How do we know that the light coming off from distant planets is as a result of red shift or the properties of the elements prevalent on the plant?


My understanding is that when an object moves away from you it increases the wavelength of the light presenting with a colour that is on the red side of the colourscale. Scientists also are able to determine what elements are present on planets based on the light it emits. How can they tell the difference?

I’m sorry for possibly using the incorrect terminology! Thanks in advance folks!

Edited the post because previously I had suggested that shorter wavelengths tended to the red side when in fact longer wavelengths tended to the red side of the colour spectrum.

In: 17

Basically different elements emit(or absorb) a specific pattern of wavelengths, this is due to the possible energy levels of electrons. So every element basically has a signature that we can look for, and if that signal is shifted towards red we can calculate very precisely how much the light has been redshifted, and the signal can also tell us about the chemical/atomic make up of whatever we are looking at.

A star emits all frequencies of visible light, so you should see a pure spectrum. Elements within the star will absorb certain frequencies, so you actually see a spectrum with lots of black dots in it.

We can measure how far away the star is, based on parallax or proximity to standard candles (objects that we know have a certain brightness).

Only distant galaxies, where individual stars are too small to be seen, have a significant redshift.

<cough the wavelength *increases* with red shift. The *frequency* reduces.>

Imagine your name was ABC and everyone knew it. Your sister’s name is LMN and everyone knew it.

Suddenly you start signing your name as BCD, and your sister MNO. It’s pretty clear that something weird is happening and for some reasons you’ve both just started writing your names exactly one letter off.

While it might take you a while to figure out *why* your writing names this way, it’ll still be clear that you are you and your sister is your sister.

That’s what’s happening with red-shifting only instead of names is the exact colors of the light the atoms give off. If we know that all the colors should be a certain way and we find a set of colors that are different, but consistently different in an ‘everything get’s a +1’ sort of way.

1. A beam of light leaves a star.
2. It arrives at our measurement device.
3. We can measure that beam’s wavelength to determine how far away it originated.

A) Elements in the star’s corona absorb some of the light as it leaves the star.
B) Those absorptions leave gaps in the beam’s spectrum.
C) We can identify the elements in the corona based on the position of those gaps in the beam’s spectrum.

Because of the structure of each atoms, the way they absorb electromagnetic radiation is unique. You could think of it as a unique finger print of the atom. If we send light through let’s say a cloud of oxygen, and then we analyze the spectrum of that light, we will see dark lines where specific wavelength of light were absorbed by the oxygen. That’s the ”finger print” of the oxygen.

Now like you said when an object moves away from us it reduce the wavelength of the light toward the red. But this is a regular shift so the pattern remain the same.

Let’s use number to give an example. Let’s say that the wavelength absorbed by oxygen is 3 and the wavelength absorbed by hydrogen is 7. So if let light go through a cloud of water, on the other side you would see black line at 3 and 7.

But what if this light had a red shift, well then maybe you see black lines at 6 and 10. You can still recognize the pattern here, there is 4 of difference between the two. And to be sure, you can calculate the red shift is indeed 4 and now you know for sure that the light passed through oxygen and hydrogen.

Of course in reality spectroscopy is much more complex, but you get the idea.