A redshift is any decrease in an electromagnetic wave’s frequency. Redshift can be caused by the Doppler effect, which is the change in wavelength and frequency of either light or sound as the source and observer are moving either closer together or farther apart. In astronomy a redshift indicates that a distant source of electromagnetic radiation is moving away, and a blueshift indicates that the source is moving closer to us. Redshifts have many important applications in astronomy. They help us to deduce the masses of stars and of galaxies and the structure and history of the universe. The redshifts for distant objects in the universe tell us that the universe is expanding.
Listen to an ambulance or police siren as it passes. You should be able to hear a higher pitch as it is moving toward you and a lower pitch as it moves away. You are hearing the Doppler effect. It works for light as well as sound. The frequency (pitch for sound) and wavelength of both sound and light change if the source is moving relative to the observer. Think of the waves as either being stretched out or squeezed together. Note that either the source or the observer can be moving. When applied to light, the Doppler effect causes light from a source moving away to be shifted to a longer wavelength and light from an incoming source to be shifted to a shorter wavelength. Because red light has a longer wavelength than blue light, the shift toward a longer wavelength is a redshift.
Closer astronomical objects, such as those within our galaxy or its near vicinity, show redshifts (or blue-shifts) caused by the Doppler affect. More distant objects, such as far-away galaxies, show a redshift with a different cause, namely, the expansion of space itself. This effect, called cosmological redshift, arises because the universe is expanding: as light traverses the interval, the interval itself is stretching, which stretches out the light. The resulting lower wavelength is a redshift. It is this form of redshift that dominates at cosmic distances, not the Doppler effect. Astronomers speak of distant galaxies “moving away” from us, but this motion is only apparent: our own galaxy is just as much (or as little) in motion as any other in the Universe.
When astronomers observe the spectrum of a star or galaxy, they see spectral lines that are produced at specific wavelengths. The wavelengths of these spectral lines are determined by the chemical composition and various physical conditions. The correct wavelengths for spectral lines produced by different elements at rest are measured in laboratories on Earth. To look for the redshift, astronomers must compare the observed wavelengths of spectral lines to the wavelengths expected from the laboratory measurements. If a spectral line is at a shorter wavelength, it is blue-shifted and the star or galaxy is moving toward us. If, on the other hand, the spectral line is at a longer than expected wavelength, it is redshifted and comes from a star or galaxy that is moving away from us.
Doppler shifts of stars within our galaxy tell us about the motions of the stars within our galaxy. In turn these motions provide clues to help us understand the galaxy. The stars in our galaxy are all orbiting the center of the galaxy, but at slightly different velocities. There are different populations of stars consisting of relatively young population I stars and older population II stars. Doppler shifts of stars belonging to these two populations tell us that the younger stars have orbital velocities fairly similar to the Sun’s. The older stars, on the other hand, have orbital velocities that differ from the sun’s because they have orbits that extend above or below the plane of the galaxy. These velocity studies tell us that younger stars are distributed in a disk and older stars have a more spherical distribution. Hence the galaxy was initially spherical but has flattened into a disk.
The spectra of some stars show two sets of spectral lines that have alternating red and blue shifts. When one set of lines is redshifted the other is blueshifted. This spectral behavior indicates that the star is really a system of two stars orbiting each other so closely that they appear as one star. As each star orbits the other, it alternates between moving toward and away from us. We therefore see alternate red and blue shifts for each star. These systems are called spectroscopic binaries because the Doppler shifts in their spectra reveal their true nature as binary systems. The orbital properties of these systems are determined by the masses of the stars in the system. Hence, studying the orbits of spectroscopic binaries allows us to find the masses of the stars in the system. Binary stars are the only stars for which we can measure the mass, so these spectroscopic binaries are quite important. Knowing the masses of stars is important because the mass of a star is the single most important property in determining its evolution.
Doppler shifts also help us to find the mass of our galaxy and other galaxies. The Doppler shifts of stars and other components in our galaxy help us find the orbital velocities of these objects around the center of
Blueshift— The Doppler shift observed when a celestial object is moving closer to Earth.
Doppler effect— The apparent change in the wavelength of a signal due to the relative motion between the source and observer.
Redshift— The lengthening of the frequency of light waves as they travel away from an observer caused by the Doppler effect.
Spectral line— In the spectrum of a celestial object a spectral line occurs when a particular wavelength is either brighter or fainter than the surrounding wavelengths by a significant amount. Each element produces its own unique set of spectral lines.
Spectrum— A display of the intensity of radiation versus wavelength.
the galaxy. The orbital velocities of objects near the edge of the galaxy are determined by the mass of the galaxy, so we can use these velocities to derive the mass of the galaxy. For other galaxies, we can find the orbital velocities of stars near the edge of the galaxy by looking at the difference in the Doppler shift for each side of the galaxy. Again, the orbital velocities allow us to find the masses of these other galaxies.
Perhaps the most significant redshifts observed are those from distant galaxies. When Edwin Hubble (1889–1953) first started measuring distances to galaxies, he noticed that distant galaxies all had a red-shift. The more distant a galaxy is, the larger the redshift. Galaxies are moving away from us, and the more distant galaxies are moving away faster. This effect, named Hubble’s law after its discoverer, allows us to measure the distance to distant galaxies. More importantly, it tells us that the universe is expanding. This expansion, and the way it appears to us on Earth, has often been likened to the inflation of a polka-dotted balloon. As the balloon grows, the dots grow farther apart and surface area of the balloon increases. Moreover, each dot sees all the other dots receding from it: all dots have exactly the same view, and dots farther away (as measured over the surface of the balloon) appear to be receding more rapidly. This is only an analogy, as in the case of the balloon the dots really are moving away from each other. Their velocities are real, not apparent.
Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.
Iye, Masanori, et al. “A Galaxy at a Redshift z = 6.96.” Nature. 443 (2006): 186-188.
Paul A. Heckert