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The motion of the star is measured by the position of the Fraunhofer lines, not the colour of the star.

Measurement of the position of the lines allows for accurate measurement of the Doppler shift of a star.Ī cool, red star which is stationary will have the Fraunhofer lines in the same position as a hot stationary blue star. The presence of these line shows the composition of the star's atmosphere. Astronomers learn an amazing number of things from the analyzing the spectra of stars, galaxies, and quasars. The Doppler effect causes a train whistle, car, or airplane to sound higher when it is moving towards you, and lower when it is moving away from you. In the illustrations below (not yet available), we have two objects: one. If the star is moving, the light, along with the Fraunhofer lines is red- or blueshifted. Astronomy is primarily concerned with how this phenomenon applies to light waves. 3 It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842. The wavelengths at which these lines appear are known very finely, for example sodium causes a line at 589.592nm. The Doppler effect or Doppler shift (or simply Doppler, when in context) 1 2 is the apparent change in frequency of a wave in relation to an observer moving relative to the wave source. The spectrum of light from a star will contain dark lines, called Fraunhofer lines, which are due to the absorption of light at particular wavelengths by gases in the star's atmosphere. causing a Doppler shift in the composite spectrum. To detect a red or blue shift you need some marker in the spectrum that can be measured accurately. Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars and other celestial objects. Except at extreme velocities, the red-shift won't affect the light enough to change the visible colour. The first Doppler redshift was described by French physicist Armand-Hippolyte-Louis Fizeau in 1848 who pointed to the shift in spectral lines seen in stars as. The colour of a star is due mostly to its temperature.