Read Any Good Spectral Lines Lately

Using the spectrum and armed with the proper instrumentation, then, astronomers can accurately read the temperature of even very distant objects in space. And even without sophisticated equipment, you can startle your friends by letting them know that Betelgeuse (a reddish star) must have a lower surface temperature than the yellow sun.

Astronomers also use the spectrum to learn even more about distant sources. A spectroscope passes incoming light through a narrow slit and prism, splitting the light into its component colors. Certain processes in atoms and molecules give rise to emission at very particular wavelengths. Using such a device, astronomers can view these individual spectral lines and glean even more information about conditions at the source of the light.

While ordinary white light simply breaks down into a continuous spectrum—the entire rainbow of hues, from red to violet, shading into one another—light emitted by certain substances produces an emission spectrum with discrete emission lines, which are, in effect, the fingerprint of the substance.

Hydrogen, for example, has four clearly visible spectral lines in the visible part of the spectrum (red, blue-green, violet, and deep violet). The color from these four lines (added together as light) is pinkish. These four spectral lines result from the electron that is bound to the proton in a hydrogen atom jumping between particular energy levels. There are many other spectral lines being emitted; it just so happens that only four of them are in the visible part of the spectrum. Hot hydrogen gas is the source of the pinkish emission from regions around young stars like the Orion nebula (Chapter 18, "Stellar Careers").

In our hydrogen atom example, a negative electron is bound to a positive proton. The electron, while bound to the proton, can only exist in certain specific states or energy levels. Think of these energy levels as rungs on a ladder. The electron is either on the first rung or the second rung. It can't be in between. When the electron moves from one energy level to another (say, from a higher one to a lower one), it gives off energy in the form of a photon. Since the levels that the electron can inhabit are limited, only photons of a few specific frequencies are given off. These particular photons are apparent as bright regions in the spectrum of hydrogen: the element's spectral emission lines.

Star Words

Spectral lines have a certain rest frequency; that is, there is a certain frequency at which we expect to see the line. If the line is somewhere else, or if a series of lines are all systematically moved, then the source of the lines might be in motion relative to us. If the source is coming toward us, the lines will occur at a higher frequency (known as a blue shift), and if the source is moving away from us, the lines will occur at a lower frequency (known as a red shift). The Doppler shift is most familiar in sound waves. We have all heard the shift in tone (frequency) that occurs when a car drives past us, especially if the driver leans on the horn as he passes. Doppler shifts are a property of all waves, light or sound.

Astronomer's Notebook

Another way spectral lines arise is when molecules rotate or vibrate. Think of it this way: Your fan has three settings, which are low, medium or high. Molecules appear to have settings as well. They can only rotate (like the fan) at particular speeds, and when the switch is turned from high to medium, they give off a photon of light. Spectral lines from molecules are very important ways to study our own Galaxy, as we'll see in Chapter 25, "What About the Big Bang?"

Astronomer's Notebook

Another way spectral lines arise is when molecules rotate or vibrate. Think of it this way: Your fan has three settings, which are low, medium or high. Molecules appear to have settings as well. They can only rotate (like the fan) at particular speeds, and when the switch is turned from high to medium, they give off a photon of light. Spectral lines from molecules are very important ways to study our own Galaxy, as we'll see in Chapter 25, "What About the Big Bang?"

Depending on how we view an astronomical source, we will see different types of spectra. A black-body source that is viewed directly will produce a continuous spectrum. But if the photons from the source pass through a foreground cloud of material, certain energies will be absorbed by the cloud (depending on its composition), and we will see a black-body spectrum with certain portions of the spectrum missing or dark. This is called an absorption spectrum. If a cloud of material absorbs energy and then re-emits it in a different direction, we see the result as emission lines, or bright regions in the spectrum. The clouds of hot gas around young stars produce such emission lines.

The light that reaches us from stars carries a lot of information. The color of the object can tell us its temperature, the wavelength of the light reaching us can tell us about the energies involved, and the presence (or absence) of certain wavelengths in a black-body spectrum can tell us what elements are present in a given source.

And all of this information reaches us in the comfort of our home planet. Who knew we could learn so much without actually going anywhere?

The Least You Need to Know

V Visible light is a rather small subset of the electromagnetic spectrum.

V The difference between visible light and other electromagnetic waves, say, radio waves, is a matter of wavelength or frequency.

V Unlike sound waves, light waves can travel through a vacuum—empty space— because these waves are disturbances in the electromagnetic field and require no medium (substance) for transmission.

V The peak wavelength in an object's spectrum tells us its temperature.

V One way spectral lines arise is by the specific energies given off when electrons jump between energy levels in an atom, or when molecules spin at different rates.

V Astronomers use spectroscopes to read the spectral "fingerprint" produced by the light received from distant objects and thereby determine the chemical makeup of an object.

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