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Scientific question...
Perhaps I would've learned this if I had continued my major in Imaging Science, but alas...

There are two basic concepts that I understand separately, and that are often explained in articles about space imaging. The first one is that the wavelengths (colors) of light seen in an image can be used to determine what chemical elements are present at the source, because different elements emit or reflect different wavelengths. The second is that the Doppler effect causes light emitted from sources moving away from the viewer to appear to have longer wavelengths, or become "red-shifted", and light from sources moving toward the viewer to appear to have shorter wavelengths, or become "blue-shifted".

I understand both of those concepts separately. But now my question is... When given an image, which shows data about brightness at specific wavelengths of light observed from our solar system, how can a scientist tell whether the wavelengths indicate the presence of one element, sitting still relative to us, or some other element, moving toward or away from us?

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I'll take a guess.

They use a reference element, say Hydrogen, which gives off a certain wavelenth and is practically everywhere. How they figured that out first, I don't know. Well, we do have the sun, which is almost still relatively. ;-)

If the wavelength corresponding to Hydrogen isn't there, they know there's a shift. So they find the closest bar wavelength, and shift the result so that it lines up with hydrogen.

I guess they could then check that other elements line up as well. If they don't, they could take the next closest and see if that lines up with the wavelengths of elements.

For example,

[Most distant object, distant galaxy, close galaxy, sun]

As the speed of the distant objects recede from us, the visible spectrum show shifting of these characteristic lines to longer wavelengths. Nearly all objects that are out of our galaxy are redshifted, meaning they are receding from us. It appears that the further the stars or galaxies are from us, the further they are redshifted. The most distant objects we have observed so far are moving away from us at over 0.6 the speed of light.

Ah, well you answered my question but not in the way you think. :)

I was thinking about the problem of looking at something like a cloud of gas in a nebula that glowed at a specific wavelength. Specifically, I was looking at today's Astronomy Picture of the Day and wondering how they knew the red gas wasn't moving away from us and the blue moving toward us, but apparently things in our own galaxy don't move fast enough relative to us to get shifted much at all. And when observing something like the "most distand object", that object would like contain every element in it anyway, so it would be easy to line up the pattern.

Thanks! :)

Sure, no problem. :-)

I just want to note that the dark bands correspond to elements, not the light ones.

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