Craig Petersen, age 12, of Omaha, Nebraska, for his question:
What is meant by the red shift?
The red refers to the red ribbon of light at one end of the rainbow colored spectrum. The bands of color in the spectrum of a star may show an orderly shift to the right. This red shift indicates that the star is receding from us at a certain rate and distance.
Astrophysicists use the speed of light as a yardstick to measure the distance of stars and also galaxies scattered through the oceans of space beyond the Milky Way. Light, of course, is an assortment of pulsing wave lengths of energy, traveling at about 186,000 miles per second. Its different wave lengths are revealed in the colored bands of the rainbow spectrum. The longest wave lengths are the red rays at the fight. The wave lengths in each band get shorter through orange and yellow, green and blue. The orderly sequence ends with the violet bands on the left, which have the shortest waves.
We can use a glass prism to separate the blended wave lengths in white light into the rainbow spectrum. Astronomers use special instruments and techniques to reveal the spectra of celestial objects. The sequence of colors is always the same. But the motion of a distant star causes the orderly bands to shift slightly towards either the red or the violet end of the spectrum. This motion is called radial velocity. It indicates whether a star is receding or approaching us, its speed and distance.
The spectrum of the sun matches the lines of the rainbow. The distance between us remains fairly constant and besides, the sun is too close to cause changes in the spectrum. The shifts increase with distance and the greatest changes appear in the far outer galaxies.
In the whirling traffic lanes of the Milky Way, many stars are overtaking us. The color bands in their spectra are shifted toward the violet. This violet shift indicates that we are getting closer together. Most galaxies beyond the Milky Way have spectra with the accent on the red side of the rainbow. Their red shifts show that we are moving apart. There seems to be a ratio between the radial velocity and distance. The red shift tells us that an object is receding and how fast, and we can use this information to estimate its distance from the earth.
The red shift has been compared to a train whistle speeding into the distance. The whistle slides down the scale. This is because the receding motion lengthens its sound waves and longer sound waves are lower in pitch. In somewhat the same way, the wave lengths of light from receding stars appear to grow longer. Instead of sliding down the scale, the short wave bands of color on the blue side of the rainbow shift toward the red side.
The red shift is a useful yardstick for measuring cosmic distances. But it is tricky. The nearest outer galaxy has a violet shift, showing that the distance between us is decreasing. This is due to our path around the Milky Way. Later we shall swerve around and this galaxy will show a receding red shift. So in using these color shifts we must allow for the movements of the earth. What's more, the red accent of a galaxy may be increased by vivid yellow stars, by white dwarfs or by tired light from vast distances. The red shift is a useful cosmic yardstick, but it is not a perfect one.