Physics question on red-shift

[TLicense]

I've just been reading this article from the BBC website about the Hubble Space Telescope being able to see galaxies that are further away than we've ever seen before:-

http://news.bbc.co.uk/1/hi/sci/tech/8401374.stm

and it reminded me of a question I had when I was in school that I don't think I ever found the answer to.

 

I understand what red-shift is and how it works, but what I've always wondered is how do you know what it red-shift, and what is just red?

 

If you use red-shift to decide how far away an object is, or how fast it is moving away from you, surely you must know what colour it was to start with?

 

For example, if you had a star that is white, but had shifted to red because it was so far away, how would you be able to see that it wasn't just a star that was red that was a lot closer?

 

[Scoboblio]

I understand what red-shift is and how it works, but what I've always wondered is how do you know what it red-shift, and what is just red?

 

Because the 'red' they refer to is infrared (short wavelength) and not the red from the visible part of the spectrum (longer wavelength). Is that what you mean, or have I got the wrong end of the stick?

[TLicense]

Yeah, I think maybe a little.

 

Red-shift shifts the frequency in the direction of longer wavelengths, but if you don't know what the wavelength was to start with, how can you say it has changed from X to Y? All you're seeing is a colour (or more correctly a wavelength of light). Without knowing what it was to start with, I don't know how it is possible to say how much it's changed by?

[imi]

They do assume that if it is a star then it emits light of certain wavelengths cause all starts are the same (other than size and the intensity of the light) and work from that assumption.

[TLicense]

They do assume that if it is a star then it emits light of certain wavelengths cause all starts are the same (other than size and the intensity of the light) and work from that assumption.

 

I thought the colour of a star changed depending on its temperature?

 

Hence the red dwarves and white giants etc...

[imi]

Fair point, however what if the wavelength of all light, invisible and visible is red-shifted, surely that way they would be able to tell.

 

other point to mention is that they are looking for elements such as hydrogen that are present in every star, they know the peaks of this element and can see whether it has shifted or not.

[Andy Blyth]

I think it's something to do with the chemical makeup of the star. Different elements give energy peaks at different wavelengths, possibly something to do with energy bands in an atom.

 

If a body emits radiation with a peak at x and you receive a wavelength (x+y) where y is small compared with x, you could assume that (x+y) started off as x and that the y component is due to red shift.

 

Spectral lines

[stevie_b]

I'm a bit rusty, but what I understand by "red shift" is something that is also known as Doppler Shift. You're familiar with Doppler shift, you just possibly don't know it...

 

Doppler shift is the effect that makes police car sirens coming towards you sound high pitched, and sirens moving away from you sound lower-pitched. Think of the sound waves: when a police car approaches you, you can think of the sound waves as bunching up in front of the car, a bit like a bow wave. They become a bit compressed, shortening their wavelength. Sound with shorter wavelengths, by definition, is high pitched. The opposite is true: when the siren moves away from you, the sound waves become stretched out like a slinky spring: this lengthens their wavelength, and they become lower pitched.

 

Now, back to light and red-shift. The same happens in space, except instead of a police car we have a galaxy, and instead of sound waves we have light waves. Red light is at the long wavelength end of the visible part of the light spectrum, so when galaxies are moving away from us (the observer), the light that they give off seems to be shifted towards the red end of the visible spectrum, hence "red-shift".

[Homer]

That's it Stevie, perfectly explained. In summary it's not about red as a colour, that's just another way of terming the doppler effect, which is used to measure relative distance of remote objects.

[Ian C]

Yep that's a great explanation of red and blue shift, but the original question was "how do you know it's shifted and not stationary with that wavelength?"

 

To re-use the siren analogy, if you hear a siren in the distance, can you tell if it's coming towards you, heading away, or stationary? If you don't know what its stationary pitch actually is and it never gets significantly closer or further away?

 

So if a star looks rather red, how do we know it's not just, well, red

 

-Ian

[imi]

Yep that's a great explanation of red and blue shift, but the original question was "how do you know it's shifted and not stationary with that wavelength?"

 

To re-use the siren analogy, if you hear a siren in the distance, can you tell if it's coming towards you, heading away, or stationary? If you don't know what its stationary pitch actually is and it never gets significantly closer or further away?

 

So if a star looks rather red, how do we know it's not just, well, red

 

-Ian

http://www.mkivsupra.net/vbb/showpost.php?p=2597400&postcount=6

[Ian C]

Sorry I thought your post would just be some anti-establishment rhetoric so I skipped it

 

(and other weak excuses for skim reading)

[RiceRocket]

I'm sure you'll find an acceptable answer somewhere in this

[Tannhauser]

A long time since I did this, but try it as an answer until a proper scientist comes along:

 

Each chemical element emits its own characteristic electromagnetic radiation, but it also absorbs radiation at particular wavelengths. This absorbed radiation bumps electrons from lower orbitals (where they have less energy) to higher orbitals (where they have more).

 

When you heat up an element and put the light through a spectroscope, you see the full spectrum of colours (different wavelengths of light) that it emits. In amongst them, you see these peculiar dark lines, which are the wavelengths of EM energy being absorbed as the electrons in that element's atoms are being pushed up to higher levels.

 

The key thing is that these absorption lines are a function of the atom's fundamental properties and therefore their wavelength never varies, whether they are in Brighton, Timbuctu, or Proxima Centurai. As far as Iknow, it also doesn't matter how much they are heated, seeing as the electron always absorbs the same wavelength of energy to make its jump, no matter much it is supplied with.

 

Therefore, the absorption lines act as a reference point. Hydrogen, for example, is going to be abundant in any star. So we can compare the wavelengths of light from hydrogen in the lab with light coming from a distant galaxy and see how much these have been red shifted.

 

This not only allows astronomers to measure red shift but also tells them about the chemical composition of stars.

[TLicense]

Therefore, the absorption lines act as a reference point. Hydrogen, for example, is going to be abundant in any star. So we can compare the wavelengths of light from hydrogen in the lab with light coming from a distant galaxy and see how much these have been red shifted.

 

Perfect! cheers Cliff!

[AJI]

yep as tannhauser explains... and also to answer how they know what it was before red-shifting, the answer here is because in the middle of each star the essential same nuclear reactions are taking place as to what is in the sun. So therefore we know what spectra to look for (or expect) in the other light sources (stars). So there is now a comparative datum to grade 'staionary' objects and 'moving' ones.