So, for the star-planet system, the star has an orbit of small radius and the planet has an orbit of larger radius. Because the star is more massive than the planet, the center of gravity will be closer to the star than the planet. Actually the planet and star orbit about the center of gravity of the system, as shown on the right. When a planet orbits a star, the star doesn't really sit still. The Doppler Effect is used to find new planetary systems. What can we deduce from this image?Ĥ) What information can we obtain from the emission spectrum shown on the right about the distant star? Emission spectrum A is that of hydrogen while spectrum B is that of a distant star.ģ) A distant galaxy is shown on the right. The type of sound is formed from the frequency while the loudness of the sound is determined by the amplitude, the height of the waves.ġ) Explain using the words frequency and wave length, why the sound of a horn, mounted on top of a car, sounds high pitched as the car travels towards and observer and quickly fades to a dull low pitch sound as the car passes.Ģ) The image on the right shows two emission spectra. We have all heard the Doppler effect but never realised it.Ī high pitched sound is formed from high frequency waves whereas a low pitched sound is formed by low frequency waves. The movie clip on the left demonstrates the familiar Doppler effect with sound. Notice how the lines are shifted towards the blue or the red depending on the speed and direction of the clouds relative to the Earth.Ĭontent on this page requires a newer version of Adobe Flash Player. The middle spectrum is that of gas cloud that is known to be stationary. The top spectrum is of a gas cloud that is moving towards Earth while the bottom is of a cloud that is moving away from Earth. On the right is an absorption spectrum of the light coming from two distant gas clouds deep in space. If we study the emission or absorption spectra originating from the galaxy we can see if it is moving away or toward Earth. The wavelength of light and hence the colour will indicate the speed of the object. We can use the Doppler effect to calculate the speed of galaxies or any other object in space moving away or towards the Earth. Red is seen when the light source is moving away and blue is the colour that is seen when the light source is moving towards us. The same happens with light waves but instead of having a different sound we have a different colour. The sound waves in front of the moving plane are compressed while the trailing waves are stretched. Since light and sound are considered waves, think of a noisy engine plane. That is because the wavelength of the light that it emits changes. If it is moving away from us it appears red. When an object in space is moving towards us the light it emits appears blue. Light of different wavelength has a different colour.īlue light has a shorter wavelength than red light. Light and sound travel in a wave as shown on the right.
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