Chapter 28 Stars and the Universe

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726 CHAPTER 28: STARS AND THE UNIVERSE Using Light as a Yardstick Redshift Distances in space are so vast that light cannot reach Earth instantaneously. For example, to reach Earth, electromagnetic energy takes about 3 seconds to travel from the moon and 8 minutes from the sun. Light takes more than 4 years to arrive from the nearest night star, Proxima Centauri. The most distant object visible to the unaided eye is the Andromeda Galaxy. Light from the Andromeda galaxy takes about 2 million years to reach us. In fact, light provides a good method to measure distances in the universe. A light-year is the distance that any form of electromagnetic energy can travel in 1 year: about 6 trillion miles, or 10 trillion km. Although the light year may sound like a measure of time, it is a measure of distance. When astronomers look at distant objects in space, they see them as the objects were when the light started its long journey toward Earth. The farther away astronomers look into space, the farther back in time they see. At present, the most distance objects visible to astronomers are estimated to be about 13 billion light-years away. Astronomers can now look at the universe about a billion years after its origin, which is estimated to be 14 billion years ago. Astronomer Edwin Hubble examined the spectra of distant galaxies in the early 1900s. He compared the dark absorption lines, or spectral lines, of light from these far away galaxies to the absorption lines of nearby stars. Nearby stars had spectral lines similar to those produced in the laboratory. The light from the distant galaxies did not show the dark lines in the same colors as the light from the nearby stars. However, the dark lines in the spectra of distant galaxies were shifted toward the red end of the spectrum. Hubble reasoned that the motion of distant galaxies away from Earth causes the redshift of spectral lines. The redshift of spectral lines is illustrated in Figure 28-9.
Figure 28-9 The sun and nearby galaxies show spectral lines similar to those produced in a laboratory. However, distant galaxies show these characteristic lines shifted toward the red and of the spectrum. Astronomers interpret this as evidence that the universe is expanding. Blue WHAT IS THE HISTORY OF THE UNIVERSE? 727 Short Waves Long Waves Blue If the galaxies were moving toward Earth, the spectral lines would shift toward the blue end of the spectrum. The shift toward the red end of the spectrum indicates that the galaxies are moving away from Earth. You can observe a similar change with sound. If you stand next to a racetrack, the high-pitched sound of the approaching car changes to a lower pitch as the car speeds past you. This apparent change in frequency and wavelength of energy that occurs when the source of a wave is moving relative to an observer is called the Doppler effect. It was named for Christian Johann Doppler, the scientist who explained it in 1842. The change in the frequency and wavelength of sound waves is similar to the changes that Hubble observed with light. The greater the redshift, the faster the object is moving away. Astronomers have found that the most distant galaxies are moving away the fastest. ACTIVITY 28-5 DEMONSTRATING THE DOPPLER EFFECT This procedure should be done only under teacher or adult supervision. This activity requires a noisemaker that can be tied to a strong cord, or string, and swung around your head. A noisemaker such as an alarm clock or a small battery-operated device from a Red Short Waves Long Waves Blue Reference Wavelength Red Red Short Waves Long Waves Sun Nearby Galaxy Distant Galaxy
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