## Identifying Elements in the Sun Using Spectral Lines

Modified from a lab from the University of Washington Dept. of Astronomy      (original)
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.

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.

The figure above shows spectral lines in light from the sun.  The wavelength scale is measured in Angstroms (1 mm = 10,000 Angstrom).

Table 1, below, lists some of the important and easily identified spectral lines.  Note, the oxygen lines are for oxygen atoms in Earth's atmosphere.

Table 1 -- "Known" Lines
Designation
Wavelength (Angstrom)
Origin
A
7594
terrestrial oxygen
B
6867
terrestrial oxygen
C
6563
hydrogen (Hα)
D1
5896
neutral sodium (Na I)
D2
5890
neutral sodium (Na I)
E
5270
neutral iron (Fe I)
F
4861
hydrogen (Hβ)
H
3968
ionized calcium (Ca II)
K
3934
ionized calcium (Ca II)

- Identify (pick out) each of these "Known" spectral lines, A through K, on the figure above.

Table 2.  More Spectral Lines.  (Note: 1 nm equals 10 Angstrom)

Wavelength (nm) Line Width (nm) Element Wavelength (nm) Line Width (nm) Element 393.3682 2.0253 Ca II 440.4761 0.0898 Fe I 394.4016 0.0488 Al I 441.5135 0.0417 Fe I 396.1535 0.0621 Al I 452.8627 0.0275 Fe I 396.8492 1.5467 Ca II 455.4036 0.0159 Ba II 404.5825 0.1174 Fe I 470.3003 0.0326 Mg I 406.3605 0.0787 Fe I 486.1342 0.3680 H 407.1749 0.0723 Fe I 489.1502 0.0312 Fe I 407.7724 0.0428 Sr II 492.0514 0.0471 Fe I 410.1748 0.3133 H 495.7613 0.0696 Fe I 413.2067 0.0404 Fe I 516.7327 0.0935 Mg I 414.3878 0.0466 Fe I 517.2698 0.1259 Mg I 416.7277 0.0200 Mg I 518.3619 0.1584 Mg I 420.2040 0.0326 Fe I 525.0216 0.0062 Fe I 422.6740 0.1476 Ca I 526.9550 0.0478 Fe I 423.5949 0.0385 Fe I 532.8051 0.0375 Fe I 425.0130 0.0342 Fe I 552.8418 0.0293 Mg I 425.0797 0.0400 Fe I 588.9973 0.0752 Na I (D2) 425.4346 0.0393 Cr I 589.5940 0.0564 Na I (D1) 426.0486 0.0595 Fe I 610.2727 0.0135 Ca I 427.1774 0.0756 Fe I 612.2226 0.0222 Ca I 432.5775 0.0793 Fe I 616.2180 0.0222 Ca O 434.0475 0.2855 H 630.2499 0.0083 Fe I 438.3557 0.1008 Fe I 656.2808 0.1020 H

Using Table 2, identify the "Unknown" spectral lines labelled a through h in the figure of the solar spectrum above.  Record your results.

Note: The wavelengths listed in the two tables have been determined in the laboratory here on Earth.  Using these known spectral lines we can identify elements found in the sun and other extraterrestrial objects.