Question #c5857

1 Answer
Oct 29, 2017

'Blue shifting' of the light spectra due to the Doppler effect.


It amazes me still that this is possible, but you can filter starlight from stars in the Andromeda galaxy through a spectroscope. This reveals characteristic signatures that are well known.

If the body being observed is in motion towards us or away from us, this characteristic signature will be "shifted". The typical analogy is sound waves from a moving object here on earth. Imagine a train in the distance blowing its horn. If it's moving towards you, the frequency of the sound will increase, and you will notice the pitch of the horn increasing. This phenomena is called "Doppler Shift", and it applies to light as well.

Characteristic signature lines in light spectra from stars in Andromeda are shifted to the right, towards the higher frequency blue end of the spectrum. This allows astronomers to deduce that Andromeda is approaching, and the amount of the blue shift allows them to calculate the velocity.

As I say, this amazes me still.

The actual collision is not due for some millions of years, so no insurance companies need be contacted just yet.