Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 17/07/2012 17:13:23
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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
Asked by Luke Bizeray
Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2467/)
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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