A spectral line is light of a single frequency, or wavelength, which is emitted (or absorbed) by an atom when an electron changes its energy level. Because the energy levels of electrons vary from element to element, scientists can determine the chemical composition of an object from a distance by examining its spectrum. In addition, the shift of a spectral line from its predicted position can show the speed at which an astronomical object is moving away from Earth. The measurement of spectral lines is the basis of much of modern astronomy.
Isaac Newton was the first to discover that light from the sun was composed of multiple frequencies. In 1666, by using a prism to break sunlight into its component colors, and then recombining them with a second prism, he showed that the light coming from the sun consisted of a continuous array of colors. Until then, some believed that the colors shown by a prism were generated by the prism itself, and were not intrinsic to the sunlight.
Later experiments showed that some light sources, such as gas discharges, emit at only certain well-defined frequencies rather than over a continuous distribution of colors; the resultant image is called an emission spectrum. Still other sources were found to produce nearly continuous spectra (i.e., smooth rainbows of color) with distinct gaps at particular locations; these are known as absorption spectra. By making observations of a variety of objects, Gustav Kirchhoff was able to formulate three laws to describe spectra. Kirchhoff’s laws can be put into modern form as follows: (1) an opaque object emits a continuous spectrum; (2) a glowing gas has an emission line spectrum; and (3) a source with a continuous spectrum which has a cooler gas in front of it gives an absorption spectrum.
The observation of spectra was used to discover new elements in the 1800s. For example, the element helium, although it exists on Earth, was first discovered in the sun by observing its spectrum during an eclipse.
Observations of particular elements showed that each had a characteristic spectrum. In 1885, Johann Balmer developed a simple formula that described the progression of lines seen in the spectrum of hydrogen. His formula showed that the wavelengths of the lines were related to the integers via a simple equation. Others later discovered additional series of lines in the hydrogen spectrum, which could be explained in a similar manner.
Niels Bohr was the first to explain the mechanism by which spectral lines occur at their characteristic wavelengths. He postulated that the electrons in an atom can be found only at a series of unique energy levels, and that light of a particular wavelength was emitted when the electron made a transition from one of these levels to another. The relationship between the wavelength of emitted light and the change in energy was given by Planck’s law, which states that energy is inversely proportional to wavelength (and hence directly proportional to frequency). Thus, for a given atom, light could only be emitted at certain discrete wavelengths, corresponding to the energy difference between electron energy levels. Similarly, only wavelengths corresponding to the difference between energy levels could be absorbed by an atom. This picture of the hydrogen atom, known as the Bohr atom, has since been found to be too simplified a model to describe atoms in detail, but it remains the best physical model for understanding atomic spectra.
Astronomers use a device called a spectrograph to disperse light into its constituent wavelengths in the same way that Newton’s prism divided sunlight into its component colors. Spectrographs may have a prism or a diffraction grating (an optical element consisting of a ruled surface that disperses light due to diffraction) as their dispersive element. The resultant spectrum may be recorded on film, electronically in a computer, or simply viewed with the eye. Because each element has a unique spectral signature, scientists can determine which elements make up a distant object by examining the often complicated pattern of spectral lines seen in that object. By recalling Kirchhoff’s laws, they can also determine the physics of the object being observed. For example, stars show an absorption spectrum, and they can be thought of as a hot object surrounded by a cooler gas.
Spectra can also be used to determine the relative abundances of the elements in a star, by noting the relative strength of the lines. Knowing the physics of the atoms involved allows a prediction of the relative strengths of different lines. In addition, because ions (atoms which have lost some of their electrons and become charged) have different characteristic wavelengths, and the ionization states are a measure of temperature, the temperature of a star can be determined from the measured spectra.
The minimum width of a spectral line is governed by the tenets of quantum mechanics, but physical processes can increase this width. Collisions between atoms, pressure, and temperature all can increase the observed width of a line. In addition, the width of the spectrograph entrance slit, or properties of the diffraction grating, provides a minimum width for the lines. The observed line widths can therefore be used to determine the processes occurring in the object being observed.
Spectrographs are characterized by their wavelength coverage and their resolution. A spectrograph normally consists of an entrance slit or aperture, a number of transmissive elements such as lenses, prisms, transmission gratings and windows, or reflective surfaces such as mirrors and reflection gratings. The configuration and types of materials used depend on the wavelength range being investigated, since different materials have different reflective and transmissive properties; typically, reflective systems are used in the ultraviolet region of the spectrum, where few materials transmit well. The resultant spectrum is an image of the entrance slit at different wavelengths.
The resolution of a spectrograph describes its ability to separate two nearby spectral lines. In a complex spectrum, there may be hundreds of spectral lines from many different elements, and it is important to be able to separate lines which may be adjacent.
Spectroscopy is also used in the laboratory. Applications include determining the composition of plasmas, and identifying chemical compounds.
Another way that spectral lines are used in astronomy is to determine the velocity of an object. An object which is moving away from Earth will have its spectral lines shifted to longer wavelengths due to the Doppler shift acting on the emitted photons. Similarly, objects moving towards Earth will be shifted to shorter wavelengths. By measuring the shift of a spectrum, the velocity with which the object is moving with respect to Earth can be determined. A shift to longer wavelengths is called a redshift, since red light appears on the long wavelength side of the visible spectrum, while a shift to shorter wavelengths is called a blueshift.
Doppler shift measurements of spectral lines have been used to measure the velocities of winds in stars, the speeds of outflowing gases from stars and other objects; the rotational motion of material in the center of galaxies, and the recession of galaxies due to the expansion of the universe. The latter measurements are particularly important, since they allow astronomers to probe the structure of the Universe.
The spectral lines in light from very distant galaxies is primariliy redshifted not by the Doppler effect, but by the expansion of space itself. This effect, called cosmological redshift, allows astronomers to tell how old events are that they are observing, and how far away: the two are really the same, since light travels at a fixed speed in vacuum.
Absorption spectrum —The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.
Bohr atom —A model of the atom, proposed by Niels Bohr, that describes the electrons in well-defined energy levels.
Doppler shift —The shift in wavelength of a spectroscopic line from its zero velocity wavelength to longer (redder) wavelengths if the source of the line is moving away from the observer or to shorter (bluer) wavelengths if the source is approaching the observer.
Emission spectrum —A spectrum containing narrow spectral lines at frequencies corresponding to the photon energies of the atoms making up the object being observed.
Energy level —An allowed energy state of an electron in the Bohr model of the atom.
Photon —A single quantum of light.
Planck’s law —A relationship describing the proportionality between the frequency of light and the energy of a photon.
Resolution —The ability of a spectrograph to separate two adjacent spectral lines.
Spectrograph —Instrument for dispersing light into its spectrum of wavelengths then photographing that spectrum.
Spectrum —A display of the intensity of radiation versus wavelength.
Kaufmann, William J. III. Universe. New York: W. H. Freeman and Company, 1991.
Abe, M., et al. “Near-Infrared Spectral Results of Asteroid Itokawa from the Hayabusa Spacecraft.” Science. 312 (2006): 1334-1338.