Spectra

As we saw above, all matter emits a spectrum of light with a peak frequency that depends on its temperature. This is how the incandescent light bulbs in your house shine, the heated filament is hot enough to radiate in the visible part of the electromagnetic spectrum (EMS). This manner of generating light is quite inefficient, however, since a great deal of energy is liberated outside the visible spectrum. You have only to touch a working incandescent light bulb to demonstrate this… Ouch! Hot!

 

As we discussed above, light can be generated in another way that is much more energy efficient. Gas tubes or energy saving light bulbs and tubes generate light by heating atoms of gases within the tubes. The electrons in the heated (or excited) atoms, absorb the energy provided by an applied electrical charge, jump into higher orbital states, and then fall back again, emitting this energy as light. The light emitted is at specific, discrete wavelengths, not as a broad spectrum of light as with incandescent bulbs or other objects emitting black body radiation. The energies of these emitted photons correspond to the difference in energy levels of the excited electrons. Because each atom is different, with different numbers of electrons in different energy states, this then becomes the unique, identifying fingerprint of the gas in the tubes. Below are the spectra of a few different gases; all different, all unique.

 

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The vertical lines are each caused by a photon being emitted as an electron falls to a lower lever in atoms of gases of the elements identified. They are called emission spectra because the result from the emission of electrons by excited atoms.

 

By breaking light into its component parts, as with a prism, scientists can view the identifying spectral lines and can tell what substances make up distant planets and stars. Further analysis of these same spectral lines can also tell us about temperature, pressure, age, wind speeds and atmospheric dynamics, magnetic field strength, abundances, evolutionary history, weather, and many other qualities of planetary and stellar systems. It is truly amazing what astronomers learn from light!

 

Another type of stellar spectrum is produced by absorption. A gas in front of a full spectrum source (a black body radiator such as a star) will absorb some wavelengths, producing a full spectrum with dark absorption lines. Stars make such spectra because the outer layers of gases absorb certain wavelengths.

 

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Stellar spectra from very hot stars (O and B) to relatively cool stars (K and M), showing atomic and molecular absorption lines. The spectral lines, in this case, are in absorption, not emission, so they appear dark. The HD, BD, etc on the right are the designations of individual stars. (NASA image)