Identifying Lines in the Solar Spectrum

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 has 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.

1. On the solar spectrum, measure the distance (in pixels) between two widely spaced, "known" lines (see Table 2, one pair has been completed as an example).
   Note: Pixels are the small dots that make up images on computer screens
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.

1. Pick one of the lines from table 1 to serve as your reference; for example, the K line of Ca II at 393.4 nm. Fill in your reference line's designation, location (in pixels), and wavelength on the worksheet.
2. Measure the location of each of the unknown lines (numbered 1-13) in pixels.
3. Subtract the location of the reference line in pixels to find the distance in pixels from each unknown line to your reference line.
4. Use the scaling factor to find the distance in nanometers.
5. Add (or subtract) the distance in nm to the wavelength of your reference line to get the wavelength of the "unknown" lines.
6. 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.
   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.