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Star Colors

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How would you describe the way a car's sound changes as it drives past you? Think of the last time a train went past you, or the last time you watched a NASCAR race. How does the pitch of the sound change when a train or race car passes you? You probably noticed the sound appears to change pitch. We call this change in pitch, due to motion, the Doppler Effect.


As an object that is producing a noise approaches you, the sound waves will be pushed together, reulting in a higher frequency. This causes a higher pitch. The frequency of the sound wave is what causes pitch.


The same principle applies to light. In order to understand this, we need to review the electromagnetic spectrum. Remember that visible light (the light we can see) can be broken down into individual colors. When passed through a prism, white light is broken into the colors of the rainbow. The differences between red light and blue light are based on wavelength and frequency. Since red light has a longer wavelength, it will have a lower frequency, meaning that fewer waves will pass a given point in a certain amount of time. Since red light has a longer wavelength, it will have a lower frequency. This means that fewer waves will pass a given point in a certain amount of time.


Now think of all the stars in the sky on a clear night. Each star is composed of different elements that are burning to produce the light that is seen. Each element produces different wavelengths of light when they are burned. These different wavelengths can be used to identify what type of gas is producing the light. For example, when hydrogen is burned, it produces light that has wavelengths of 410.17 nanometers (nm), 434.05 nm, 486.13 nm, and 656.28 nm. Astronomers use devices called spectroscopes to view those four wavelengths of light. Whenever you use a spectroscope and see light produced at the wavelengths listed above, you know that they came from hydrogen. Look at the pictures below and notice what different elements look like when they burn and what scientists see when they look at that light through a spectroscope. Each of the lines you see when looking through a spectroscope are called spectral lines, and each line represents one of the wavelengths of light emitted by the element. Below you can see two different gases burning and the spectral lines you would see if you were to look through the spectroscope pictured next to the lines.


Quickly review the Doppler Effect. When the car was approaching, the waves were pushed together, resulting in a higher frequency, or higher pitch. It was just the opposite when the car was moving away. Waves were moved apart, giving a lower frequency, or lower pitch.

The same basic principles apply to light. If a light source moves toward you, the spectral lines produced by the light will be shifted. In this case, the spectral lines will move toward the blue end of the visible light spectrum, however the distance between the spectral lines will not change. This is known as a blue shift. If, however, the light source is moving away from you, the spectral lines will be pushed toward the red end of the spectrum, something we call a red shift.

The picture below shows examples of a red and blue shifted hydrogen spectrum. Notice that in each case the distance between the spectral lines does not change.

Ah Hah! The picture above shows minor shifts in the spectrum of hydrogen. What would happen to the color of the spectral lines if the light were moving fast enough to cause a major shift toward the blue or red end of the visible light spectrum?
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The color of the spectral lines would change with the direction of the shift. For example, a spectral line that is normally green might turn yellow under a large red shift.

Now you are the astronomer. When looking at a certain star through a spectroscope, you notice that the spectral lines that should be present are all shifted toward the red end of the spectrum. What can you say about the direction that star is moving in relation to Earth? Check your answer.Just the opposite would be true if you saw a blue shift. The star would then be moving toward Earth.


Check your understanding by looking at the pictures below. Determine which set of spectral lines represents a star moving toward Earth. Then choose which set of spectral lines represents a star that is moving away from Earth. Check your answers.


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Updated October 24, 2008 by: Glen Westbroek

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