|Energy | Pages 136-137 ||
of energy levels for each element is unique. Scientists have charted these spectral "fingerprints" in their laboratories for decades by studying patterns of light from hot and cool gases. Because many elements coexist within gases and other objects in space, each spectrum can be a collage of dozens of elemental fingerprints. The same technique can be applied to molecules such as water vapor or carbon monoxide. Those assemblages of atoms also leave characteristic imprints on spectra.
To picture in another way how this works, think back to the last fireworks display you saw. Exploding rockets lit the night with incandescent blues, reds, and other colors. Pyrotechnicians burn different materials for each color--magnesium for white, iron for red, copper for green, and so on. Now imagine what happens when those elements vaporize on the hot surface of a distant star. They produce colors in exactly the same way. When we take a photograph of the star, the colors blend together like paints on a long-unwashed palette. However, when we measure the spectrum of the star, the light spreads into its components and reveals the contributions of each element and compound.
Soon after astronomers first used spectroscopy, they discovered one-quarter of the visible universe. In 1868 the first detailed spectrum of the Sun's atmosphere exposed many familiar elements, such as hydrogen and oxygen. But no known atoms of the day could produce other features that arose. One set of spectral features was so prevalent that astronomers attributed them to a new element. Since it first appeared in the Sun--known to the Greeks as Helios--the element was dubbed helium. Today, we know that almost 25 percent of the atomic matter in the universe is helium. Here on Earth we encounter it in party balloons and Super Bowl blimps. Planetary atmospheres, the surfaces of asteroids and comets, the remnants of supernova explosions, and other objects reveal their unique mixtures of ingredients through their spectra in the same way.
Beyond composition, spectra contain a storehouse of other astrophysical data. One quantity we can deduce is temperature. Each element and compound burns at a certain temperature. For example, you could judge the temperature in your kitchen oven without a thermometer by tossing some junk mail inside. When that useless paper bursts into flames, you know the oven has crossed the magic threshold made famous by Ray Bradbury's science-fiction novel Fahrenheit 451. In a similar way, when we see a particular set of emission lines from a glowing object, we know that the object's temperature is at least as high as the burning point of that substance. Absorption lines also reveal temperatures. A cool star emits photons with relatively low energies. Certain atoms and molecules in the star's atmosphere absorb light at just those levels. If the star's temperature increases, the electrons jump to higher levels. We then see a different set of dark lines across the spectrum.
The strength of an object's magnetic field is another characteristic we can learn from a spectrum. Some spectral lines split into two parts in the presence of a magnetic field. As the field grows stronger, more lines show this effect. We also gauge field strength by observing the motions of nearby charged gas, which a magnetic field steers through space. However, spectral splitting is our most direct probe.
Finally, spectra have proven invaluable as tracers of the motions of objects. Spectral lines are not static. Rather, they shift when the glowing object moves toward or away from us. Our Earthly analogy is the Doppler effect (page 139). The pitch of an approaching race car sounds higher than it actually is, while one receding from you sounds lower. In the case of light, its wavelength--a property directly related to its energy--undergoes a "blue shift" to shorter wavelengths (higher energies) if the object approaches. A receding object displays a "red shift" to longer wavelengths of light (lower energies). The entire spectrum shifts, but the easiest changes to spot are the shifted positions of the object's bright emission lines and dark absorption lines. Even so, these changes are subtle. Unless the object moves at a fair fraction of the speed of light, we must use precise instruments to measure the shifts.
The rewards are well worth the effort. For example, when a star spins, half of it turns away from you while the other half turns toward you. As a result, part of each spectral line has a slightly shorter wavelength, while the other part has a slightly longer one. The net effect is that each line gets wider. The faster the star spins, the wider its lines become. The Doppler effect works on larger objects as well, no matter how far away they are. It shows us that spiral galaxies, millions of light-years away, spin on their axes at more than 100 miles per second. Even at that clip our gigantic Milky Way takes more than 200 million years to go around once. Moreover, when galaxies reside in rich clusters containing hundreds or thousands of similar galaxies, they dart around within those clusters at speeds up to 1,000 miles per second.