How do we actually measure the Doppler shift? 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 Doppler shift 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 blueshifted 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 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 first started measuring distances to galaxies, he noticed that distant galaxies all had a redshift. 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. Think of making a loaf of raisin bread. As the dough rises, the raisins move farther apart. If your pet ant named Hubble was on one of the raisins as you made the bread, it would look at the other raisins and see them moving away. If the raisins are like galaxies, and the dough like the space between galaxies, we see the same effect as the universe expands. Distant galaxies have large redshifts because they are moving away from us. Hence the universe is expanding. An apparently simple effect observed on the Earth has far-reaching implications for our understanding of the cosmos.
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Paul A. Heckert