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If Sun = 98% H & He, why continuous spectrum?

  1. Nov 28, 2004 #1
    If the Sun is 98.4 % Hydrogen and Helium by mass, and 99.8% by number of atoms, why does it radiate a continuous spectrum of light?

    Why doesn't it radiate a bright emission line spectrum only at the Hydrogen alpha, beta, gamma, etc. lines plus the Helium lines, and little or no lines at other wavelengths corresponding to the other insignificant traces of elements in the Sun?

    Losing sleep in Cincinnati.
  2. jcsd
  3. Nov 28, 2004 #2


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    Again, the question contains the answer: those are emission lines. They happen when light passes through a filter (such as an emission nebula). While there is quite a bit of light emitted at the hydrogen line, the vast majority is emitted due to black body radition. Hot objects emit radiaion at all frequencies.
  4. Nov 28, 2004 #3
    Hey Russ. I appreciate your quick responses. And I know you're a smart guy, I've seen your posts here before. But please read these questions a little slower.

    My question is : How can a star that is almost completely made up of TWO elements, radiate at ALL frequencies? I'm not asking what H and He emission lines are. I'm asking why isn't that all you get when taking a spectroscopic reading of the Sun. Or why isn't that MOSTLY what you get, since that's MOSTLY what's in the Sun. What you actually get is a smooth distribution of light at all frequencies, with a peak somewhere in the yellow. I would expect you to get overwhelmingly strong peaks at the H and He emission lines. But you don't. Why?
  5. Nov 28, 2004 #4


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    I dont have anything to say that contradicts what russ said, but I would just like to talk about the thermal radiation from the surface of a star

    suppose for simplicity the star is ALL hydrogen and suppose it has about the same temperature profile as the sun----in the outer partially ionized layers

    the average temp they quote for sun is around 5700 kelvin but the outer layers are really a layercake of different temps----like any ocean or any atmosphere---so that 5700 they quote is some kind of "effective" temperature which gives a good one-number handle on things but does not tell the whole layercake story----for that you look in the handbook

    so near the surface (the handbook says) there is a 7000 K layer and a 6000 layer and 5000 and a 4000 layer----the surface is not sharp defined and it has an "optical depth"

    5000 is not hot enough to ionize all the H or even to keep all the atoms in an excited state! So it is a mixed bag.
    in the surface there is some molecule H2 and also some molecule broken into some atomic H and also some
    ionized, and some FREE ELECTRONS.

    So be reductive-minded and think about a cloud of free electrons all by itself, in thermal motion, colliding and bouncing and buzzing around like crazy because very hot. A simple hot cloud of electrons, say 5700 kelvin, has a simple theory and it is going to for sure tell part of the story.

    THAT CLOUD OF FREE ELECTRONS, in frantic thermal motion, IS GONNA DO THERMAL PLANCK BLACKBODY RADIATION: like one of thos suckers does not even know about a hyrdogen atom and its energy levels,he has totally forgotten the hydrogen atom context with its levels and spectral lines.

    it is just loose, in random motion

    and the most basic thneory of light says that random accelerations of charges radiates random wavelength photons

    and plancks 1900 or 1901 theory says black body is a continuous curve like a lopsided mound or a woman's breast
    like a bell curve but lopsided skewed, so not like a bell curve

    so think about thermal motion of hot electrons and think about
    black body and come back and ask

    it is a good question!
    Last edited: Nov 28, 2004
  6. Nov 28, 2004 #5


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    I don't know if it's relevant, but don't forget that while the orbital energy of the bound electrons is quantized, the kinetic energy of the atoms is not.
  7. Nov 28, 2004 #6


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    that is right, thermal motion of any charged species will radiate thermal (continuous spectrum)
    so not only the electrons are involved but also the H+ ions and other stuff
    (and for sure there is doppler broadening as well)

    however for simplicity we can focus on the free electrons
    at a given temp, they will be moving around a lot more and doing much
    more collisions and (the key thing) accelerations
    because the electron is only 1/1836 of the proton mass-wise
    means that to be in equilibrium with the temperature the electron
    has to move around---roughly by a factor of 1836---more frantically

    so I think that is the species that is radiating most of the light
  8. Nov 28, 2004 #7


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    I did answer the question: The answer is that all hot objects radiate at all frequencies regardless of their composition. Composition is only relevant to absorption/emission specra and has nothing to do with blackbody radiation. I think you're hung up on emission spectras when they aren't relevant here.
  9. Nov 28, 2004 #8


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    The problem, I think, is that the only way to "create" light that is explained in introductory texts is emission -- the mechanism of black body radiation is not described at all. (At least I don't remember it from my freshman physics courses)
  10. Nov 28, 2004 #9


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    The sun produces energy primarily by the PP reaction and about 1-2% by the CNO cycle -

    In addition to H and He, there are quantities of Li, Be and B, and they will be producing their emission spectra as well.

    There are numerous reactions generating photons. And in at least one case, where a positron is produced, the positron will annihilate with a electron to produce 2 gamma-rays of 0.511 MeV.

    Clearly, the recombination and de-excitation of the protons and He atoms and ions will produce characteristic frequencies (emission spectrum). But consider the fact that the atoms/ions are moving in various directions (i.e. they are not static), so due to Doppler broadening, the emission lines are broadened.

    In addition to the gamma-rays produced, charged particles moving perpendicular to magnetic fields lines (and there are strong magnetic fields in the sun) produce cyclotron radiation (X-rays). As charged particles move through matter, Brehmsstrahlung radiation is produced. Now all these gamma-rays and X-rays are scattered by the electron fields in atoms, and consequently a broad spectrum of radiation is produced, of which visible is a small part. The sun's outer layers are relatively transparent to the light frequencies which do not precisely match the characteristic energy levels of the elements in the sun.

    Also, look at the solar structure - http://www.physics.nau.edu/~lavery/A180/Lectures/A180week12.html

    The Photosphere: The "surface" of the Sun - Temperature is about 6000 K

    The Chromosphere:
    Thin, cooler transparent (nearly clear) layer above the Photosphere. Temperature T = 5000 K (though regions get down as low as 4000 K and as high as 1,000,000 as one gets close to the Corona).

    The Corona:
    Very tenuous (the density is very low), very hot, outer region of the Sun's atmosphere.
    The temperature is about 2,000,000 K
  11. Nov 28, 2004 #10


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    hurkyl you have put yr finger on the main pedagogical point!
    they should tell you very early how hot objects actually manage to glow
    (with a continuous spectrum planck black body lopsided mound glow)

    but they mostly DONT tell you
    so people like Smith are going around trying to apply the only model they know for producing light-----namely an electron jumps down one notch---and that is not how most light is produced!

    so if Smith were still around, here is what I suggest----turn him on to the CalTech online animation that shows how an electron accelerates or decelerates and by that very act it emits a photon----and you can control the electron motion with the mouse---and you can see the photon coming out.


    so if Smith comes back to the thread lets give him the link

    and also the pluscharge nuclei in a crystal lattice are jiggling obviously, with the heat, and that is acceleration, and acceleration of any charge maketh light

    but the main thing is, first get a good vision of the simple case of an electron doing it
    Last edited: Nov 28, 2004
  12. Nov 28, 2004 #11


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    Stellar Spectroscopy

    This might also be of interest -

    Spectroscopy: Unlocking the Secret in Starlight

    Spectral classes

    Information from Astronomical Spectra http://outreach.atnf.csiro.au/education/senior/astrophysics/spectra_info.html

    Effective (Surface) Temperature and Wien's Law
    The Doppler Effect
    Translational Motion
    Rotational Motion
    Stellar Density and Pressure Broadening
    Chemical Composition
    Other Information
  13. Nov 28, 2004 #12
    YES, YES, YES !!! Russ, Hurkyl, Marcus, Astronuc, you've just nailed my problem. I was assuming that the only way to produce a photon was for an excited electron to drop a level or two. And that is all I was taught in my freshman physics class. We did study blackbody radiation, but only in generalities, the Ultraviolet catastrophe, etc.

    Never did we learn, as Russ said, that a hot object will radiate at all frequencies, regardless of its composition. I assumed that such black bodies were made of a mixture of lots of elements, and that a continuous spectrum was produced only because so many elements were all producing a variety of emission spectra and they "filled up the spectrum".

    I'd love to get the link to the CalTech site Marcus mentioned. Couldn't find it on Google.

    And I will check out Astronuc's links as well and also go back and re-read my text books more thoroughly. Maybe I just missed it.

    Thanks all! I'll let you know when I finally "get it."

    Last edited: Nov 28, 2004
  14. Nov 28, 2004 #13


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    one place is in this big list of EM physics resources links

    some ways down that list you see the Calteck MovingCharge applet

    try selecting different types of motion
    circular motion (contstant acceleration) is nice
    any acceleration causes the charge to radiate
    the ripples in the electric field spread out
    at the speed of light (which is slow in this applet so you can see)
  15. Nov 28, 2004 #14


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    in hot solids the atoms of the crystal lattice jiggle
    like an array of balls connected by springs
    so it is lke a great mob of harmonic oscillators

    the nuclei are going to jiggle somewhat independently of the electrons (especially the spread out conduction electrons) because
    they are more massive and held more firmly on location
    so what is jiggling IS NOT electrically NEUTRAL

    whenever an electrically charged thing jiggles it is accelerating so you can
    expect a photon now and then

    but at the surface of the sun my intuition says that most of the radiating is done by loose electrons, because they are like a swarm of bees and really dancing-----a short mean free path, a high speed, many collisions
    I could be wrong. but to first order approx I would guess you can just picture it as a hot "gas" of electrons
  16. Nov 28, 2004 #15
    Thanks !! Let's review.

    Thanks Marcus, that's very enlightening. That demonstration makes it easy to see how an accelerating charge could produce almost any wavelength of light.

    Let's see if I'm learning this correctly.

    Visible Light waves are caused by the motion of electrons in the following ways: One way is when atomic orbital electrons drop from one energy state to a lower one, thereby producing photons at discrete (quantized) wavelengths, corresponding to the change in energy levels.

    Another way is by an accelerating electron. (I'm assuming a quantum leap of an electron is not considered "acceleration"). That acceleration can be caused by a number of reasons, I assume. In the case of heat, the physical collisions of atoms changes the direction of atomic electons (potentially causing light). Can heat also be manifested in vibrations of atoms. Or they can be accelerated by changes in other electric or magnetic fields present (like in the Sun's corona).

    Any other ways visbile light is produced?

    Further, other wavelengths of E-M waves outside the visible spectrum can be produced in those same ways, plus a few others (did some reading since last night). Gamma rays can be produced by a decaying neutral pion, or mutual annihilation of an electron and a positron.

    Net, E-M waves can be produced in three general methods: 1) drop in electron energy level, 2) acceleration of an electron due to any cause (blackbody radiation is an example of this kind), 3) nuclear reactions (like the two examples above).

    All three methods occur in the Sun:

    Mostly #3 happens in the Sun's core;

    It's mostly number 2 that is responsible for the majority of the VISIBLE light that we see that comes from the photosphere. I think Russ said that it was thermal blackbody radiation that caused most of the light from the Sun. You suspect it is from freely moving and accelerating electrons (not with an atom). But both are of due to accelerating electrons.

    And we're mostly familiar with #1 in the Sun's chromosphere where Hydrogen atoms absorb and then re-emit photons in the spectral lines of H-alpha, H-beta, and so on.

    What in the above is not right?

    What have I left out that is important?

    Thanks everyone for all your help in this very complicated matter (at least to me.)

    Now I can finally get some sleep.

    Sleeping in Cincinnati
    Last edited: Nov 28, 2004
  17. Nov 28, 2004 #16


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    I didnt see any mistakes or omissions. Maybe someone else will
    look and find some. Im sometimes careless and dont catch things.

    I really urge playing with that moving charge toy. there are some deeper things there. if something is in steady motion (no accel) then the field somehow knows where the charge is going to be and the lines are straight
    acceleration causes a kind of shearing kink in the fieldlines (if it is abrupt) like a whipcrack

    I like the old Central Railroad Station in Cinn'ti. It has 1930s mosaics.
    Is it still there. beautiful old building. Ohio river nice along there, or used to be. A very old iron bridge looking like it was built before 1910 (rough guess). Wonder if it has fallen into the river yet. City laced thru with municipal parks. Cool town, or used to be.

    the shear is in the opposite direction. So if one electron is abruptly slowed down it sends a message to the one immediately beside it, in the next lane, telling it to speed up------thats why coils make sparks because of selfinductance----electricity flowing in a coil does not like to have to stop
    and so if you break flow it will arc.

    you can tell this from watching the Caltech moving charge animation.

    and you can see what radio waves coming off an antenna look like.
    just tell it to do sinewave motion back and forth in a straight line segment (the antenna)

    you can learn a lot of E&M from the bottom up just by playing with that.

    You did a good job asking questions and summarizing. What kind of products does your team marketresearch?
  18. Nov 29, 2004 #17
    Thanks again Marcus (and others). I've learned a lot.

    Incidentally, I originally asked this question because I recently purchased a new telescope with a Hydrogen-alpha filter (Coronado PST), and it got me to thinking about what I was actually looking at on the Sun and why this filtering mechanism worked, etc. So, now I have a much better idea.

    Marcus, I'll send you a personal post on the other questions.

    Thanks again all!

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