In this series, we explore the weird and wonderful world of astronomy jargon! Pay attention to today’s topic: Doppler shift!
We all heard the howl of the ambulance as it rushed by. Not only does it get louder as it gets closer, but it also changes in pitch. The closer the ambulance gets, the louder the howl becomes. After it’s over, it deepens again.
If you were to ride in the ambulance, the howling would be exactly the same: not only the same volume, but also the same pitch.
The difference is due to the Doppler shift. When the ambulance comes towards you, the sound waves coming from the siren are literally crushed – they are squeezed by the forward movement of the vehicle. When you squeeze sound waves, they shift to higher frequencies and create a higher tone. On the way out, the opposite happens.
What is true of sound is true of light. When a distant star moves towards us, the light it emits is shifted to higher frequencies. These higher frequencies correspond to blue light, hence the name blueshift for this type of Doppler shift and redshift for objects moving away from us.
Doppler shift provides astronomers with a super simple way to measure the motion of stars. First, they identify certain spectral lines known as fingerprints due to the presence of elements and molecules in the light from the object. Then they compare the wavelengths of those fingerprints with the same fingerprints given off by a light in a terrestrial laboratory. Measuring the displacement of this light allows them to calculate the speed of the object (at least the speed in the direction of our line of sight; constructing full 3D speed is much more difficult).
This Doppler shift has been used to measure the speeds of over a billion stars and is a common technique for finding exoplanets. When an exoplanet orbits its parent star, the star wobbles back and forth, which we can see as a periodic blue and red shift.
Note that the Doppler shift is not responsible for the redshift caused by the expansion of the universe. It’s because of the stretching of space-time, not the speed of the object itself. But that’s another lesson.
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