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Doppler effect
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Displacement of the lines in the red or in the blue depending on whether the star moves away from or gets closer to the observer.
Crédit : Paris Observatory / UFE

Measure of the radial velocity

The "spectroscopic" technique consists of measuring the radial velocity of the star with high accuracy through the Doppler effect, observing it with a spectrograph and measuring the displacements of the spectral lines caused by this effect.

The stellar oscillations are detectable with this method because they generate velocity fields on the surface of the stars.

The "reflex" movement of the star, due to the rotation of a planet around the star, is also detectable by Doppler effect.

Doppler-Fizeau effect

When the emitting source of a wave gets closer to the observer, the frequency appears to be higher (the wave is "packed"), and when it moves away, the frequency seems to be lower (the wave is "dilated").

  • That's the reason why the siren of an ambulance is more high-pitched when it gets closer to the observer, and deeper when it moves away from him.
  • That's also the reason why race cars make the noise "Niiiiiiiiaaaaaaaoooooooum" when they pass in front of you.
  • The Doppler effect can also be applied to luminous waves emitted by a moving object. If this object gets closer to us, its spectral lines are slightly displaced towards the purple. If it moves away, the lines are displaced towards the red. It is thanks to this principle that we can measure the relative movement of a star compared with the observer.
  • In the case of solar-type stars, the variations of radial velocity are only about 10 cm/s, very difficult to detect. The spectroscopic observations are thus limited to very bright stars in slow rotation.