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Author Topic: How are the spectra of different elements discriminated in starlight?  (Read 2479 times)

Moshe Levy

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Moshe Levy  asked the Naked Scientists:
   
Hello naked astronomers!
 
my question is regarding spectroscopy, since the light coming to earth from the stars is coming in "one go", how do we separate the different materials composing that star? lets say that the absorption light spectrum of oxygen and carbon put 'one on top of the other' makes the same pattern as helium, how then do you know if your light spectrum from that star shows that the star has helium or you are seeing oxygen and carbon through the spectrograph, since again the light comes in in "one go" towards us.
 
thanks very much, keep up the great work,
Levy (Israel)

What do you think?
« Last Edit: 20/04/2011 22:01:02 by _system »


 

Offline yor_on

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You're thinking of it as energy quanta now right :)

But they are thought on, and treated, as waves. Every substance burning will emit waves of radiation, and suns burn. Temperature is, to its simplest, just 'particles' in motion, exchanging kinetic energy in 'interactions', and radiating it out as photons/waves.

As I understand it you won't find two 'elements' having the same spectra in one source. But if you could prove that a same source would have two different elements giving of the exact same spectra there would be a problem as you point out.

And the cosmological redshift will shift all frequencies of a source by the same amount as I understands it?

"Planetary nebulae are hot glowing gas clouds ejected by dying low- to intermediate-mass stars. The nebulae glow because they are heated by energetic ultraviolet photons from the exposed stellar core. According to Kirchhoff's laws, the light produced by a planetary nebula should be an emission spectrum, with spikes of emission at specific wavelengths corresponding to the elements in the gas. A spectrum can be displayed as a picture showing stripes of color at the wavelength of each emission line, or as a graph, plotting the amount of light at each wavelength. "

And the spectrum of an object is the variation in the intensity of its radiation at different wavelengths. Objects with different temperatures and compositions emit different types of spectra. By observing an object's spectrum, then, astronomers can deduce its temperature, composition and physical conditions, among other things.
==

And Kirchhoff's Laws are:

"
A hot solid, liquid or gas, under high pressure, gives off a continuous spectrum.
A hot gas under low pressure produces a bright-line or emission line spectrum.
A dark line or absorption line spectrum is seen when a source of a continuous spectrum is viewed behind a cool gas under pressure.

The wavelength of the emission or absorption lines depends on what atoms are molecules are found in the object under study.

    What atoms or molecules exist depend on:

        temperature
        chemical composition.

Where each atom or molecule exhibits a different pattern of lines (rather like a fingerprint or DNA signature)."
==

From Emission Lines and Central Star Temperature.
 

Offline RD

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I think the question is: can the spectra from two different elements have an emission line in common, i.e. both spectra include exactly the same line.

I don't know the answer*, but even if this is coincidence is possible the other lines present (which don't coincide) would enable both elements to be identified.


[* Even if all the lines are inherently unique two could become matched via red-shifting one light source]
« Last Edit: 21/04/2011 15:23:21 by RD »
 

Offline yor_on

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Yes, maybe that was the question :)
Da*n.
 

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