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When we think of astronomy, spectacular, colorful pictures of swirling galaxies, collapsing stars, and giant clouds of interstellar gas come to mind. In reality, however, some of the most useful observational data in astronomy does not involve images at all. Spectroscopic techniques are powerful tools that allow scientists detect the presence of certain elements or processes in faraway galaxies.

In spectroscopy, incoming light—such as that from a star—is passed through a grating or a prism that splits the light up into its constituent wavelengths, or colors. Normally, a very bright, hot star will emit a continuous spectrum of light that spreads like a rainbow across the electromagnetic spectrum. In the case of lower density gas masses such as nebulae, however, the light will be emitted only at certain specific wavelengths defined by the elements found in the nebula—hydrogen atoms, for example—generate vivid yellow lines at characteristic wavelengths. The spectra will thus consist of a collection of bright lines in an otherwise dark background; this is called an emission spectrum. Similarly, if a star is surrounded by a cooler atmosphere, the atoms in the atmosphere will absorb certain wavelengths, leaving dark lines in what would otherwise be a continuum. This is known as an absorption spectrum.

Scientists study absorption and emission spectra to discover the elements present in stars, galaxies, gas clouds, or planet-forming nebulae. By monitoring the amount by which spectroscopic lines shift toward red wavelengths or toward blue wavelengths, astronomers can determine whether objects are moving toward or away from the Earth. This technique, based on the Doppler shift, is not only used to help astronomers study the expansion of the universe, but to determine the distance or age of the object under study. By studying the Doppler shift of stellar spectra, astronomers have been able to monitor faint wobbles in the motion of stars that indicate the presence of a companion star or even of extrasolar planets.

Although the sophisticated instruments and analysis techniques of astrophysics assist in the understanding of universe, astronomy is essentially about the observation of light. Using the data produced by a multitude of telescopes around the world and in orbit, astronomers are making new discoveries on a daily basis, and just as often exposing new puzzles to solve. The basic tools described above help scientists to extract information about stellar objects, and thus about the processes at work in the Universe.



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Crosswell, Ken. See the Stars: Your First Guide to the Night Sky. Boyds Mills PA: Boyds Mills Press, 2000.

Hawking, Stephen. The Illustrated Brief History of Time, Updated and Expanded. New York: Bantam, 2001.

Rees, Martin J. Our Cosmic Habitat. Princeton, NJ: Princeton University Press, 2001.

Sagan, Carl. Cosmos New York: Random House, 2002.


Nemiroff, Robert, and Jerry Bonnell. National Air and Space Administration and Michigan Technological University.
"Astronony Picture of the Day" [cited February 5, 2003]. <http://antwrp.gsfc.nasa.gov/apod/astropix.html>.

K. Lee Lerner

Kristen Lewotsky


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Absolute magnitude

—The apparent brightness of a star, measured in units of magnitudes, at a fixed distance of 10 parsecs.

Absorption spectrum

—The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.

Apparent magnitude

—The brightness of a star, measured in units of magnitudes, in the visual part of the electromagnetic spectrum, the region to which our eyes are most sensitive.

Emission spectrum

—A spectrum consisting of bright lines generated by specific atoms or atomic processes.


—The amount of light emitted from a source per unit area.


—A technique for studying light by breaking it down into its constituent wavelengths.

Additional topics

Science EncyclopediaScience & Philosophy: A-series and B-series to Ballistic Missiles - Categories Of Ballistic MissileAstronomy - History And Impact Of Astronomy, The Science Of Astronomy, Quantifying Light—luminosity And Spectral Classes, Spectroscopy