The sun in the visible band. Courtesy of SOHO/MDI consortium. SOHO is a project of international cooperation between ESA and NASA.

Adapted from Learning Astronomy by Doing Astronomy by Ana Larson

Summary

The student will identifies lines of the solar spectrum, using interpolation from "known" Fraunhofer lines.

Background and Theory

The brightest star in our sky is the Sun. Absorption lines in the solar spectrum were first noticed by an English astronomer in 1802, but it was a German physicist, Joseph von Fraunhofer, who first measured and cataloged over 600 of them about 10 years later. These lines are now known collectively as the "Fraunhofer lines." In the 1800's, scientists did not know that these lines were chemical in origin. Thus, the letters used by Fraunhofer to identify the lines have no relation to chemical symbols nor to the symbols used to designate the spectral types of stars. Today's astronomers use some of the designations simply for convenience and ease in identifying lines.

Now we know that each absorption line is caused by a transition of an electron between energy levels in an atom. Each element has a distinct pattern of absorption lines. Once the pattern of the lines of a particular element have been observed in the laboratory, it is possible to determine whether those elements exist elsewhere in the universe simply by pattern matching the absorption lines.

The strongest Fraunhofer lines of the Sun can easily be seen with even the most primitive spectroscope. By viewing a bright sky (of course, never look directly at the Sun -- the lines you see will be those of your retina burning), or the full Moon with a spectroscope or diffraction grating, you should be able to see at least a few absorption lines. In this exercise, we work with the solar spectrum between approximately 390 and 660 nm (3900 - 6600 Angstroms) and identify some of the strongest Fraunhofer lines.

Procedure

Table 1 -- "Known" Lines
Designation Wavelength (nm) Origin
A 759.4 terrestrial oxygen
B 686.7 terrestrial oxygen
C 656.3 hydrogen (Hα)
D1 589.6 neutral sodium (Na I)
D2 589.0 neutral sodium (Na I)
E 527.0 neutral iron (Fe I)
F 486.1 hydrogen (Hβ)
H 396.8 ionized calcium (Ca II)
K 393.4 ionized calcium (Ca II)
Part A: Determine the Scaling factor (completing Table 2)
  1. On the solar spectrum, measure the distance (in pixels) between two widely spaced, "known" lines (see Table 2).
  2. Find the distance between these lines in nanometers using Table 1.
  3. Divide the distance in nanometers (step 2) by the distance in pixels (step 1) to get the number of nanometers per pixel.
  4. Average the results of the four measurements to get the scaling factor.

Part B: Calculate the wavelengths of the "unknown" lines (completing Table 3)
  1. Pick one of the lines to serve as your reference; for example, the K line of Ca II at 393.4 nm. Measure the distance from this line to each of the numbered lines (1-13) in pixels.
  2. Use the scaling factor and convert the distances in pixels to distances in nm.
  3. Add the distance in nm to the wavelength of your reference line to get the wavelength of the "unknown" lines.
  4. Compare these wavelengths to the list of lines in Table 4 and identify the "unknown" lines. Your values may not exactly match those given in the table. Why?
    Note: If you find that some of your calculated wavelengths do not seem to match any of those in the table, find the closest match and the corresponding element. "Flag" these wavelengths in Table 3.

© 1999 University of Washington
Revised: 3 February, 2000