Energy | Pages 138-139 | See Linked Version

Edwin Hubble tracked red shifts in the spectral lines of his galaxies to determine that the entire universe is expanding. Here, our Doppler shift analogy suffers a bit. Distant galaxies are not roaring away from us like the race car in our Earthbound comparison. Rather, all of space expands, and galaxies get carried along for the ride. The relentless expansion stretches a galaxy's light waves to longer and longer wavelengths as the waves travel toward Earth. Consequently, their spectral lines are redshifted just as though the galaxies are blasting through space at high speeds. Galaxies near the limits of the observable universe appear to recede so quickly from us that many of their spectral lines disappear from the spectrum of visible light. They shift all the way into the infrared.

Extremely tiny Doppler shifts in the spectra of stars are the key tools to finding planets outside our solar system. A star does not sit motionless at the center of its planetary kingdom. Rather, a planet's gravitational pull tugs its star to and fro--just as when you jump into the air, you induce Earth to move slightly in the opposite direction. Both phenomena are displays of Newton's third law: For every action there is an equal and opposite reaction. Thus, a star with planets in orbit around it jiggles slightly in space, leading to minuscule but periodic red shifts and blue shifts in the star's spectral lines. Starting in the mid-1990s, astronomers used ultrasensitive spectrographs to search for such jiggles in other stars. Current technology allows them only to detect planets close to or exceeding Jupiter in size. However, even that high limit has yielded more planets outside our solar system than inside it. New search programs will extend this quest down to planets with masses like that of Saturn, then Uranus and Neptune. Within a decade or two, planets like Earth may be within range. As tempting as it was for us to think of our solar system as something special, it's already clear that planets in the cosmos are far from rare.


The light from distant galaxies and from stars within our own galaxy is a form of electromagnetism, one of nature's four basic forces. Other types of electromagnetic energy are all around us, although they may seem radically different from starlight. For instance, the electricity that powers our society arises from the same force that governs light. So too does the magnetism that guides sailors and scouts and encodes the strips (continued)

The Doppler Effect: Key to Motion and Energy

Astronomers use the absorption and emission lines of spectra of celestial bodies as a key to their motions and energy levels, clues to unveiling the nature of even the most distant galaxies in the sky. Scientists can do this because of the so-called Doppler effect. The sound emitted by a race car has a higher pitch as the car approaches because the wavelengths of sound are compressing in the direction of the observers (top). A race car moving away has a lower-pitched sound because the intervals between waves stretch. The same principle applies to electromagnetic radiation and is the key to reading the spectra of stars and other celestial objects. By analyzing the location on the spectrum of certain patterns of spectral lines (bottom), scientists can determine whether an object is moving toward or away from Earth and how fast it is moving. This information in turn figures into calculations of the object's mass and the kind of energy it is emitting. Added together, these separate clues can suggest, for example, that a powerful source of radio waves harbors a supermassive black hole (page152) or that a star 50 light-years away is actually home to a small system of planets (page 140).

The Doppler Effect and Red Shift

Absorption lines for a particular chemical element appear at a certain place on the spectrum for an object at rest in relation to an observer (left, top). (The lines used here have been simplified for clarity.) If the object is moving away from the observer (middle), the wavelengths stretch; some lines move off into the infrared, while lines normally in the blue range shift toward red. For an object moving toward the observer, the wavelengths compress and lines shift toward the blue end of the spectrum (bottom). The spectra of rotating objects show both blue and red shifting (page 155).