Could red shift just be the colour of the star?

15 July 2012

Question

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 it's 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 it's definitely hydrogen, and that's 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 it's moving away from us. Dominic - Exactly, yes.

Add a comment