Doppler Shifts

When I first learned about the Doppler effect, I found that the easiest way to think about it was to imagine an ambulance driving past me at a very high speed.  When the ambulance is approaching me, the sound its siren makes is higher because its waves are more compressed, and when it passes me, I hear the sound get lower in pitch because when it is traveling away from me, the sound waves are less compressed.  I found this concept relatively easy to translate into my understanding of Doppler shifts in terms of spectroscopy and wavelengths of light.

However, one thing that puzzled me was what occurs when a star, for example, isn’t moving directly toward or away from Earth, but away at a diagonal.  What sort of shift do we observe then?  Once I discovered that the observed shift had to do with radial velocity, I was surprised to discover that Doppler shifts are actually key to our observation and discovery of extrasolar planets, as seen in the image above.

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2 responses to “Doppler Shifts

  1. Just to clarify-when something is moving both somewhat away or towards us and somewhat across our field of view we can still observe Doppler shift but it will only depend on the speed towards or away, so we don’t know anything about the speed across. This makes getting the exact mass of exoplanets difficult, because we only know how much the star moves towards or away.

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