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The spectrum of sunlight is a continuous spectrum as in the rainbow. But superimposed on the spectrum are dark absorption lines of some specific frequencies. From the absorption lines, we are able to tell that the main elements of the sun is hydrogen, iron, carbon, helium and some others.

My question is why the continuous spectrum ? Elements have characteristic emission line spectra. So every specific wavelength in the sun's continuous spectrum is associated with one (?) element which has that wavelength in its line spectrum. Does it mean that the sun has all 108+ known elements?      

[edit] Or the emission lines of the main elements in the sun - iron,carbon, etc. - sufficient to form the continuous spectrum.

Edited by avicenna
further info.

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21 minutes ago, avicenna said:

The spectrum of sunlight is a continuous spectrum as in the rainbow. But superimposed on the spectrum are dark absorption lines of some specific frequencies. From the absorption lines, we are able to tell that the main elements of the sun is hydrogen, iron, carbon, helium and some others.

My question is why the continuous spectrum ? Elements have characteristic emission line spectra. So every specific wavelength in the sun's continuous spectrum is associated with one (?) element which has that wavelength in its line spectrum. Does it mean that the sun has all 108+ known elements?      

[edit] Or the emission lines of the main elements in the sun - iron,carbon, etc. - sufficient to form the continuous spectrum.

The continuous emission spectrum is due to the surface temperature of the Sun.

This is obviously hotter than that of a steel bar heated to red or white heat, but the principle is the same.

see stephans law

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

 

The black lines are absorbtion lines.

That means certain wavelengths this thermal luminescence are removed.
This happens as the light travels outwards from the Sun's surface through its atmosphere.

And yes they are specific to the elements that are in the very outer layers of the Sun and in its atmosphere.

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"he continuous emission spectrum is due to the surface temperature of the Sun.

2 minutes ago, studiot said:

The continuous emission spectrum is due to the surface temperature of the Sun.

This is obviously hotter than that of a steel bar heated to red or white heat, but the principle is the same.

 

According to the Bohr model, light of a specific frequency is emitted due to the difference of two energy states of one (? or a few) element. Why a white hot iron bar can emit white light?  

 

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Thermal agitation of the particles.

I say particles because in the case of the steel bar the particles are the atoms bonded to each other, forming the steel alloy structure.

In the case of the Sun the particles are in the plasma state and in sigfnificant motion because of their temperature.

 

The Bohr spectra come from promoting electrons within an atom between discrete energy levels, just as you say.

The thermal spectra come from vibration of the bonded atoms about the bond length/angle without breaking the bond.

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On 10/3/2019 at 4:20 AM, avicenna said:

"he continuous emission spectrum is due to the surface temperature of the Sun.

According to the Bohr model, light of a specific frequency is emitted due to the difference of two energy states of one (? or a few) element. Why a white hot iron bar can emit white light?  

The blackbody process does not depend on electrons being excited; that will give a discrete spectrum, and studiot has already pointed out that excitations are responsible for the absorption lines.

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The Sun is so hot that most of the gas is ionised.

Free electrons and protons are allowed to have (almost ) any energies.

When they crash into eachother some fraction of the energy- depending on angles of impact etc, is sent out as em radiation.

 

Also, at those temperatures the "lines" are broadened by doppler shift and at the high pressures involved they are also broadened by collisions.

https://en.wikipedia.org/wiki/Spectral_line#Line_broadening_and_shift

 

The overall effect is a mass of overlapping emission bands which  approximate (rather well) to a black body spectrum.

 

 

 

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5 minutes ago, John Cuthber said:

The Sun is so hot that most of the gas is ionised.

Free electrons and protons are allowed to have (almost ) any energies.

When they crash into eachother some fraction of the energy- depending on angles of impact etc, is sent out as em radiation.

 

Also, at those temperatures the "lines" are broadened by doppler shift and at the high pressures involved they are also broadened by collisions.

https://en.wikipedia.org/wiki/Spectral_line#Line_broadening_and_shift

 

The overall effect is a mass of overlapping emission bands which  approximate (rather well) to a black body spectrum.

 

 

 

Excellent point I should have mentioned. +1

The atoms in the Sun are in the 'fourth state of matter' - the plasma state.

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Posted (edited)
On 10/3/2019 at 9:44 AM, avicenna said:

The spectrum of sunlight is a continuous spectrum as in the rainbow. But superimposed on the spectrum are dark absorption lines of some specific frequencies. From the absorption lines, we are able to tell that the main elements of the sun is hydrogen, iron, carbon, helium and some others.

My question is why the continuous spectrum ?

Any object with temperature T is emitting photons. How much energy have these photons depend on temperature of the body. Your biological body is also emitting infra-red photons (therefor one can use IR thermometer to remotely check temperature, or IR camera to detect people or animals from longer distance). If body is enough hot, there are emitted visible photons (and your eyes are able to detect them as light).

Please read black-body radiation article on Wikipedia for more details about this subject:

https://en.wikipedia.org/wiki/Black-body_radiation

 

Edited by Sensei

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I think correct about plasma.

7 hours ago, Sensei said:

Any object with temperature T is emitting photons. How much energy have these photons depend on temperature of the body. Your biological body is also emitting infra-red photons (therefor one can use IR thermometer to remotely check temperature, or IR camera to detect people or animals from longer distance). If body is enough hot, there are emitted visible photons (and your eyes are able to detect them as light).

Please read black-body radiation article on Wikipedia for more details about this subject:

https://en.wikipedia.org/wiki/Black-body_radiation

I think this is the answer. Thanks.

 

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On 10/4/2019 at 7:45 PM, Sensei said:

Any object with temperature T is emitting photons. How much energy have these photons depend on temperature of the body. Your biological body is also emitting infra-red photons (therefor one can use IR thermometer to remotely check temperature, or IR camera to detect people or animals from longer distance). If body is enough hot, there are emitted visible photons (and your eyes are able to detect them as light).

Please read black-body radiation article on Wikipedia for more details about this subject:

https://en.wikipedia.org/wiki/Black-body_radiation

 

That's interesting.

If, like the Sun, I was made from hydrogen and helium, what would my emission spectrum be like? (assuming I'm at 37C and 1 atmosphere pressure).

Would it look like black body radiation?

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You also have to be in approximate thermal equilibrium. Though as the link mentioned you can do a first order approximation 

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It's a matter of opacity too (and of reflectance). Stars are opaque because they're big. If they can absorb any incoming wavelength, they can radiate it perfectly too, hence like a blackbody does. The temperature varying with the depth makes this more complicated.

More reasons to emit or absorb other wavelengths than the atom's discrete spectrum:

  • In a solid, electrons are shared among many atoms. They get many new energy levels. In a metal, the levels are extremely close to an other.
  • A transition line is fine only if the emission is slow enough. Transitions have their own duration, which is usually shortened by collisions, in a gas, plasma, liquid.

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2 hours ago, Enthalpy said:

It's a matter of opacity too (and of reflectance). Stars are opaque because they're big. If they can absorb any incoming wavelength, they can radiate it perfectly too, hence like a blackbody does. The temperature varying with the depth makes this more complicated.

Does it? The sun's blackbody spectrum is consistent with the surface temperature, not the internal temperature. Photons from the hotter interior regions are absorbed before they reach the surface, and thus make no contribution to the observed spectrum.

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And?

The chromosphere is thick enough to be opaque, so we see only the chromosphere, which radiates like a black body at its temperature. Nothing to change in my previous post.

The line absorption spectrum needs atoms cooler than the blackbody radiation and between the emission depth and us. Hence "temperature varying with the depth".

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28 minutes ago, Enthalpy said:

.The line absorption spectrum needs atoms cooler than the blackbody radiation and between the emission depth and us. 

Why?

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