Harvard University Astronomy Lab and Clay Telescope

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The Spectrum of the Blue (or cloudy) Sky

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. We are not going to point the telescope at the Sun because the Sun is much too bright for our detectors. We will instead point away from the Sun to capture the scattered Sunlight!

Procedure:

• With the spectrograph ready to go, point the telescope AWAY from the Sun. Take a short (~few seconds) exopsure and check that the spectrum is not overexposed. Click here if you need a refresher on how to take spectra. When you have determined a good exposure time, take 3 exposures. It is always good to have a few observations in case one is flawed for some reason. Don't forget to take calibration spectra so you can determine a wavelength solution!

Analysis:

• Now that you have your spectrum of the blue (or cloudy) sky, you need to determine a wavelength solution.
• 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?