# Binary system and doppler effect

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#1
In order to find out what elements are present in a star it's absorption spectrum's are observed and wherever the black lines shows indicates the elements in the star.

However due to the doppler effect in a binary system of stars how can you figure out the elements present with absorption lines? Do you observe the stars when they are at a total eclipse a there is no doppler shift because there relative motion to the earth is entirely perpendicular? Or is there another way?

Lastly, when observing the doppler shift do you use an absorption spectrum or do you directly observe the wavelength of light that you see and plot it on the spectrum and hence giving spectrum lines?

I might have mixed ideas and concepts as I am confused on how this works :P

Thank you in advance for anyone who can explain this to me
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6 years ago
#2
(Original post by MSB47)
In order to find out what elements are present in a star it's absorption spectrum's are observed and wherever the black lines shows indicates the elements in the star.

However due to the doppler effect in a binary system of stars how can you figure out the elements present with absorption lines? Do you observe the stars when they are at a total eclipse a there is no doppler shift because there relative motion to the earth is entirely perpendicular? Or is there another way?

Lastly, when observing the doppler shift do you use an absorption spectrum or do you directly observe the wavelength of light that you see and plot it on the spectrum and hence giving spectrum lines?
The stars will both recede and advance as they orbit each other. The spectrum therefore oscillates about a mean (tangential) value from which the true spectral composition of both stars can be extracted.

The answers should be self evident from these diagrams:

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#3
(Original post by uberteknik)
The stars will both recede and advance as they orbit each other. The spectrum therefore oscillates about a mean (tangential) value from which the true spectral composition of both stars can be extracted.

The answers should be self evident from these diagrams:

So do the black lines represent the elements present? How come the lines keep appearing and dissappear as the doppler shift occurs?
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6 years ago
#4
(Original post by MSB47)
So do the black lines represent the elements present?
Yes. The example shows the same 3 elements present in both stars.

(Original post by MSB47)
How come the lines keep appearing and dissappear as the doppler shift occurs?
The lines are always present. The spectral lines are shifted towards the infra red end of the spectrum for the receding star, and towards the ultra violet end of the spectrum for the approaching star.

There are two stars and hence all 3 spectral lines of the approaching star will shift progressively towards the blue end, whilst simultaneously, all 3 spectral lines of the receding star will shift progressively to the red end.

i.e. The examples represent the nominal position of the same spectral lines (3 positions at the transverse motion points closest to and furthest away from the observer). These then appear to split into 6 lines and gradually move apart, finally reaching a maximum shift corresponding to when one star achieves maximum approach speed and the other simultaneously achieves maximum receding speed as viewed by the observer.

Hence the spectral lines appear to split and follow a sinusoidal oscillation motion about a mean position .

Have a look at this example for a binary system showing two spectral lines:

https://youtu.be/CEqpqghD-Ds

(In the video, the observers position is at the green spot on the LHS)
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#5
(Original post by uberteknik)
Yes. The example shows the same 3 elements present in both stars.

The lines are always present. The spectral lines are shifted towards the infra red end of the spectrum for the receding star, and towards the ultra violet end of the spectrum for the approaching star.

There are two stars and hence all 3 spectral lines of the approaching star will shift progressively towards the blue end, whilst simultaneously, all 3 spectral lines of the receding star will shift progressively to the red end.

i.e. The examples represent the nominal position of the same spectral lines (3 positions at the transverse motion points closest to and furthest away from the observer). These then appear to split into 6 lines and gradually move apart, finally reaching a maximum shift corresponding to when one star achieves maximum approach speed and the other simultaneously achieves maximum receding speed as viewed by the observer.

Hence the spectral lines appear to split and follow a sinusoidal oscillation motion about a mean position .

Have a look at this example for a binary system showing two spectral lines:

https://youtu.be/CEqpqghD-Ds

(In the video, the observers position is at the green spot on the LHS)
Ahh so the original 3 black lines when there is no doppler shift it shows 3 elements present in both stars and they will shift either way depending on the movement of the stars in the system?
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6 years ago
#6
(Original post by MSB47)
Ahh so the original 3 black lines when there is no doppler shift it shows 3 elements present in both stars and they will shift either way depending on the movement of the stars in the system?
You got it!
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#7
(Original post by uberteknik)
You got it!
thank you very much for ur help and the links
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#8
(Original post by uberteknik)
You got it!
Sorry I'm still slightly confused, if the black lines represent the elements present wouldn't the black lines always have the same wavelength because it indicates the energy levels of that particular element and the only thing that shifts is the light that was not absorbed by those elements and is observed due to the doppler effect?
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6 years ago
#9
(Original post by MSB47)
Sorry I'm still slightly confused, if the black lines represent the elements present wouldn't the black lines always have the same wavelength because it indicates the energy levels of that particular element and the only thing that shifts is the light that was not absorbed by those elements and is observed due to the doppler effect?
The whole spectrum shifts but don't forget, the absorption lines are compared with a spectrum in the observers frame of reference as well as against the stars shifted spectrum to determine which elements are present.

It therefore makes no difference because the light starts out within the core of the star and is absorbed by the intervening elements as it makes the journey to the surface.

The original absorption properties of the stars atoms have not changed and neither has the original wavelength of the light being absorbed. The whole lot has Doppler shifted but the absorption lines shift
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#10
(Original post by uberteknik)
The whole spectrum shifts but don't forget, the absorption lines are compared with a spectrum in the observers frame of reference as well as against the stars shifted spectrum to determine which elements are present.

It therefore makes no difference because the light starts out within the core of the star and is absorbed by the intervening elements as it makes the journey to the surface.

The original absorption properties of the stars atoms have not changed and neither has the original wavelength of the light being absorbed. The whole lot has Doppler shifted but the absorption lines shift
By observers frame of reference do you mean the measured wavelength that we have calculated in labs on earth for each element? So when the stars are at an eclipse the black lines merge to the true value of the wavelength?

I thought the absorption black lines means that light was absorbed by that element so it wouldnt shift because it wasnt emitted in the direction of the earth in the first place
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6 years ago
#11
(Original post by MSB47)
By observers frame of reference do you mean the measured wavelength that we have calculated in labs on earth for each element? So when the stars are at an eclipse the black lines merge to the true value of the wavelength?
In essence, as an illustration, then yes. However that statement excludes the binary pairs common velocity (as part of their local cluster) either moving towards or away from the observer simultaneously.

(Original post by MSB47)
I thought the absorption black lines means that light was absorbed by that element so it wouldnt shift because it wasnt emitted in the direction of the earth in the first place
By default, we only see the light that was directed at the earth in a direct path. i.e. absorption must have occurred in the line of sight in the outer layers of the star.

A picture is worth a thousand words:

The absorbed wavelengths appear to shift because the unabsorbed wavelengths either side are shifted.

http://regentsprep.org/Regents/physi...um/default.htm
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#12
(Original post by uberteknik)
In essence, as an illustration, then yes. However that statement excludes the binary pairs common velocity (as part of their local cluster) either moving towards or away from the observer simultaneously.

By default, we only see the light that was directed at the earth in a direct path. i.e. absorption must have occurred in the line of sight in the outer layers of the star.

A picture is worth a thousand words:

The absorbed wavelengths appear to shift because the unabsorbed wavelengths either side are shifted.

http://regentsprep.org/Regents/physi...um/default.htm
Right so because the elements are also moving they will have a doppler shift in the black absorption lines? I'm not sure If ive got the right idea
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6 years ago
#13
(Original post by MSB47)
Right so because the elements are also moving they will have a doppler shift in the black absorption lines? I'm not sure If ive got the right idea
Yes.

It's a case of absorption spectra is the same as emission spectra.
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#14
(Original post by uberteknik)
Yes.

It's a case of absorption spectra is the same as emission spectra.
Could you also say that because of the doppler effect that the elements would absorb wavelengths of higher/lower value depending on the relative motion of the sun which they are present in as it will appear to those elements coming towards them as a different value?

EDIT: never mind that didn't make any sense...

So just to conclude an absorption spectrum and line spectrum are the same in the sense that the line spectrum only shows the discrete wavelengths of that element it has emitted where as the absorption spectrum just shows the discrete wavelength values that have been absorbed by that element? Hence because that binary star is either moving towards us or away from us it will either red shift or blue shift and if because the absorbed wavelength are re emitted by that element in random directions if we assume it was directed towards us it will still shift due to the fact that element is moving because the star it is present under is also moving?
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