What Causes the Lines in the Spectrum of a Star?

[shirin]

I have 3 questions:
1)Does the lines in the spectrum of a star indicate the elements in its surface or also the inner parts?
2)when an electron goes to a higher state, it emits a photon with a wavelength according to that transition, and we can see that line in the spectrum of that star? But what about the ionization? Do we see the wavelength related to energy released because of ionization in the spectrum?

 

[Astronuc]

The bulk of 'visible' light originates in the photosphere, with some light originating in the chromosphere.
http://en.wikipedia.org/wiki/Photosphere
http://csep10.phys.utk.edu/astr162/lect/sun/photosphere.html
http://csep10.phys.utk.edu/astr162/lect/sun/chromosphere.html

Photon emission occurs when electrons move to lower energy states in an atom. The light one sees is determined by the temperature and composition of those regions of the atmosphere that produce the light.

http://hyperphysics.phy-astr.gsu.edu/hbase/mod5.html
http://csep10.phys.utk.edu/astr162/lect/light/bohr.html

See this about the hydrogen spectrum and note which series of lines are visible.
http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html

 

[Ken G]

And in terms of the ionization question, I would add that we do see the energy associated with ionization, although it would be more correct to say we see the energy associated with the inverse process of ionization (which is called "recombination"). When an atom takes in what used to be a free electron, there can be light energy released. This is a very important process, because the light energy released reflects not just the structure of the atom you were just told about, but also the excess energy carried by the free electron initially. Since that excess energy can take on a range of values, recombination emission produces a continuous spectrum of wavelengths-- not just specific wavelengths the way the line transitions do.

For example, when you go outside and are warmed by sunlight, the light that is doing that warming most likely came from the capture of a free electron by a neutral hydrogen atom, creating a bizarre but important ion called the negative hydrogen atom (or "H minus" atom). Thus, sunlight falls under the heading of the type of emission you asked about in regard to ionization processes.

 

[shirin]

Ok, so there is no specific line in the spectrum of stars to tell us about ionization.
Regarding to my first question, I think as the cooler gas of photosphere is between us and hotter gas of internal layers of star, we see absorption lines of elements in the photosphere. But are we sure that the spectrum of internal layers is complete, without any absorption line?

 

[Ken G]

I'm not exactly sure what you are asking. There are specific lines that help a lot in determining the nature of the ionization of the surface layers-- indeed, the 3<-->2 transition in hydrogen (the "Balmer alpha" line) is one such line. That single line is largely responsible for the letter in the "spectral type" of a star, where letters near the beginning of the alphabet (A star, B star, etc.) mean the Balmer alpha line is very strong, and letters later on (M star, O star) mean the Balmer alpha line is virtually absent. To get a letter like A or B, you need a T that is about 10-20 thousand K, because then you get lots of excitation of level 2 but not so much that you completely ionize the hydrogen. Some later letters, like O, are because you got too hot and ionized everything, and other later latters, like M, are because you are too cold and cannot even excite hydrogen to the second level. That's what saddled us with "OBAFGKM" for the rest of astronomical eternity.

As for the internal layers, you are right that we don't see those-- we only see the surface layers. What's underneath just has to be modeled using the laws of physics and some helioseismological constraints, and it all works out well, to the point that neutrino oscillations were first indicated from solar interior models, not particle accelerator experiments. Under the surface, the density is so high that individual lines form a "line blanketed" continuum, so we just have essentially blackbody physics ruled by a kind of average opacity called the "Rosseland mean opacity," rather than particular absorption lines. But there's still a lot of opacity variation from frequency to frequency, and the radiative flux of the star tends to be carried in the wavelength regions where the opacity is lower.

 

[shirin]

How does relative flux follows low opacity? I mean if the relative flux tends to wavelength regions where the opacity is lower, and also it is governed by blackbody model (its temperature determines the wavelength associated to maximum flux), how is the outcome?

 

[ibysaiyan]

I have 3 questions:
1)Does the lines in the spectrum of a star indicate the elements in its surface or also the inner parts?
2)when an electron goes to a higher state, it emits a photon with a wavelength according to that transition, and we can see that line in the spectrum of that star? But what about the ionization? Do we see the wavelength related to energy released because of ionization in the spectrum?

Much of what you are inquiring about has been answered,above.
Look for these terms to help you out in the understanding of Spectral emission.
Emission Spectra,Dark line (fraunhofer lines) Spectra

2) When an electron goes to a higher level,it excites and as such the whole system loses a unit photon. No photon is released.The cases in which photon emission occurs is by Spontaneous emission or Stimulated emission (in lasers).The absorbed lines on the spectra are the "missing wavelengths" which were re-emitted by the atmospheric ions of the star (in random direction)

Regards,
ibysaiyan

 

[Ken G]

How does relative flux follows low opacity? I mean if the relative flux tends to wavelength regions where the opacity is lower, and also it is governed by blackbody model (its temperature determines the wavelength associated to maximum flux), how is the outcome?

It's a strange kind of combination-- there's a kind of "envelope function" based on the wavelengths that correspond to the temperature, but on top of that, there's the increased tendency to get more flux at wavelengths where the opacity is lower. You can find more details if you google "Rosseland mean opacity."