Harvard University Astronomy Lab and Clay Telescope

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The Solar Spectrum

Spectral lines are universal signatures of elements; the NaD lines have exactly the same wavelengths from our lamp (in emission) as in the Sun (in absorption) if we measured in the center of the Sun where there is no radial velocity componet of rotation. Precise measures of the wavelengths of lines from stars uniquely identify the elements and velocities of stars, and emission vs. absorption lines yield the relative temperature and density of the source regions.

Procedure:

• Direct light into the lab and through the spectrograph slit using the heliostat mirrors. With the room lights off and the shades down, take turns looking at the solar spectrum with the eyepiece lens. You should notice the Solar continuum and the dark vertical absorption lines.
• Take particular note to the NaD lines (which are marked on the spectrograph) and adjust the spectrograph slit. Notice how the sharpness of the lines depends on the width of the slit. Adjust the slit for maximum sharpness of lines yet acceptable level of brightness.
• Each person should identify 3 lines in the Solar spectrum.
  • This Wikipedia link is a nice visual reference to the Solar spectrum and line ID list. It is a nice guide to some of the major Fraunhofer Lines: CaII H&K lines, the group of G lines (Fe, H and Ca), and F and C lines (Hβ and Hα lines from H), the two neightboring D1 and D2 lines of Na, the D3 line of He, and the b lines of Mg.
  • Using an online version of the Solar spectrum look up 3 different lines. Do this by either clicking on the colored spectrum or typing in the wavelngth in the middle search box. Enter your observations in table 1(pdf or docx).
  • Note the line profiles, or shapes, when you observe them. Why are some lines broader than others?
• Note that all the lines are in absorption. Why? You MIGHT be able to see emission from the solar chromoshpere by doing the following:
  • Find the Hα line (in the red) and mark it (with pencil) on the spectrograph
  • Now use the N-S heliostat guiding button to move the Sun downwards along the slit so the bright solar "limb" (edge of Sun) is just below the bottom edge of the slit. What do you see now at the position where Hα should be? An emission line (glowing "pinkish red"). Why?