Summary

Concepts

In 1842, Christian Doppler pointed out that the same thing happens with light waves. In astronomy, we study the light (and its spectra) coming from stars. If the spectral lines are shifted, then we know that the star and earth are moving relative to one another. A shift in the spectral lines toward shorter wavelengths is called a blue shift. A shift toward longer wavelengths is called a red shift because red light has a longer wavelength than blue light. By measuring the amount that the lines are shifted, we can determine how fast the earth and star are approaching or receding from one another. For an illustration, look at the following picture:

However, there is a fundamental difference between the Doppler Shifting of light and that of sound; that is, why do we not commonly experience the Doppler Shifting of light in our everyday experiences? The answer lies in the fact that Doppler Shifting is not observed unless the relative motion of the source is moving at a substantial fraction of the speed of the waves (sound or light). To put this into perspective, the speed of sound is ~350 m/s whereas the speed of light is 3.00 x 10^8 m/s!

The Doppler Effect is still being used today to find new planetary systems. When a planet orbits a star, the star doesn't really sit still at one focus of the ellipse (Kepler's Law). Actually the planet and star orbit the center of gravity of the system. To understand the center of gravity, think of two different masses on the end of a yardstick. The point along the yardstick where you can balance it on your finger is the center of gravity of that system. Because the star is much more massive than the planet, the center of gravity will be closer to the star than the planet. So, for the star-planet system, the star has an orbit of small radius and the planet has an orbit of larger radius. Astronomers label the motion of the star as a wobble and have been looking for many years for stars exhibiting this wobble.

The wobble is too small to be seen visually. To detect these massive planets, astronomers look for a modulation in the spectra. In understanding what scientists look for, we must differentiate between the two causes of Doppler shift. The first type was described previously. Depending on whether the star is moving towards or away from the observer, the motion of the star will cause an increase or decrease in the frequencies of the observed light. It is caused by the motion of the center of mass (in other words the motion of the whole star system and the planet). The second type of Doppler shift is caused by the rotation of the star about the center of mass of the star and planet system. The rotation will cause a periodic change in the spectra. For half of the stellar orbit, it will be moving away from us, decreasing the observed frequencies slightly, and for the other half of the orbit, the star will be moving towards us increasing the observed frequencies.

From the observed spectra scientists can figure out the spectral type of the star as well as its luminosity class. This data gives the characteristics of the star such as the mass and surface temperature. Using this data and the frequency modulation, scientists can then figure out the mass of the planet as well as the distance between the planet and the star, which they then use to estimate other characteristics like the temperature of the planet.

Last updated on November 5, 2000.