The Doppler Effect

What is the difference between a red shift and a blue shift?

The visible color spectrum ranges from the low-frequency red, orange, and yellow, to the higher-frequency green, blue, indigo, and violet. Astronomers observing the planets, stars, and galaxies use the Doppler Effect to measure the velocity at which objects are moving, rotating, or revolving. The faster the object is moving, the more the frequency is shifted. Most galaxies are moving away from us and their light is red-shifted. In general, the further away, the greater the red shift. Recently astronomers have detected more than 400 planets revolving about other stars using the Doppler Effect. The gravitational force of the planet on the star causes the planet and star each to circle around a common point, usually close to, but not in the center of, the star. As a result the star “wobbles” with the same period as the orbital period of the planet.

The effect is truly tiny. Jupiter causes the sun to wobble in a circle with a speed of 12 meters per second. By measuring the Doppler Effect in the light from the star they can find its velocity and how it changes over time. With that information they can determine the period, distance from the star, and mass of the planet. Most discoveries have been of extremely massive planets, but recently a planet with a mass only a few times that of Earth was detected. It is close to a dim reddish star and the temperature of its surface is estimated to be tens of degrees below the freezing point of water. If the planet has greenhouse warming it might be able to sustain life.


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