Back to Jeff's Pages

The Doppler Shift

Think about standing near railroad tracks when a train passes blowing its horn (it works with ambulances and sirens too). As the train passes the horn goes Eeeee-owwwww! It starts out high pitched and as it passes the tone seems to go down. If the train was just parked there (or if you were on the train) the tone on the horn would be halfway between the "Eeee" and the "owww".

The change of pitch because of movement is called the Doppler Shift (named for Christian Andreas Doppler). Sounds from objects moving towards you sound higher pitched than when stationary and sound lower pitched when moving away. If you know what the train horn sounds like standing still, you can figure out if a train in moving towards or away from you and how fast just by measuring the frequency (pitch) of the sound you hear! Cool! Of course, to do this you MUST know the exact tone of the train horn "at rest".

The Doppler Shift works with light exactly the same as with sound! Just as sound seems higher or lower in pitch as the object is moving towards or away from you, light appears higher in frequency (color) when the object is moving towards you, and lower in fequency/color when moving away from you. The order of colors/frequency of light that we can see is:

Lower Frequency Higher Frequency
red - orange - yellow  - green - blue - violet

So a far away object moving quickly away from you would appear more reddish than if it were standing still. The same object moving quickly towards you would appear more blueish than if it were standing still.

If it weren't for those Fraunhofer lines we wouldn't be able to ever tell if a star were moving towards or away from us because we wouldn't know the exact "tone" (frequency) of the star's light "at rest". Since we can measure Fraunhofer lines here on Earth, we do know the "at rest" frequency!

Determing Movement from the Doppler Shift

In a lab we can measure the 2 yellow lines of sodium (like our salt and alcohol demonstration) and can see that they are at 5880 and 5900 Angstroms (a measure of the wavelength of light but that's another lesson). If we see sodium lines in the spectra of a star but the lines are at 5890 and 5910, like the lower pitch of the train whistle, these are a lower wavelength of light! So we would know the star was moving away from us. If we measured 5800 and 5820, the star would be moving toward us!

Sodium spectrum redshifted (6000 and 5980 A.) -- object is moving away from us

Sodium spectrum (5900 and 5880 A.)

Sodium spectrum blueshifted (5800 and 5780 A.) -- object is moving towards us

OK, so far we've learned that that spectral lines ("Fraunhofer lines") tell us what gases are in the star or light source. We also know that we can learn something about how a star moves from it's Doppler Shift. Don't Panic! Don't worry about things like "Angstroms". Here's the game: if a star is coming towards you, its Fraunhofer lines will move toward the blue end of the spectrum ("rainbow"). If a star is moving away from us, its Fraunhofer lines will move towards the red end of the spectrum. Remember our bright yellow sodium lines? If we saw them on a star (or nebula) coming towards us, the lines might be green instead of yellow. If the star or nebula was traveling away from us, the lines might be orange or red instead of yellow. Thus the bluer or redder the sodium lines, the faster the star or nebula is moving toward or away from us!

Do remember that the star (or nebula) isn't just emitting "light" (photons) that we can see.  It also emits photons that are of less energy -- below" red, which we call infra-red and radio waves. And, it also emits photons that are of greater energy, past or above the violet we can see, hence ultraviolet, x-rays, and maybe even gamma rays. There are also Fraunhofer lines in these wavelengths that we cannot see with our eyes. If a star is moving towards us, its light "blue shifts" and Fraunhofer lines that have been hiding down in the infra-red are now visible in the red or orange. In the same way, a star moving away from us may have "red shifted" its light and Fraunhofer lines from up in the UV (ultraviolet) to make then now visible down in the blue. Simple!