Science Questions

Could red shift just be the colour of the star?

Sat, 14th Jul 2012

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Question

Luke Bizeray asked:

In regards to the red shift and blue shift of stars to measure their movement away or towards us: If the only information we have to recognise distant stars is the light emitted by them, how do we know that the red shift or blue shift isn't just the colour of the star and not due to movement? How do scientists find the value from which to measure the Doppler shift?

 

Regards,

Luke

 

 

Answer

Dominic -   Stars certainly do come in wide variety of colours.  Hotter stars will tend to be bluer than cooler stars like Betelgeuse which are comparatively red.  But when you're looking for the red shift, looking at the frequency shift which is associated with the velocity of that star, what you're looking at are spectral lines.  These are very specific wavelengths in the spectrum of that star where particular elements produce light in a very small range of frequencies and this produces a bump in the spectrum which is always in exactly the same place.  And what you see when a star is red shifted is that characteristic bump is shifted to a different frequency.  You can be sure itís the same bump you're looking at because they form a pattern across the spectrum of different elements emitting different wavelengths.

Chris -   So, you basically know if your hydrogen bump is shifted a little bit that itís definitely hydrogen, and thatís just offset by certain amounts and the amount is going to be offset by is proportional to the amount the star has been red shifted or blue shifted, and that tells you how fast itís moving away from us.

Dominic -   Exactly, yes.

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Hi Luke

well, if you looked purely at the colour of a star and saw that it looked redder than another star, you could conclude that it is just the star's composition, rather than it's accelerating away from you, that accounts for this.

But, astronomers don't rely on the absolute colour of a star when making this assessment. Instead, they look at discrete wavelengths (colours) of light that correspond to absorption and emission by specific chemical elements, each of which has a unique "colour fingerprint" of its own. This is how spectroscopy works.

And because the elements are the same throughout the Universe, their absorption spectra are also the same. So when red-shifting occurs, you see the same spectra corresponding to each element, but now they are altered slightly towards the red corresponding to the light being stretched out as it travels towards you. The amount by which the spectral lines have been displaced like this away from their normal absorption wavelength then tells you the extent of the redshift.

Chris chris, Sat, 14th Jul 2012

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