30 Moving Objects, Spectra, and the Doppler Effect

The Doppler Effect is a change in an object’s spectrum because the object is moving closer or away from the observer. In sound we experience this change in a siren’s pitch. When it approaches us, the waves are being compressed, then becoming longer as it moves away. This increase in pitch can be heard as a police car approaches and then decreases as it moves farther away.What are the Implications of Doppler Shift?

  1. You can determine an object’s velocity by the amount the spectrum is shifted.
  2. The amount of the shift is the source’s velocity relative to the observer. This major clue tells astronomers if an object is moving towards us or away from us, and at what speed.
  3. The largest red shift recorded, that of one specific Quasar, is close to the speed of light.
Image of Car police top view. Doppler effect. Change of wavelength caused by motion of the source.
CC BY-SA 3.0 | Image courtesy of Wikimedia Authors:

Car_Overhead.svg: Inkwina
Old_microphone.svg: rg1024
Doppler-effect-two-police-cars-diagram.png: Jeremykemp at English Wikipedia
derivative work: kismalac

Image of Spectral Blueshift, Redshift, and At Rest. Blueshift is a blue line that is wavy and really close together. At Rest is a green line what is a little bit more separate. Redshifted is a wavy line more separate than the rest.
Image courtesy of Florida State College at Jacksonville.

Spectral Blueshift, Redshift, and At Rest

In light, this is referred to as the Spectral Red or Blueshift . For objects coming towards us, a blueshift; the entire object’s spectrum shifts towards the blue end of the spectral scale. For objects moving away from us, a redshift; the entire object’s spectrum shifts towards the red end of the scale. So, how can we tell if an object is moving towards or away from us? By comparing an object’s spectrum to the spectrum of an object that is at rest (an object that is not moving). What is called its rest wavelength?

What are The Astronomical Implications and Importance of Spectra?

  1. How do we know what a star is made up of?
    • From the star’s spectra
  2. How do we know if an object, star, galaxy, etc., is moving towards or away from us?
    • Look for a red or blue shift in the object’s spectra
  3. Can we tell how fast an object is moving towards or away from us?
    • Yes, by the extent of red or blue shift

Consider this…

Image of astronomer Frederick William Herschel.
Astronomer Frederick William HerschelPublic Domain

For most of history, visible light was the single most recognized portion of the electromagnetic spectrum, EMS. The ancient Greeks noted that light traveled in straight lines and studied its properties. Over the years the study of light continued. During the 16th and 17th centuries there were conflicting theories which regarded light as either a wave or a particle. In 1800, astronomer Frederick William Herschel discovered infrared radiation while using a thermometer to measure temperatures of visible light frequencies. A year later Johann Ritter discovered ultraviolet radiation.


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