Astronomy Lab: Stellar Spectra

Goal: To analyze stellar spectra and identify spectral type

Background

Stellar spectra information is gathered both analogically (a photograph of the spectra) and digitally (measured intensities of light over a range of wavelengths). This lab will look in detail at the latter, but to get a sense of the analogical classification, try your hand at this practice site. This site allows you to match known spectral types against that of a single star. Record the star name, your best guess for its spectral type, and the actual star type.

If you have fun with this kind of thing, you can also play Guess the Elements at the science of light site. After reading the brief background information, click on Go! at the bottom of the page to test your ability to recognize different elements in star spectra.

For this lab, we will use actual stellar spectra taken with the ELODIE at the Observatoire de Haute-Provence echelle spectrograph between 1993 and 2006. Spectra were captured in the range 390-680nm. The intensity range (vertical axis) varies depending on the magnitude of the star.

Materials and tools

• Stellar Spectra from the ELODIE archive (see below).
• Comparison spectral line information for H, He, C, O, N, Ca.

Raw Data

• Sun:

• Star 1:

  

• Star 2:

  

• Star 3:

  

Data Analysis

1. Start by comparing the four spectra. What do you notice about them?
2. How might you explain the difference in slope or shape of the four spectra?
3. How might you explain the difference between the spectra of the Sun and Star #2, and the spectra of stars #1 and #3?
4. What is the peak wavelength for star #3? Use Wien's law to predict a possible value for surface temperature for this star. The actual surface temperature is 9940K. What wavelength does Wien's law predict for this temperature?
5. Identify the four main dips for star #1. What would cause dips like these in a stellar spectrum?
6. Identify the major dips in the spectrum for star #3. How do they compare with the dips for star #1? What conclusions might you be able to draw about these two stars despite their obvious "spectrum shape" differences?
7. The Balmer hydrogen spectra has emission lines at the following wavelengths, due to the specified electron transitions:
   • H-α: 656.3nm (level 3 to level 2)
   • H-β: 485.1 (level 4 to level 2)
   • H-γ: 434.1 (level 5 to level 2)
   • H-δ: 410.2 (level 6 to level 2)
   • H-ε: 397.0nm (level 7 to level 2)

   Compare these values with the dips for stars #1 and star #3. Also compare the widths of the dips. What might you conclude about the relative temperatures of these stars?

8. What conclusions can you make about the comparative surface temperatures for stars #1, #2, and #3 from their spectra?

Report

This time, since you didn't actually collect the data, we are more concerned with your analysis of it. Your report should include your answers to the questions in the data analysis section above, plus any other observations you care to make on the data presented.

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