In this series we are exploring the weird and wonderful world of astronomy jargon! Watch out for today’s topic: doppler shift!
We’ve all heard the wailing of the ambulance as it rushes by. Not only does it get louder as it approaches, but changes in pitch. As the ambulance gets closer, the wailing shifts to a higher frequency. After it passes, it deepens again.
If you were to ride along with the ambulance, the wailing would stay exactly the same: not just the same loudness, but also the same pitch.
The difference is due to Doppler shift. When the ambulance is coming towards you, the sound waves coming out of the siren literally get squished – they get pressed together from the forward movement of the vehicle. When you squish sound waves together, they shift to higher frequencies, making a higher-pitched sound. The reverse happens on the way out.
What goes for sound goes for light. If a distant star is moving towards us, the light it emits gets shifted into higher frequencies. Those higher frequencies correspond to bluer light, hence the name blueshift for this kind of Doppler shift, and redshift for objects moving away from us.
The Doppler shift gives astronomers a super-easy way to measure the movement of stars. First they identify certain spectral lines, which are known fingerprints in the light coming from the object due to the presence of elements and molecules. They then compare the wavelengths of those fingerprints to the same fingerprints given by a light in an Earth-bound laboratory. Measuring the shift in that light allows them to compute the velocity of the object (at least, the velocity in the direction of our line of sight; constructing the full 3D velocity is much more challenging).
This Doppler shift has been used to measure the velocities of over a billion stars, and is a common technique for finding exoplanets. As an exoplanet orbits its parent star, the star will wiggle back and forth, which we can see as a periodic blue- and red-shifting.
Note that the Doppler shift is not responsible for the redshift caused by the expansion of the universe. That’s due to the stretching of spacetime, not the velocity of the object itself. But that’s a different lesson.