Spectrometer Used in Astronomy
by Ron Kurtus (revised 5 February 2017)
A spectrometer (or spectroscope) is a device that spreads out a incoming beam of light into its spectrum of different colors or wavelengths. It is one of the most useful devices in astronomy, because it allows astronomers to determine the chemical composition of planets and stars, as well as to indicate the speed and direction of a star or galaxy.
Questions you may have include:
- How does a spectrometer work?
- How does a spectrometer determine chemical composition?
- How does a spectrometer determine speed?
This lesson will answer those questions.
Spectrometer spreads light waves
A spectrometer is a scientific instrument that uses glass prism or diffraction grating to spread an incoming beam of light into its spectrum. The different parts of the spectrum can then be measured as to its wavelength and intensity.
Spectrometers are combined with astronomical telescopes—both on Earth and in space—to examine light from stars, galaxies, and even other planets.
Determines chemical composition
What makes this device especially useful is that when a chemical element is heated, it gives off light in discrete spectral lines, as opposed to a continuous range of colors. A spectral line is a single wavelength or frequency of light.
By using a spectrometer and measuring the wavelengths of the spectral lines, you can determine what materials are being heated. The intensity of the spectral line also gives an indication of the amount of the element in the object.
A good example can be seen by sprinkling table salt (Sodium Chloride or NaCl) in a candle flame. The flame turns yellow, because of the two strong yellow spectral lines of Sodium.
Yellow spectral emission lines of Sodium
By knowing the spectral lines of the various elements, you can determine what the chemical composition is of a distant star.
Calculates velocity of object
You can also use a spectrometer to determine the velocity of a distant object by the Doppler Effect on the spectrum.
(See Doppler Effect Equations for Light for more information.)
When a star is moving away from Earth, its spectral lines will shift toward lower frequencies. This is called a red-shift, because the lines are moving toward the red end of the spectrum. The velocity away from Earth can then be calculated from the amount of red-shift.
When the star or galaxy is moving toward the observer, the spectral lines shift toward the blue end. This is called a blue-shift. Again, the velocity can be calculated from the amount of shift.
A spectrometer uses a prism or diffraction rating to spread out a incoming beam of light into its spectrum of different colors or wavelengths. The tool allows astronomers to determine the chemical composition of planets and stars, as well as to indicate the speed and direction of a star or galaxy.
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Spectrometer Used in Astronomy