What is the Doppler shift, and why is it important to astronomers?

In classical physics, when the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency #f# and emitted frequency #f_0# is

where #c# is velocity of waves in the medium;
#v_r# is velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction);
#v_s# is velocity of the source relative to the medium; positive if the source is moving away from the receiver (and negative in the other direction).

• Use in Astronomy * :

• The Doppler effect results in something known as the red and blue shift..
  Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blueshift and those of a receding astronomical light source exhibit a redshift.

• It is also used to measure the speed of stars and galaxies.

The doppler effect just states that, to an observer, a sound appears to sound higher in pitch (frequency) as it gets closer to them and lower in pitch (frequency) as it moves farther away.

Think of a fire truck or police vehicle siren getting very high pitched right as it passes by you, and then getting lower in pitch as it drives away from you.

Astronomers can use this because you can use emitted sounds to calculate the distance or velocity of an object.