will be less bright than a black body that is hotter. So when photons are radiated from a cooler part of the photosphere, they will appear as dips in the spectrum.
Not dissimilar to a black body, except for the absorption lines. Did you read what I wrote regarding temperature at different levels of the photosphere? Follow those links I provided.
I add Figure16.9 to my post. We just compare the total amount of Halpha photons at different layers. The amount of Halpha photons at A is larger than B. If the main reason is not your 'including back down!', where are the reduced Halpha photons?
@questionhang Where did Figure 16.9 come from? We want it in the correct context and not have to guess what the author is showing us.
@LDC3 from Jeremy's second link. (http://astronomy.nju.edu.cn/~lixd/GA/AT4/AT416/HTML/AT41603.htm)
I'm going to delete that from the question. It doesn't contribute to the question, and we don't have rights to use it. It is in the linked material.