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Spectral Lines

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In order to study electromagnetic radiation from space, astronomers need to separate the radiation according to each of the frequencies present. They use an instrument called a spectrometer to do just that. For example, an optical telescope detects visible light comprising of electromagnetic waves the frequencies of which range between 500 and 700 nanometers. After the light is collected by the telescope, a spectrometer breaks it up into its constituent colors. The resulting spectrum provides a unique signature of the frequencies present in the light.

Sodium Spectral Lines

These are the spectral lines of burning sodium atoms, as measured by a light spectrometer. When heated, sodium atoms always emits spectral lines at the same frequencies; the electromagnetic "barcode" unambiguously demonstrates the presence of the element.

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Spectrometers are also used with non-optical telescopes, including radiotelescopes. Radio spectrometers allow researchers to examine the spectral lines that characterise molecules, such as hydrogen cyanide or HCN, a molecule commonly found in space.

HCN Spectral Lines

Unlike sodium gas which is comprised of atoms, HCN and other molecules exhibit spectral lines that fall within the radio portion of the electromagnetic spectrum. The peaks in the radio emissions define the spectral lines of such molecules.

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What Causes Spectral Lines?

Most of the spectral lines emitted by a single atom are associated with changes in energies of its orbiting electrons. For instance, as sodium atoms gain energy when heated, their electrons eventually release the extra energy in the form of visible light. The light exhibits unique frequencies or spectral lines characteristic of this energy change.

Molecules, on the other hand, can display spectral lines linked not only to energy changes in their constituent atoms, but also to their molecular motions . For example, HCN produces spectral lines corresponding to changes in how fast it rotates or vibrates. As molecules gain energy, their internal motions or vibrations increase. This energy is released as electromagnetic radiation possessing characteristic spectral lines. As energy is lost, the molecular motions and vibrations slow down.

Lewis Snyder, University of Illinois, on-camera
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But what causes the changes in energy associated with spectral lines? A variety of processes operate here. These include collisions between atoms and molecules, their interactions with electromagnetic radiation at different wavelengths, and with different types of charge particles found in space. Such particles are accelerated by powerful electrical and magnetic fields prevalent near some stars, particularly when they are born or dying.

The Spectral Barcode

The spectral line emissions or "cosmic barcodes" associated with each of these processes releases provides a unique signature. The study of spectral lines is known as spectroscopy and is heavily used in astronomy.

Aside from yielding much information about structure, composition and changes in energy, spectroscopy also reveals the motions relative to Earth of a host of astronomical objects. Stars, galaxies and giant molecular clouds for example. By studying how their spectral lines are shifted by their motions with respect to Earth, scientists can determine not only their characteristic motions but also estimate the forces responsible, particularly gravity.

Finally, since spectral lines reveal the presence of specific atoms and molecules in space, these same components can serve as tracers of the physical conditions in their surroundings, particularly those of temperature, density, and pressure.

All this from barcode? Amazing.

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NCSA. Last modified 11/10/95