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Have astronomers ever observed a violet shift like they have blue shifts and red shifts?

Category: Space
Published: June 27, 2013

Violet shifts happen all the time. We call them blue shifts. When a star emits light, the color of its light as observed on earth depends on its motion relative to earth. If a star is moving towards the earth, its light is shifted to higher frequencies on the color spectrum (towards the green/blue/violet/ultraviolet/x-ray/gamma-ray end of the spectrum). A higher frequency shift is called a "blue shift". The faster a star moves towards the earth, the more its light is shifted to higher frequencies. In contrast, if a star is moving away from the earth, its light is shifted to lower frequencies on the color spectrum (towards the orange/red/infrared/microwave/radio end of the spectrum). A lower frequency shift is called a "red shift". The faster a star moves away from the earth, the more its light is shifted to lower-frequency colors. This effect is known as the "Doppler shift". It is the same principle at work when an ambulance siren coming towards you is high pitched and then switches to a lower pitch sound once the ambulance passes you and is traveling away from you. The Doppler shift is also used in police radar guns to measure how fast your car is going based on how the radio wave shifts in frequency when it bounces off your car.

redshift diagram
A blue shift does not mean that the object ends up blue. It just means that the entire spectrum is shifted up in frequency. Note that this is a schematic diagram and not actual data. Public Domain Image, source: Christopher S. Baird.

The book titled The Handy Astronomy Answer Book by Charles Liu states,

"When an object emitting light – or any kind of electromagnetic radiation, for that matter – moves toward someone, the wavelength of its emitted light is decreased. Conversely, when the object moves away, the wavelength of its emitted light is increased. For visible light, the bluer part of the spectrum has shorter wavelengths, and the redder part of the spectrum has longer wavelengths. Thus, the Doppler effect for light is called a ‘blueshift' if the light source is coming toward an observer, and a ‘redshift' if it is moving away. The faster the object moves, the greater the blueshift or redshift."

Simply by looking at the colors of light from a star, astronomers can figure out how fast that star is moving relative to earth using the Doppler shift. Any spectral color of light can shift to any other spectral color if the motion of the source is right. For this reason, if an orange light beam has been blue shifted, that does not mean that its final color is blue. It just means that its final color has been shifted towards the blue end of the spectrum; i.e. its color has been shifted up in frequency. An orange light that has been shifted so that it ends up as a yellow light has been "blue shifted". An orange light that has been shifted so that it ends up as a violet light has also been "blue shifted." In contrast, a violet light that is shifted so that it ends up as an orange light has been "red shifted". When it comes to the Doppler effect, "red shifted" should be heard as "down shifted" and "blue shifted" should be heard as "up shifted". For instance, if an ultraviolet ray (which is higher in frequency than blue) is shifted up in frequency so that is ends up as an X-ray, we still call it a blue shift, even though it actually has shifted away from the blue. This case only makes sense if you interpret "blue shift" to mean "up shift" and not "towards blue".

Now an interesting question arises. We call a down shift a "red shift" because red is the color at the bottom edge of the visible part of the electromagnetic spectrum. Red is the first color of a rainbow. With this type of reasoning, we should call an up shift a "violet shift" because violet is the color at the top edge of the visible part of the electromagnetic spectrum. Violet is the last color of a rainbow. But we don't. We call an up shift a blue shift and not a violet shift. Why? The reason is that humans do not see violet very well. Even though humans can technically see violet, and therefore it is the color at the upper edge of the visible spectrum, we do not see it very well. As a result, blue is the de facto upper edge of the visible spectrum. This is why an up shift is called a blue shift. Such a confusing state of affairs seems worth avoiding, but the fact is that having blue be the upper edge of the visible spectrum is part of the human experience.

The sky contains all colors of light at all times, put its light peaks in one color at different times of the day. Scientifically, the peak color of the sky shifts from sunrise to noon in the order: infrared, red, orange, yellow, green, blue, violet, ultraviolet. But humans experience the color of the sky from sunrise to noon as: red, orange, yellowish-white, white, blue. The difference between the scientific version of the sky's color and what humans experience are due to three facts: 1) our eyes can't see ultraviolet or infrared, 2) our eyes can't see violet very well, and 3) our eyes experience a nearly even mix of all colors as white. In terms of how we experience the sky, the color spectrum seems to start at red and end at blue. The same goes for incandescent flames such as in a campfire or on a candle. Scientifically, flame colors go from coldest to hottest in the order of: infrared, red, orange, yellow, green, blue, violet, ultraviolet. But humans experience the color of flames from coldest to hottest as red, orange, yellow-white, white, blue. From everyday experience, blue seems to be the upper end of the visible color spectrum, even though we can technically see violet. That is why an upwards Doppler shift is called a blue shift. A "violet shift" would therefore mean the same thing as a blue shift if the phrase were ever used: an upwards shift.

Use of the Doppler shift has allowed astronomers to make some interesting observations. On average, the light from all stars outside our local group of galaxies is red shifted. Also, the farther away a star is, the more its light is red shifted. This fact indicates that our universe is expanding and all of the stars outside our local group of galaxies are moving away from us. Also, when a star rotates, one edge of the star is moving towards us relative to its center while the other edge is moving away. As a result, light from one edge of a star is slightly red shifted while light from the other edge is slightly blue shifted. Astronomers can use these two shifts in order to calculate how fast a star is rotating. The same approach can be used to calculate how fast a galaxy is rotating.

Topics: Doppler shift, blue shift, color, expanding universe, light, movement, red shift, violet shift, wave, waves