Like the spectroscope, new tools of science nearly always increase our understanding of the universe. About one hundred years ago, the first really large telescopes were manufactured. One of these was the one-hundred-inch reflecting telescope on top of Mt. Wilson in Southern California. An American astronomer named Edwin Hubble used this new and powerful scope to discover that the universe was much bigger than had been thought in the past. Prior to that time, it was believed that the whole universe was made of just our own Milky Way Galaxy. Hubble was able to see that many fuzzy, dim patches of light (thought to be clouds of dust within our own galaxy) were in fact distant collections of millions upon millions of stars. These clusters of stars were not part of our own galaxy. Hubble realized that he was seeing separate, distant galaxies like our own. He realized that the universe was full of millions of galaxies beyond our own.
Hubble went on to discover that the spectral lines from the light of these galaxies were not in the expected places. Look at the two bright lines for sodium in the figure below. The normal position for these two sodium lines is at about 5,900 angstroms, as shown in the first spectra. When Hubble saw sodium lines in the spectra of the distant galaxies, they were not in the normal location at 5,900 angstroms; they were closer to the 6,000 angstrom line as shown in the second spectra. The spectral lines had been shifted toward the red side of the spectrum. This shifting of the spectral lines was consistent for all of the elements—the spectral lines were always shifted toward the red side.
Hubble called this change in position of the spectral lines a redshift because the spectral lines were shifting towards the red side of the spectrum. Today we know of thousands more galaxies than Hubble did, yet when we measure the spectra of all these newly discovered galaxies, Hubble’s redshift is confirmed. Nearly every galaxy in our universe has a redshift in its spectrum.
Astronomers wondered why the spectral lines were shifted toward the red side of the spectrum. The answer is provided by the work of Christian Doppler, who studied the properties of sound waves produced by a moving source like a train or car horn in the 1800s. Doppler knew from daily experience that the sound of a train rushing away from the station is lower in pitch than the sound of one approaching. This is due to the fact that the receding train stretches out the sound waves. This stretching causes the waves to have longer wavelengths and thus lower pitch. The approaching train bunches waves together ahead of it. The wavelengths of these compressed waves are shorter, and we hear this as a higher pitch. This change in pitch of a sound wave caused by stretching or compression is called the Doppler effect.
A moving sound source can be detected by measuring its change in pitch, which is determined by the wavelength. The faster the object moves away, the longer the wavelength, resulting in a lower pitch. The faster the object approaches, the shorter the wavelength, resulting in a higher pitch.
For a refresher of how the Doppler effect functions with sound waves, please view the course video for this unit.
Light waves from moving light sources (like stars and galaxies) will also show this effect. If the light source is moving away from Earth, the light will have wavelengths that are stretched out. Remember, the color red represents the visible light with longer wavelengths—the longer waves, then, appear shifted to the red side of the spectrum. They will show a redshift.
If the galaxy were moving toward us, the wavelengths of its light would be shorter. We would thus see a blue shift because shortened waves appear bluer to our vision.
The faster the galaxy or star moves away, the more the lines are shifted toward the red end of the spectrum. The same is true for objects coming toward the earth. The faster the star or galaxy is approaching Earth, the more the lines are shifted toward the blue end of the spectrum. The Doppler effect provides a way to measure the speed of objects as they move toward or away from Earth.
Remember these rules for the Doppler effect: