Question on stellar absorption spectra

[ddoctor]

Why don't stellar spectra show absorption lines for carbon, oxygen or nitrogen if they are such abundant elements. Is there a 'spectral type' that does?
Thanks
Dave

 

[Labguy]

Why don't stellar spectra show absorption lines for carbon, oxygen or nitrogen if they are such abundant elements. Is there a 'spectral type' that does?
Thanks
Dave

It is because H is most predominant in any star's photosphere and some other elements can, and some can't, make their way to the photosphere. It is only from the photosphere that the emissions we can see are produced, and any absorption lines. It is the "Binding Energy" of each elements propensity to "hang on" to electrons that allows, or doesn't allow, that element to be in enough quantity at the photosphere to permit absorption lines (that we can measure).

At:
http://www.astronomynotes.com/starprop/s12.htm#msprop there is a decent explanation about half-way down the page:

Hydrogen lines will be strong for temperatures = 4,000 to 12,000 K. Helium atoms hang onto their electrons more strongly and, therefore, require higher temperatures of 15,000 to 30,000 K to produce absorption lines in the visible band. Calcium atoms have a looser hold on their electrons so calcium lines are strong for cooler temperatures of 3000 to 6000 K. The strengths of each element's absorption lines are sensitive to the temperature. A given strength of an element's lines will give you either two possible temperatures for the star or a range of possible temperatures. But using two or more element's line strengths together narrows the possible temperature range. Cross-referencing each elements' line strengths gives an accurate temperature with an uncertainty of only 20 to 50 K. This technique is the most accurate way to measure the temperature of a star.

 

[Janus]

Class O stars show lines of doubly and triply ionized oxygen and nitrogen.
Class N stars show carbon bands

 

[ddoctor]

So basically what you are saying is that C, N, O are not present in the photosphere in sufficient quantities under the right temperature conditions in relation to their respective binding energies to produce significant absorption spectra? However there are a couple of spectral types where you might see them? O stars are the hottest stars, so I am surprised that O and N can be seen. Any references would be terrific.

I greatly appreciate your responses!
Dave

 

[SpaceTiger]

To add to what has already been said...

Why don't stellar spectra show absorption lines for carbon, oxygen or nitrogen if they are such abundant elements. Is there a 'spectral type' that does?

One could just as well ask why hydrogen lines are weaker than calcium lines when hydrogen is a million times more abundant. Unfortunately, there is no simple answer for any given ion or transition, but in general, the strength of an absorption line is determined by:

1) The abundance of the atom or molecule in question.
2) The frequency of the ionization state from which the transition can be produced.
3) The frequency of the energy state of the ionization state from which the transition can be produced (for example, the n=2 state of hydrogen vs. n=1).
4) The oscillator strength of the transition vs. other transitions from the same energy state (e.g. n=2->1 vs. n=3->1).

The functions that govern these things are by no means simple, so one shouldn't take the abundances as a sole indicator of line strengths. Items 2 and 3 are determined primarily by the density and temperature in the photosphere and item 4 is determined by atomic physics.

 

[Astronuc]

According to http://csep10.phys.utk.edu/astr162/lect/energy/cno-pp.html

In the case of the Sun, detailed considerations suggest that it is producing about 98-99% of its energy from the PP chain and only about 1% from the CNO cycle.

and there is obviously a temperature dependence on the CNO cycle - as the start temperature increases, so does the rate of CNO fusion - as well as age, which would determine composition, besides the initial composition.

Now, is there a database that describes the proportion of PP / CNO energy generation for say, the 200 brightest stars?

http://www.palmbeachastro.org/stars.htm (200 brightest stars)

http://www.seds.org/Maps/Stars_en/ (brightest stars up through M=2.5)

Presumably, compositions have been determined from optical spectroscopy.

 

[SpaceTiger]

as the start temperature increases, so does the rate of CNO fusion - as well as age, which would determine composition, besides the initial composition.

At the moment, the sun can't dredge up the products of nuclear burning from its core because it's only convective about a third of the way down, so the initial conditions are pretty much the whole story for abundances in the photosphere.

 

[Astronuc]

So the comment about the sun being 98-99% PP and 1-2% CNO is not necessarily valid? In which case, the core may have (or does have) a greater proportion of fusion occurring via CNO?

I am not up on stellar structure as I would like to be, but how does the convective layer of the sun (G2, or G2V) compare with the convective layer of other stars of its type/size and with other largers stars?

 

[SpaceTiger]

So the comment about the sun being 98-99% PP and 1-2% CNO is not necessarily valid? In which case, the core may have (or does have) a greater proportion of fusion occurring via CNO?

No, I'm sure that number is fine, I'm saying that the chemical composition of the core doesn't mix with the photosphere, where these lines are produced.

I am not up on stellar structure as I would like to be, but how does the convective layer of the sun (G2, or G2V) compare with the convective layer of other stars of its type/size and with other largers stars?

Low-mass stars, like M dwarfs, are convective all the way from surface to core, while very high mass stars are convective only in the core. The convection zones of stars of the same spectral type would all be about the same, though there would be some small variations with metallicity.

 

[Labguy]

Now, is there a database that describes the proportion of PP / CNO energy generation for say, the 200 brightest stars?

http://www.palmbeachastro.org/stars.htm (200 brightest stars)

http://www.seds.org/Maps/Stars_en/ (brightest stars up through M=2.5)

The "brightness" of the stars you link to are all given as "Apparent Magnitude" and would have nothing at all to do with their intrinsic brightness; Absolute Magnitude. It is often the case where a dim star is so near that it appears bright, and vice-versa.
http://www.astronomynotes.com/starprop/s4.htm

Low-mass stars, like M dwarfs, are convective all the way from surface to core, while very high mass stars are convective only in the core. The convection zones of stars of the same spectral type would all be about the same, though there would be some small variations with metallicity.

This paragraph is true, but does not mention "medium-small" stars like our sun and a bit of the main sequence on either side of the sun. These sun-like stars have a radiative core and convective outer layers. So, we have:
- Low mass stars: Convective cores and mantels.
- Mid mass stars: Radiative cores and convective mantels.
- High mass stars: Convective cores, radiative inner mantels, convective outer mantels.
- All types, of course, have radiative photospheres.

Anyone care to guess why the massive stars would have convective cores? It is (should be) in all the books.

 

[SpaceTiger]

This paragraph is true, but does not mention "medium-small" stars like our sun and a bit of the main sequence on either side of the sun.

That's because I mentioned it in the previous post.

 

[Labguy]

That's because I mentioned it in the previous post.

I didn't see anything about

- Low mass stars: Convective cores and mantels.
- Mid mass stars: Radiative cores and convective mantels.
- High mass stars: Convective cores, radiative inner mantels, convective outer mantels.

except that the sun is convective about 1/3 of the way down..

The high-mass stars are the most interesting in terms of energy transfer; agreed?

 

[SpaceTiger]

I didn't see anything aboutexcept that the sun is convective about 1/3 of the way down..

Astronuc was talking about the chemical abundances in the core of the sun and I was pointing out that it wouldn't matter because the convective layer did not go all the way to the core. Unless I'm misunderstanding something, this is the same thing that you're saying in the "Mid mass Stars", though I've never heard the term "mantle" used in reference to the sun. Is this to be the convective layer?

The high-mass stars are the most interesting in terms of energy transfer; agreed?

Well, I don't disagree, per se, but perhaps you can expand on why you find them so fascinating.

My research has been heavy in the ISM, so I find radiative transfer fascinating in general. I'll be happy to respond to the question you posed in the previous post, but it wouldn't really be a guess, so I didn't want to spoil the fun.

 

[Labguy]

Astronuc was talking about the chemical abundances in the core of the sun and I was pointing out that it wouldn't matter because the convective layer did not go all the way to the core. Unless I'm misunderstanding something, this is the same thing that you're saying in the "Mid mass Stars", though I've never heard the term "mantle" used in reference to the sun. Is this to be the convective layer?

True, the term "mantel" is used most often to describe outer layers of planets, etc. In my post I should have used the terms core, radiative envelope and convective envelope.

The high-mass stars are the most interesting in terms of energy transfer; agreed?

Well, I don't disagree, per se, but perhaps you can expand on why you find them so fascinating.

My research has been heavy in the ISM, so I find radiative transfer fascinating in general. I'll be happy to respond to the question you posed in the previous post, but it wouldn't really be a guess, so I didn't want to spoil the fun.

Fascinating only because the densities of the core are so high that even the most energetic EM radiation (gamma) can't pass freely until a less dense area is encountered, so they have convective cores, radiative inner envelopes and convective outer envelopes. Just a more complicated mechanism to transfer the core's fusion energy to the photosphere. This isn't analogous to radiative transfer in the ISM is it, ie hindered by high densities? Otherwise, the question was rhetorical, as was the previous sentence.

 

[SpaceTiger]

This isn't analogous to radiative transfer in the ISM is it, ie hindered by high densities?

There are certainly much higher densities in stars, but then radiative transfer is radiative transfer. Whether or not its analogous depends on exactly which characteristics you wish to highlight and which parts of the ISM you're talking about. I hope this isn't a rhetorical question as well...

 

[Chronos]

It appears we are talking about Jean's mass limits here. In that case, it is not that difficult to quantify. The only difficulty is constraining the parameters, as I understand it.

 

[Chronos]

Are we not back to the surface of last scattering thing when it comes to photon emissions from stars? I was under the impression convective layers captured and suppressed photons from escaping from stellar cores for a huge number [perhaps millions] of years.

 

[Astronuc]

The "brightness" of the stars you link to are all given as "Apparent Magnitude" and would have nothing at all to do with their intrinsic brightness; Absolute Magnitude. It is often the case where a dim star is so near that it appears bright, and vice-versa.
http://www.astronomynotes.com/starprop/s4.htm

Labguy, thanks for the link. Yes, I was aware that I was referring to stars by apparent rather than intrinsic magnitude. Those are the easiest for observers on the Earth to 'see', and I am sure that there are databases with a much greater quantity of stars. If anyone could point me to such a database, I would appreciate it.

Here's what NASA has about the sun's structure - http://genesismission.jpl.nasa.gov/science/mod3_SunlightSolarHeat/SolarStructure/

Then there is a table of the structure of the sun according to http://zebu.uoregon.edu/~js/ast121/lectures/lec22.html

My old textbook (undergrad), from about 30 years ago, had precious little on the details. Iwould appreciate any recommendation on texts (and papers) about stellar structure.

Now from what I have read, the core of the sun is He-rich. So much of the rest of the sun is H, with traces of C, N, and O. Clearly the sun's color and spectrum reflect the relatively cool photosphere (~6000 K) and it's predominantly H-rich composition.

 

[SpaceTiger]

My old textbook (undergrad), from about 30 years ago, had precious little on the details. Iwould appreciate any recommendation on texts (and papers) about stellar structure.

If you're not interested in the really nitty-gritty theory, then The Physics of Stars is an excellent text. It's aimed at the undergraduate level. For a more sophisticated treatment, see Stellar Structure and Evolution.

 

[SpaceTiger]

It appears we are talking about Jean's mass limits here. In that case, it is not that difficult to quantify. The only difficulty is constraining the parameters, as I understand it.

Can you expand on the connection you're drawing to the Jean's mass? I'm afraid I'm not seeing your point.

Are we not back to the surface of last scattering thing when it comes to photon emissions from stars? I was under the impression convective layers captured and suppressed photons from escaping from stellar cores for a huge number [perhaps millions] of years.

I've heard a range of numbers, ranging from 50,000 to 10 million years, but it's certainly much less than the time to the surface of last scattering (~10 billion years). Perhaps Labguy knows some more precise numbers from the latest models.

If you're instead referring to the photosphere as the "surface of last scattering" for solar radiation, then you're right. However, the timescale for the diffusion of atoms from the center much longer than this. Mixing is basically impossible without convective processes.

 

[Labguy]

There are certainly much higher densities in stars, but then radiative transfer is radiative transfer. Whether or not its analogous depends on exactly which characteristics you wish to highlight and which parts of the ISM you're talking about. I hope this isn't a rhetorical question as well...

The only "characteristic" I'm referring to here is opacity due to extreme densities. I don't know if the ISM has such dense areas or not. http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy213/phy213_opacity.html and: http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy213/phy213_convection.html

The criterion for convection derived above can be satisfied in two ways: either the ratio of specific heats is close to unity or the temperature gradient is very steep. If a large amount of energy is released in a small volume at the centre of a star, it may require a large temperature gradient to carry the energy away. This means that convection may occur at the centres of stars where nuclear energy is being released. Such regions are known as convective cores. Alternatively, in the cool outer layers of a star, where the gas is only partially ionized, much of the heat used to raise the temperature of the gas goes into ionization and hence the specific heat of the gas at constant volume is nearly the same as the specific heat at constant pressure and ~1. In such a case a star can have an outer convective layer.

and: http://www.shef.ac.uk/physics/people/vdhillon/teaching/phy213/phy213_detailed.html

In stars with masses below about 1.3M , the surface layers are cool enough to be partially ionised and they are hence unstable to convection. The structure of solar-type stars then consists of a hot radiative core, at the centre of which the nuclear energy is produced, surrounded by a cool convective envelope. The envelope is shallow for stars near the upper-mass limit but becomes gradually deeper for lower-mass stars, and stars with masses less than about 0.3M are believed to be fully convective. In stars with masses greater than about 1.1M , the central temperature is high enough for the CNO cycle to operate. The CNO cycle is much more sensitive to temperature than the proton-proton chain and requires a much steeper radiative temperature gradient, which is unstable to convection. Stars considerably more massive than the Sun therefore have a convective core, which contains the energy-generating regions, and a radiative envelope. Stars just a little more massive than the Sun may have both a small convective core and a shallow convective envelope. The mass in the convective core increases with the total mass of the star and very massive stars, like very low-mass stars, may be fully convective.

which we have already discussed.

Iwould appreciate any recommendation on texts (and papers) about stellar structure.

Here are a few but not all are just about "sun-sized" stars. Sorry for the low-tech content of some of them.
http://www.ucolick.org/~woosley/lectures_winter2004/lecture16.pdf#search='stars%20burning%20shells' ; pages 19,20 and 21 specifically, and:
http://www.ifa.hawaii.edu/~szapudi/astro110/ch22.pdf#search='stars%20burning%20shells' ; pages 2,5 and 14 for stars of <4 M_sun, and:
http://www.astro.sunysb.edu/fwalter/AST101/death_of_stars.html specifically for the sun. There are tons of other pages out there, but I like Woosley's the best.

 

[SpaceTiger]

The only "characteristic" I'm referring to here is opacity due to extreme densities. I don't know if the ISM has such dense areas or not.

Of course not, but an analogy implies the similarity of ideas, not the exact matching of behavior. My only statement was:

My research has been heavy in the ISM, so I find radiative transfer fascinating in general.

...implying that although I found the inner regions of massive stars interesting in terms of energy transport, I found the other extremes (low densities and pressures) to be interesting as well. I don't really see how anyone can object to that statement.

 

[Labguy]

...implying that although I found the inner regions of massive stars interesting in terms of energy transport, I found the other extremes (low densities and pressures) to be interesting as well. I don't really see how anyone can object to that statement.

I guess I simply don't understand your language. I was NOT objecting to anything. And, all I was saying is that I find the energy transport mechanisms in massive stars interesting also, after you stated that:

My research has been heavy in the ISM, so I find radiative transfer fascinating in general.

Something wrong with us both being interested in radiative transfer of energy...

That question doesn't actually require an answer! (Please)

 

[SpaceTiger]

I guess I simply don't understand your language.

Likewise.

In particular, I don't understand why you seem to be acting so stand-offish. Rhetorical questions are difficult on the internet because you can't hear the person's tone of voice. I'm sorry if I misinterpreted you, but it's really hard for me to tell which questions you mean to be answered, so please bear with me.

 

[Chronos]

A careless remark. My intent was to compare the CMB surface of last scattering with the tortured path a solar photon must weave through to 'free' itself from our sun. I find the analogy compelling. Perhaps it's similar to what the CMB photons encountered in the 'primordial soup' of the big bang... just a thought.

 

[Nereid]

Somewhat OT, but if you're interested in this sort of thing, one of the challenges for those doing stellar models was (I believe it's now solved) He ionisation in massive stars. Apparently there is a region of composition/mass/(maybe other factors) space where this ionisation occurs at the convective/radiative transport boundary - leading to all kinds of modelling challenges (the relevant stars, of course, just go do their thing, oblivious to the simulation difficulties of astrophysicists).

Something like 'if the boundary is here, then He is ionised in the convective zone; but it's unstable, a small change and it's ionised in the radiative zone, but that throws some equilibrium conditions off, so ... doggone it, how *do* we set this puppy up to avoid these conundrums?!'